Pacific Southwest 

Research Station

General Technical 

Report

PSW-GTR-242

December 2013

United States Department of Agriculture 

Forest Service

A Reference Manual for 

Managing Sudden Oak Death 

in California

Tedmund J. Swiecki and Elizabeth A. Bernhardt
This publication is available online at www.fs.fed.us/psw/. You may also order 

additional copies of it by sending your mailing information in label form through 

one of the following means. Please specify the publication title and series 

number.

Fort Collins Service Center

Web site http://www.fs.fed.us/psw/

Telephone (970) 498-1392

FAX (970) 498-1122

E-mail rschneider@fs.fed.us

Mailing address Publications Distribution 

Rocky Mountain Research Station 

240 West Prospect Road 

Fort Collins, CO 80526-2098

Pacific Southwest Research Station

800 Buchanan Street

Albany, CA 94710

Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been 

registered. All uses of pesticides must be registered by appropriate state or federal 

agencies, or both, before they can be recommended. 

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable 

plants, and fish or other wildlife—if they are not handled or applied properly. Use 

all pesticides selectively and carefully. Follow recommended practices for the 

disposal of surplus pesticides and pesticide containers.

Authors

Tedmund J. Swiecki and Elizabeth A. Bernhardt are plant pathologists and 

principals of Phytosphere Research, 1027 Davis Street, Vacaville, CA 95687 

(email: phytosphere@phytosphere.com). 

This work was developed with support from the U.S. Department of Agriculture, 

Forest Service, State and Private Forestry, Forest Health Protection Agreement No. 

08-DG-11052021-144, with additional support provided by the Pacific Southwest 

Research Station.

Opinions expressed may not necessarily reflect the position of the U.S. Department 

of Agriculture.

The use of trade or firm names in this publication is for reader information and 

does not imply endorsement by the U.S. Department of Agriculture of any product 

or service.

Cover photograph: Canyon live oak, Russian Ridge Open Space Preserve, San 

Mateo County, California, by Ted Swiecki. All photos and drawings are by the 

authors except where indicated otherwise.
Abstract

Swiecki, Tedmund J.; Bernhardt, Elizabeth A. 2013. A reference manual for 

managing sudden oak death in California. Gen. Tech. Rep. PSW-GTR-242. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station. 129 p.

This publication contains background information and guidance for resource 

management professionals and landowners to understand and manage sudden 

oak death (SOD) in California forests. The publication is divided into three chapters: Chapter 1 discusses the epidemiology of SOD in California and includes 

information on biology of the pathogen Phytophthora ramorum, host-pathogen 

interactions, disease spread, and environmental conditions that affect disease 

development. An understanding of these relationships is needed to choose the most 

appropriate strategies for managing SOD at a given location. Chapter 2 describes 

how to develop a plan to manage SOD within a stand and how to identify and 

prioritize areas that may be suitable for SOD management activities. Options for 

managing SOD are presented by stage in the disease epidemic: before the SOD 

pathogen has reached a susceptible forest; during the local epidemic, while disease 

is active in an area and many hosts are still at risk of becoming diseased; and after 

SOD has killed so many host trees that forest restoration needs to be considered. 

Chapter 3 provides descriptions of management techniques.

Keywords: Sudden oak death, Phytophthora ramorum, tree disease 

management, tanoak, coast live oak, mixed-evergreen forests, invasive species. 
ii

Contents

1 Chapter 1—Sudden Oak Death in California: An Overview

1 1.1—Diseases Caused by Phytophthora ramorum

2 Box 1-1—Taxonomic Relationships Between Susceptible Trunk Canker Hosts

3 Box 1-2—Native Foliar Hosts of Phytophthora ramorum

4 1.1.1—Pathogen Biology

6 Box 1-3—Host Susceptibility Versus Spore Production

7 1.1.2—Origin and Distribution of Phytophthora ramorum in California

8 Box 1-4—Strains of Phytophthora ramorum

9 1.2—Sudden Oak Death Disease Cycle in California Forests

9 1.2.1—Sudden Oak Death in Tanoak Stands

12 1.2.2—Sudden Oak Death in Oak/California Bay Forests

17 1.3—Factors That Influence Sudden Oak Death Development

17 1.3.1—Environmental Conditions

18 1.3.2—Variation in Host Susceptibility

19 1.3.3—Pathogen Dispersal

19 1.4—Sudden Oak Death Development on Trunk Canker Hosts

19 1.4.1—Symptom Development

28 1.4.2—Patterns of Disease Progression

31 1.4.3—Survival of Trees With Sudden Oak Death

32 Box 1-5—Diagnosing Sudden Oak Death

33 1.5—Stand-Level Sudden Oak Death Impacts

35 Chapter 2—Managing Stands Before, During, and After Sudden 

 Oak Death

35 2.1—Developing a Sudden Oak Death Management Plan

35 2.1.1—Why is Management Needed?

36 Box 2-1—Costs Associated With Sudden Oak Death

36 2.1.2—Why Develop a Management Plan?

37 2.1.3—Elements of a Management Plan

38 Box 2-2—Disease, Damage, and Mortality Related to Other Agents

39 2.2—Management Strategies for Different Stages of the Sudden 

 Oak Death Epidemic

40 Box 2-3—Time Considerations and Sudden Oak Death

40 2.2.1—Before: Reducing Sudden Oak Death Potential in Areas Not 

 Currently Infested

44 2.2.2—During: Managing Disease in Areas Currently Affected by Sudden 

 Oak Death

46 2.2.3—After: Dealing With Impacts of Sudden Oak Death

58 Box 2-4—Restoration in Developed Versus Wildland Settings
iii

58 2.3 Prioritizing Areas for Sudden Oak Death Management Activities

59 2.3.1—Prioritization Categories

62 2.3.2—Developing Overall Priority Rankings

62 Box 2-5—Developing a Numeric Priority Ranking Scale

65 Chapter 3—Technical Management Guidelines

65 3.1—Excluding Phytophthora ramorum From Noninfested Areas

65 3.1.1—Nursery Stock

66 3.1.2—Plant Material—Pruning Waste, Wood

67 3.1.3—Firewood

68 3.1.4—Soil

68 3.1.5—Water

69 Box 3-1—Cleaning Contaminated Tools and Equipment

70 3.2—Reducing Disease Risk in Susceptible Stands

70 3.2.1—Timing of Pruning or Other Wounding

70 3.2.2—Removing California Bay

81 3.2.3—Removing Other Sporulating Hosts

82 3.2.4—Use of Chemical Protectants to Prevent Infection

84 Box 3-2—Selecting and Using Fungicides

95 3.2.5—Nonrecommended Treatments

96 3.3—Monitoring Oaks With Sudden Oak Death to Assess Survival and 

 Failure Potential

96 3.3.1—Overview

100 3.3.2—Guidelines for Assessing Mortality and Failure Risk in Trees Affected 

 by Sudden Oak Death

102 3.4—Assessing Fire Hazard Related to Trees Killed by Sudden 

 Oak Death

102 3.5—Restoring Forests Affected by Sudden Oak Death

109 3.5.1—Species Selection

111 3.5.2—Factors That Constrain Regeneration

114 3.5.3—Natural Regeneration

114 3.5.4—Planting

121 3.5.5—Monitoring Restoration Outcomes

121 Acknowledgments

122 English Equivalents

122 References
iv

Preface

This publication is intended to help resource management professionals and landowners understand and manage sudden oak death (SOD) in California forests. The 

publication is divided into three chapters. 

Chapter 1 discusses the epidemiology of SOD in California. This includes 

information on biology of the pathogen Phytophthora ramorum, host-pathogen 

interactions, disease spread, and environmental conditions that affect disease. 

An understanding of these relationships is needed to choose the most appropriate 

strategies for managing SOD at a given location. 

Chapter 2 describes how to develop a plan to manage SOD within a stand. We 

discuss how you can identify and prioritize areas that may be suitable for SOD 

management activities. Options for managing SOD are presented by stage in the 

disease epidemic: 

• Before the SOD pathogen has reached a susceptible forest.

• During the local epidemic, while disease is active in an area and many 

hosts are still at risk of becoming diseased.

• After SOD has killed so many host trees that forest restoration needs to be 

considered. 

Depending on the size of the landscape, you may need to manage different 

stages of the epidemic in different parts of your forest. 

Chapter 3 provides detailed descriptions of the management techniques that are 

discussed in Chapter 2. 

Our goal in developing this publication was to synthesize what is currently 

known about SOD into a framework to inform disease management. Readers who 

are interested in a detailed review of the published research on SOD may consult 

the literature summary compiled by Kliejunas (2010), which is available at http://

www.fs.fed.us/psw/publications/documents/psw_gtr234/. In addition, the California 

Oak Mortality Task Force (http://www.suddenoakdeath.org) Web site contains a 

comprehensive catalogue of SOD-related publications and information.
1

A Reference Manual for Managing Sudden Oak Death in California

1.1—Diseases Caused by Phytophthora ramorum

The introduced pathogen Phytophthora ramorum Werres, de Cock & Man in’t Veld 

causes a lethal trunk canker disease known as sudden oak death (SOD) (Rizzo and 

others 2002). Tanoak (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H. Oh), coast live oak (Quercus agrifolia Née), Shreve oak (Q. parvula

Greene var. shrevei (C.H. Mull.) Nixon), California black oak (Q. kelloggii Newberry), and canyon live oak (Q. chrysolepis Liebm.) are native California trees that 

are killed by SOD (see box 1-1). In areas invaded by P. ramorum, SOD has quickly 

become the most common cause of mortality of these trees (Swiecki and Bernhardt 

2010). Mortality rates are greater than 50 percent in some areas and continue to 

increase (Maloney and others 2005, Swiecki and Bernhardt 2008a). 

Phytophthora ramorum also causes lesions on leaves, twigs, and small stems 

of many native species, including tanoak, California bay (Umbellularia californica 

(Hook. & Arn.) Nutt.), Pacific madrone (Arbutus menziesii Pursh), California huckleberry (Vaccinium ovatum Pursh), manzanita (Arctostaphylos spp.), toyon (Heteromeles arbutifolia ) (Lindl.) M. Roem., poison oak (Toxicodendron diversilobum 

(Torr. & A. Gray) Greene ), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var.

menziesii), and coast redwood (Sequoia sempervirens (Lamb. ex D. Don) Endl.) 

(box 1-2). In addition, P. ramorum causes foliar and stem diseases in a wide variety 

of non-native plants, including many ornamentals (box 1-3). This wide host range 

contributes to the ability of P. ramorum to invade and become established in new 

areas. Phytophthora ramorum causes only minor damage on most of these foliar 

hosts. 

Disease can result when P. ramorum comes in contact with appropriate tissues 

of a susceptible host. However, environmental conditions must be favorable for 

a sufficient length of time for infection to occur. The rest of chapter 1 describes 

the processes that contribute to disease development. By understanding the SOD 

disease process, you will be able to determine what steps can be taken to manage 

this disease at a particular site. 

Chapter 1—Sudden Oak Death in California: An Overview
2

general technical report psw-gtr-242

Table 1-1—Susceptibility of California native tree species in the Fagaceae (oak family) to lethal trunk 

cankers caused by Phytophthora ramorum under field conditions 

Group Species P. ramorum susceptibility

Non oaks Notholithocarpus densiflorus

(=Lithocarpus densiflorus)—tanoak

Lethal trunk cankers and twig/foliar 

infections 

Quercus section 

Lobatae

red (or black)

oaks

Q. agrifolia—coast live oak 

Q. kelloggii—California black oak

Q. parvula var. shrevei—Shreve oak 

Q. wislizeni—interior live oak

Lethal trunk cankersa

Lethal trunk cankersa

Lethal trunk cankersa

Susceptible in greenhouse tests; disease 

not yet observed in field

Quercus section

Quercus

white oaks

Q. douglasii—blue oak 

Q. engelmanii—Engelmann oak

Q. garryana—Oregon white oak

Q. lobata—valley oak

Not susceptible

Not susceptible (?)b

Not susceptiblec

Not susceptible

Quercus section

Protobalanus

intermediate

oaks

Q. cedrosensis—Cedros Island oak

Q. chrysolepis—canyon live oak

Q. tomentella—island oak

Unknown

Lethal trunk cankersa

Unknown

Note: Natural oak hybrids and shrub oak species are not listed. a

 Twig or foliar infections are occasionally observed where shoots are exposed to high spore loads from California bay (Vettraino and others 

2008).

b Quercus engelmannii does not occur in areas currently infested by P. ramorum. Based on the lack of susceptibility of other related California 

white oaks (Q. douglasii, Q. garryana, Q. lobata) under field conditions, Q. engelmannii is likely not susceptible. c Quercus garryana develops cankers in laboratory inoculations, but disease has not been observed in the field (Hansen and others 2005).

In California, Phytophthora ramorum is capable of killing tanoak (Notholithocarpus densiflorus), coast live oak 

(Quercus agrifolia), Shreve oak (Q. parvula var. shrevei), 

California black oak (Q. kelloggii), and canyon live oak 

(Q. chrysolepis). These species are all members of the 

family Fagaceae.

Tanoak, native to California and southern Oregon, 

is found only in North America. Until recently, it was 

considered a member of the genus Lithocarpus. In 2008, 

tanoak was assigned to the newly created genus Notholithocarpus as its only member (Manos and others 2008). 

Under current taxonomy, all Lithocarpus species are 

native to eastern and southeastern Asia. Tanoak is unique 

among the trunk canker hosts in that it readily develops 

P. ramorum infections on twigs and leaves.

The true oaks, genus Quercus, include about 600 

species of trees and shrubs. The genus has been divided 

into two subgenera: Quercus and Cyclobalanopsis. All 

North American and European oaks are in subgenus 

Quercus, as are oaks of Africa, South and Central 

America, and many oaks of Asia. Oaks in subgenus 

Cyclobalanopsis, which is treated as a separate genus by 

some taxonomists, are native to eastern and southeastern 

Asia (Nixon 1993). 

The subgenus Quercus has been divided into varying 

numbers of sections. Representatives of three of these 

sections (table 1-1) occur in California and include at 

least 21 oak species plus a number of natural hybrids and 

varieties. Oaks in the red oak section, including species 

that are native to the Eastern United States, are susceptible to P. ramorum trunk cankers. Canyon live oak, the 

only widespread tree species from the intermediate oaks 

in California, is also susceptible to lethal trunk cankers. 

Oaks in the white oak section have not been found 

infected by this pathogen in the field. Table 1-1 summarizes information on the susceptibility of California’s 

tree species in the Fagaceae to P. ramorum trunk cankers. 

Most of California’s shrub oak species occur outside of 

the known range of SOD and have not been tested for 

susceptibility. 

Box 1-1—Taxonomic Relationships Between Susceptible Trunk Canker Hosts
3

A Reference Manual for Managing Sudden Oak Death in California

Box 1-2—Native Foliar Hosts of Phytophthora ramorum

As research on P. ramorum has continued, the list of plants known to be susceptible 

to it has expanded. An up-to-date host list is available at http://www.suddenoakdeath.

org. Table 1-2 lists native trees, shrubs, and herbaceous plants that have been found 

with P. ramorum infections on leaves or twigs under field conditions. These hosts 

develop minor to significant canopy dieback, but are not killed by P. ramorum. 

Table 1-2—California native forest species that develop Phytophthora 

ramorum leaf or twig infections (foliar hosts)

Scientific name Common name

Abies concolora White fir

Abies grandisa Grand fir

Abies magnificaa Red fir

Acer circinatuma Vine maple

Acer macrophyllum Bigleaf maple 

Adiantum aleuticum Western maidenhair fern 

Adiantum jordanii California maidenhair fern 

Aesculus californica California buckeye 

Arbutus menziesii Pacific madrone 

Arctostaphylos columbianaa Hairy manzanita

Arctostaphylos manzanita Whiteleaf manzanita 

Berberis diversifolia (= Mahonia aquifolium) Oregon-grape

Calycanthus occidentalisa Spicebush 

Ceanothus thyrsiflorusa Blueblossom 

Clintonia andrewsianaa Andrew’s clintonia bead lily 

Corylus cornutaa California hazelnut 

Dryopteris argutaa California wood fern 

Frangula californica (= Rhamnus californica) California coffeeberry 

Frangula purshiana (= Rhamnus purshiana) Cascara 

Fraxinus latifoliaa Oregon ash 

Garrya ellipticaa Silk tassel tree, coast silktassel 

Gaultheria shallona Salal, Oregon wintergreen 

Heteromeles arbutifolia Toyon 

Lonicera hispidula California honeysuckle 

Mahonia nervosaa Creeping Oregon grape

Maianthemum racemosum (= Smilacina racemosa) False Solomon’s seal 

Osmorhiza berteroia Sweet cicely 

Phoradendron serotinum subsp. macrophyllumb Colorado Desert mistletoe

Pseudotsuga menziesii var. menziesii Douglas-fir 

Continued on next page
4

general technical report psw-gtr-242

Table 1-2—California native forest species that develop Phytophthora 

ramorum leaf or twig infections (foliar hosts) (continued)

Scientific name Common name

Rhododendron spp. Rhododendron 

Rosa gymnocarpa Wood rose 

Rubus spectabilisa Salmonberry 

Sequoia sempervirens Coast redwood 

Taxus brevifoliaa Pacific yew 

Torreya californicaa California nutmeg 

Toxicodendron diversilobuma Poison oak 

Trientalis latifolia Western starflower 

Umbellularia californica California bay, California laurel, pepperwood,

 Oregon myrtle 

Vaccinium ovatum California huckleberry 

Vancouveria planipetalaa Redwood ivy, redwood insideout flower

Source: USDA APHIS 2012.

a

 Plants have been found naturally infected, and P. ramorum has been cultured or detected using 

polymerase chain reaction, but Koch’s postulates (formal proof of pathogenicity) have not been completed 

as of February 2012.

b

 Riley and Chastagner 2011.

1.1.1—Pathogen Biology 

Phytophthora ramorum and other Phytophthora species are members of the 

Oomycota, or water molds. Based on DNA analysis, the Oomycota are placed in the 

kingdom Stramenopila, which also includes diatoms and kelp. Most Phytophthora 

species attack living plants, and many are serious plant pathogens affecting forests, 

agricultural crops, and horticultural plants worldwide.

The Oomycota were previously grouped with the fungi based on their overall 

form and life habits. Like the true fungi, Phytophthora and related organisms are 

made up of a network of filaments (referred to as hyphae, or collectively as mycelium). As P. ramorum hyphae penetrate and grow into a host plant, they secrete 

chemicals that kill and degrade plant tissues. The hyphae absorb nutrients released 

from dead and dying host cells to fuel their continued growth. 

Phytophthora ramorum is disseminated via several spore types that are produced by the hyphae. These spores can only be observed with the aid of a microscope. During periods of rainy weather, especially when temperatures are relatively 

warm (see section 1.3.1, “Environmental Conditions”), sporangia (fig. 1-1) are 

produced on the surfaces of infected leaves and twigs of some hosts. The sporangia 
5

A Reference Manual for Managing Sudden Oak Death in California

Figure 1-1—Sudden oak death disease cycle in tanoak stands. Phytophthora ramorum survives over the dry season primarily in tanoak 

twig infections. Sporulation occurs during the wet season primarily on infected twigs. Rain-splashed spores initiate new leaf and twig 

infections within the trees and on nearby tanoaks. Under prolonged wet conditions, high numbers of spores produced in the canopy are 

splashed onto trunks and initiate trunk cankers (dashed arrow).

Phytophthora ramorum survives

in infected twigs on overstory

and understory tanoaks

DRY

SEASON

WET

SEASON

Spores splashed

from twigs onto

trunk initiate

cankers

Phytophthora ramorum

does not spread from

trunk cankers on tanoaks

Phytophthora ramorum survives

in infected twigs on overstory

and understory tanoaks

Spores splashed

from twigs onto

trunk initiate

cankers

Phytophthora ramorum

does not spread from

trunk cankers on tanoaks

Spores

dispersed by

rain splash 

infect more

tanoak leaves

and twigs

Spores

dispersed by

rain splash 

infect more

tanoak leaves

and twigs

Sporangia and zoosphores produced

on infected tanoak twigs

detach readily and can be dispersed via splashing water and wind-blown droplets. 

Relatively few hosts support production of sporangia (see box 1-3).

Sporangia can germinate directly, producing hyphae that can penetrate the host. 

However, sporangia in water, including water films on plant surfaces, can instead 

release multiple zoospores, about 30 zoospores from each sporangium (Widmer 

2009). Zoospores are motile and can swim small distances, up to about 3 cm, using 

their threadlike flagella (Duniway 1976). They can be carried large distances by 

flowing water (at least tens to hundreds of meters, depending on the type of watercourse). Zoospores actively swim toward certain chemicals released by host plants, 

which helps them aggregate on the host. Upon reaching the host, each zoospore 

transforms into a nonmobile cyst that germinates by producing a single hypha that 

can penetrate and infect the plant. If zoospores aggregate before they encyst, host 

tissue may be attacked by many hyphae in a small area. This mass attack increases 

zoospores’ ability to overwhelm host defenses. 
6

general technical report psw-gtr-242

Box 1-3—Host Susceptibility Versus Spore Production

Hosts that develop severe foliar symptoms when infected by Phytophthora ramorum

may not support production of significant numbers of spores. Among native species, 

Pacific madrone is an example that shows this pattern. In contrast, some species that 

are not overly susceptible to P. ramorum support production of high numbers of spores. 

Known relationships between host susceptibility and spore production are shown in 

table 1-3. 

Table 1-3—Comparison between susceptibility to Phytophthora ramorum foliar infection 

and P. ramorum spore production on various hosts 

Amount of Susceptibility

sporulation High Moderate Low to resistant

High Ceratonia siliqua Castanea sativaa Acer pseudoplatanus

Larix kaempferia,b Cistus salvifolius Hedera helix

Rhamnus alaternus Lonicera implexa Lonicera periclymenum

Rhododendron

 catawbiensea Michelia doltsopaa Quercus petraea

Syringa vulgarisa Pistacia lentiscus Quercus robur

Umbellularia californicaa Pistacia terebinthus Rosa canina

Quercus ilexa Rubus fruticosus

Quercus cerrisa

Rosa sempervirens

Vaccinium myrtillus

Moderate Arbutus unedoa Aesculus hippocastanum Viburnum opulusa

Camellia japonicaa Corylus avellana

Fraxinus excelsiora Malus sylvestris

Magnolia sp.a Ulmus glabra

Pieris japonicaa Ulmus procera

Quercus ilexa

Rhododendron catawbiensea

Rhododendron ponticuma

Viburnum tinusa

Low Arctostaphylos uva-ursi Calluna vulgaris

Kalmia latifolia Taxus bacattaa

Sambucus nigra Vaccinium × intermedium

Vaccinium vitis-idaea

None Empetrum nigrum

Pinus halepensis

Quercus pyrenaica

Source: Moralejo and others 2007.

a

 Found naturally infected in the United Kingdom.

b

 Added to list based on Webber and others 2010. 
7

A Reference Manual for Managing Sudden Oak Death in California

Zoospores are short-lived, typically swimming for a few hours. Zoospore cysts 

persist a short while longer, but do not tolerate drying. Sporangia can persist for 

days, but are relatively thin-walled and do not survive dry conditions. Phytophthora 

ramorum also produces thick-walled, long-lived spores (chlamydospores) within 

and on various infected plant tissues. Chlamydospores are relatively resistant to 

environmental degradation and help P. ramorum persist through periods unfavorable for growth.

Sporangia, zoospores, and chlamydospores are asexual spores. When these 

spores germinate, they give rise to new clones (genetically identical copies of the 

individual that produced the spores). Phytophthora species also can reproduce 

sexually by forming oospores. Oospores give rise to individuals that are genetically 

distinct from the parental strains. Oospores are thick-walled and can persist in the 

environment for extended periods. Phytophthora ramorum can only form oospores 

when strains of opposite mating types come in contact with each other (Werres and 

others 2001). Only a single mating type of the pathogen is currently widespread in 

California, and sexual reproduction of P. ramorum has not yet been observed to 

occur in California forests (see Box 1-4). 

1.1.2—Origin and Distribution of Phytophthora ramorum

in California 

Sudden oak death was first widely recognized in California in 1995, when large 

numbers of tanoaks began dying in areas of Marin, Santa Cruz, and Monterey 

Counties (Svihra 2001). It was not until 2000 that P. ramorum was diagnosed as the 

cause of SOD (Rizzo and others 2002). 

The pathogen was introduced into California on contaminated nursery stock or 

other plant materials, possibly in the 1980s. Genetic analyses and other data suggest 

that P. ramorum moved via infested nursery stock to multiple locations in several 

counties, including Santa Cruz, Marin, and Sonoma, where it spread from nurserygrown plants into the adjacent forests. Eventually, the larger forest infestations 

served as sources for continued spread of the pathogen to other forests (Croucher 

and others 2013). 

Phytophthora ramorum is now found in coastal California forests from Humboldt to Monterey Counties, and in Curry County in southern Oregon. It is also 

found in the counties bordering San Francisco Bay. All counties with confirmed 

P. ramorum in natural settings (tablulation below) are under state and federal 

quarantine. Quarantined counties are subject to regulations regarding the movement and use of susceptible plants. County agricultural commissioners enforce 

both California and federal regulations related to this pathogen.
8

general technical report psw-gtr-242

Box 1-4—Strains of Phytophthora ramorum

Phytophthora ramorum was first detected in Europe in 1993 and has been found 

in nursery, horticultural, and wild settings in many European countries since then. 

However, the strains of P. ramorum in Europe (EU1 and EU2), are different from 

the strain originally introduced into California (strain NA1) (Ivors and others 2006). 

By some measures, the EU1 strain is more aggressive than the NA1 strain. Another 

unique strain of P. ramorum was initially known only from nurseries in Washington 

state and is designated as NA2. This strain has since been found in California and 

nurseries in British Columbia (table 1-4). A second European strain (EU2) is present on Japanese larch (Larix kaempferi (Lam.) Carrière) in the United Kingdom 

(including Northern Ireland, southwest England, Wales, and southwest Scotland). The 

distribution of these four strains and other genetic data indicate that P. ramorum is 

an introduced pathogen in both North America and Europe. The native range of P. 

ramorum is unknown at this time.

In addition to the NA1 strain, both the EU1 and NA2 strains have been found 

in California nurseries. To date, the EU1 strain has been detected outside of a retail 

nursery site in Humboldt County. From a disease management standpoint, it is 

important to avoid widespread introduction of the European strains into California 

because the European and California strains are of opposite mating types (A1 and 

A2, respectively). The A2 mating type strain in California forests has been limited to 

asexual reproduction only. Introduction of the A1 mating type into California forests 

or nurseries could allow the pathogen to reproduce both sexually and asexually. This 

could introduce new variability into the P. ramorum population, possibly increasing the pathogen’s virulence, host range, or adaptation to a wider range of climatic 

conditions.

Table 1-4—Summary of currently known Phytophthora ramorum strains 

P. ramorum strain Mating type Description

NA1 A2 Original strain introduced into California and 

Oregon forests and currently most widely 

distributed. Also common in U.S. nursery 

detections. 

NA2 A2 Second strain detected in U.S. nurseries, 

originally in Washington; not closely related 

to NA1. Also in nurseries in British Columbia, Canada, and California. Has spread from 

nurseries to waterways in Washington.

EU1 A1 Strain originally found in European nurseries 

and in landscape introductions in Europe. 

Found in U.S. nurseries, especially in Washington. Has spread from nurseries to waterways in Washington, also found in a river in 

Humboldt County.

EU2 A1 Strain identified from Northern Ireland and 

western Scotland from isolates collected in 

2011, with an earliest collection date in 2007. 

This is the second strain detected from Japanese larch in the United Kingdom.

Sexual reproduction is possible if A1 and A2 strains are crossed, but does not occur if only one mating 

type is present. 
9

A Reference Manual for Managing Sudden Oak Death in California

California counties with confirmed Phytophthora ramorum

and under state and federal quarantine (September 2013)

Alameda Mendocino Santa Clara

Contra Costa Monterey Santa Cruz

Humboldt Napa Solano

Lake San Francisco Sonoma

Marin San Mateo

County, state, and federal agencies in cooperation with nursery managers have 

made considerable efforts to eliminate P. ramorum from nursery plants. These 

efforts have greatly reduced the risk of spread of the pathogen in nursery plants. 

Nonetheless, contaminated nursery stock remains a potential means of pathogen 

spread within California as well as between states and countries. 

The range of P. ramorum in California (fig. 1-2) continues to expand. Within 

infested counties, P. ramorum is not uniformly distributed. Stands of uninfected 

trees commonly occur in counties where SOD is common. The distribution of 

SOD-affected trees is typically patchy, even within infested stands. 

1.2—Sudden Oak Death Disease Cycle in 

California Forests 

The disease cycle of P. ramorum in California differs between forests containing 

tanoak and those containing susceptible oaks. P. ramorum sporangia are produced 

abundantly on tanoak twigs (Davidson and others 2008), so inoculum produced 

within tanoak canopies is sufficient to initiate trunk cankers on tanoaks (fig. 1-1). 

Tanoaks can also be infected by spores produced on California bay (Davidson and 

others 2005, 2008) where these two species occur together. 

In California forests that contain SOD-susceptible true oaks, infected California bay leaves typically serve as the primary source of P. ramorum spores (fig. 1-3). 

In oak stands that also contain tanoak, susceptible oaks can be infected by spores 

produced on tanoak. Because SOD-susceptible oaks are normally infected by 

spores produced on other hosts, oaks require different disease management strategies than tanoaks.

1.2.1—Sudden Oak Death in Tanoak Stands

Phytophthora ramorum commonly infects leaves and twigs of tanoak, causing 

twig dieback on both mature trees and understory seedlings and saplings (fig. 1-1). 

Phytophthora ramorum sporangia form readily on tanoak twig cankers (Davidson 

and others 2008). They can be dispersed by splashing rain to other parts of the 
10

general technical report psw-gtr-242

Figure 1-2—Approximate distribution of Phytophthora ramorum in California as of July 2012. Note Oregon location, just north of the California border. Upright blue triangles are positive plant and water samples from SODMAP 

(University of California–Berkeley 2012). Inverted red triangles are officially confirmed records from OakMapper 

(Kelly and others 2012).

OREGON

N E VA D A

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CALIFORNIA
11

A Reference Manual for Managing Sudden Oak Death in California

canopy, initiating additional leaf and twig infections. Spores can also be splashed 

from infected tanoak twigs to canopies of adjacent tanoaks, so P. ramorum can 

spread between trees in a tanoak stand.

Under wet conditions, as more twig cankers form in tanoak canopies, increasingly large numbers of P. ramorum spores are produced and dispersed. These 

spores splash onto tanoak trunks and can also move down the trunk in the rainwater that flows along the stems. These spores can initiate trunk cankers that can kill 

infected trees. Cankers on the trunk commonly form within 1 to 2 m of the ground 

but may occur higher on the trunk or on branches. Tanoak trunk cankers do not 

produce P. ramorum spores. 

Figure 1-3—Sudden oak death disease cycle in oak-California bay forests. Phytophthora ramorum survives over the dry season primarily 

in infected California bay leaves on trees. Sporulation occurs on bay leaves during the wet season, and rain-splashed spores initiate new 

leaf infections on nearby California bay foliage. Under prolonged wet conditions, many spores are produced in California bay canopies 

and are splashed onto nearby oaks (dashed arrow). Cankers develop on infected oak trunks but P. ramorum does not sporulate on trunk 

cankers. 

Phytophthora ramorum

does not spread from

cankers on oaks

Phytophthora ramorum survives 

in infected bay leaves on trees 

Many infected 

bay leaves

senesce and 

drop over

dry season

P. ramorum can survive

in fallen leaves

Sporangia and zoospores are

produced on infected bay leaves

Spores 

dispersed

by rain 

splash

infect 

more bay

leaves

Spores 

splashed 

from bay

leaves

infect oaks

DRY

SEASON

WET

SEASON
12

general technical report psw-gtr-242

Various studies have shown that in California forests, SOD risk in tanoak 

is usually greater if California bay is nearby (Davidson and others 2008). When 

P. ramorum initially spreads into tanoak stands, trees located near California bay 

are commonly among the first to develop lethal trunk cankers. Inoculum from other 

host species has not been shown to be significant for causing trunk infections in 

tanoak stands.

1.2.2—Sudden Oak Death in Oak/California Bay Forests

In most California forests, the P. ramorum spores that infect susceptible oak species 

(see Box 1-1) are produced on the leaves of California bay (fig. 1-3) (Davidson and 

others 2005). However, oaks present in tanoak stands may be infected by spores 

produced on tanoak twigs.

Phytophthora ramorum causes dark lesions on California bay leaves, typically 

on downward-hanging leaf tips and leaf edges where water collects (fig. 1-3). 

The edge of the lesion is usually jagged rather than smooth and has a chlorotic 

(yellowish) border. Under wet and somewhat warm conditions (see section 1.3.1, 

“Environmental Conditions”), large quantities of sporangia and chlamydospores 

can be produced on infected leaves.

During the summer dry season, California bay leaves with P. ramorum infections commonly turn yellow (fig. 1-3) and drop earlier than healthy leaves. However, some infected leaves remain attached for at least a year. Consequently, 

P. ramorum can persist over the dry summer in the canopy of California bay. 

Typically, the number of infected California bay leaves remaining in the canopy 

after the summer is relatively low, so few spores are produced during the first 

rains of the season in the fall and early winter. However, small numbers of spores 

can initiate new leaf infections on California bay. As the rainy season progresses, 

spores produced on infected leaves are splashed to other leaves where they start 

new infections. If rainfall is frequent, the number of foliar infections on California 

bay can increase explosively (Swiecki and Bernhardt 2008b). Peak P. ramorum 

spore production on California bay typically occurs when frequent rains occur in 

the spring, after temperatures have warmed up (Davidson and others 2005, 2008) 

(see section 1.3.1, “Environmental Conditions”). 

Sudden oak death in susceptible oaks is largely a byproduct of the P. ramorum 

foliar disease cycle on California bay. When conditions are favorable for foliar 

disease development in California bay, large numbers of spores from infected leaves 

are deposited on nearby oaks by dripping and splashing water. Water running down 

oak stems concentrates spores in the lower trunk area, where P. ramorum cankers 
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A Reference Manual for Managing Sudden Oak Death in California

typically develop (fig. 1-4). The lower trunk also tends to dry more slowly after 

rain. This provides an extended period of moisture, which favors P. ramorum spore 

germination and infection.

Sudden oak death is not known to spread from infected oaks to healthy oaks. 

Phytophthora ramorum does not produce spores on infected oak trunks; oaks are 

a “dead end” host for the pathogen (Davidson and others 2005, but see also section 3.1.3, “Firewood”). Leaf infections caused by P. ramorum rarely develop on 

oaks that are susceptible to trunk cankers (California black, coast live, Shreve, 

and canyon live oak). Oak foliar infections are only seen in trees exposed to high 

spore loads from nearby infected California bay foliage. Consequently, infected oak 

leaves appear to have little or no role in pathogen spread.

California bay–oak distance and disease risk— 

Sudden oak death incidence, severity, and mortality rates increase as the distance 

from oak trunk to California bay foliage decreases (figs. 1-5 and 1-6) (Swiecki and 

Bernhardt 2007, 2008b). Oaks with California bay foliage directly over or within 

1.5 m (5 ft) of the trunk have the highest risk of infection and mortality (fig. 1-5). 

Disease risk also increases as the total amount of California bay cover within 2.5 to 

5 m (8 to 16 ft) of the oak trunk increases. 

Proximity of California bay to oaks strongly influences disease risk because 

few P. ramorum spores travel more than 5 m horizontally via rain splash. Spores 

in water drops splashing from leaves higher in the canopy are more influenced by 

wind and may travel greater horizontal distances. Spores present on the soil surface 

and in low foliage (less than about 0.5 m above the ground) are only dispersed 

about 1 m by rain splash alone (Tjosvold and others 2006). 

Other factors may also influence spore deposition on oaks from infected 

California bays (Swiecki and Bernhardt 2007, 2008b). 

• Height of California bay canopy relative to the oak. Especially under 

windy conditions, average spore dispersal distance increases with foliage height. California bay canopy that is very tall or located upslope from 

oaks can disperse spores greater distances than low understory canopy. 

In dry locations with widely spaced trees, this effect may be less important because fewer leaf infections are present in the tops of California bay 

canopies.

• Prevailing storm wind direction. Because most spore dispersal occurs 

during rain events, greater numbers of spores are dispersed downwind. 

This effect is more important when the spore source is extensive and high 

(tall canopy or upslope) relative to the target oaks.
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Figure 1-4—Bleeding Phytophthora ramorum canker at the base of a coast live oak. 

0

10

20

30

40

50

60

70

80

90

100

0 >0 – 2.5 >2.5 – 5 >5 – 10 >10

Coast live oaks (percent)

Minimum bay-oak trunk distance (m)

Sudden oak death incidence

Sudden oak death mortality

Figure 1-5—Sudden oak death incidence and mortality in coast live oak decreases as the 

minimum distance between the oak trunk and California bay foliage increases. Minimum 

California bay-oak trunk distance is the shortest distance between the main stem of an 

oak and a vertical line dropped from the nearest California bay foliage. Data shown were 

collected in 2004 from coast live oaks in Marin and Napa Counties (Swiecki and Bernhardt 

2007, 2008b). 
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A Reference Manual for Managing Sudden Oak Death in California

Figure 1-6—Example of minimum distance between California bay foliage and oak trunk 

(white arrow). 

• Oak canopy size. Large oak canopies that have many large branches may 

serve to collect and concentrate P. ramorum spores. Large trees with dominant canopies are typically among the first trees to develop severe SOD 

infections.

• Interception (screening) by nonsusceptible species. If nonsusceptible 

species, especially those with dense foliage, are located between infected 

California bay foliage and an oak, many spores will be intercepted and 

fewer will deposit on the oak. Similarly, the spread of foliar infections 

throughout a stand containing California bay may be slowed if individual 

California bays are widely separated from each other by nonsusceptible 

species. 
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Other sources of inoculum— 

Limited data suggest that poison oak (Toxicodendron diversilobum) can serve as a 

source of P. ramorum spores. Oaks that are relatively far from infected California 

bay (>10 m) have an elevated SOD infection risk if poison oak is climbing in their 

canopies (fig. 1-7). Climbing poison oak is not associated with SOD unless infected 

California bay is present in the area. Poison oak that occurs as a small shrub or 

ground cover is not associated with SOD risk in oaks (Swiecki and Bernhardt 2005, 

2007, 2008b).

Other than California bay, tanoak, and possibly poison oak, none of the other 

known native P. ramorum foliar hosts (see box 1-2) have been associated with 

disease risk in oaks or tanoak in California forests. Infections on many of these 

other foliar hosts typically develop only under very wet conditions when inoculum 

levels from California bay and tanoak sources are very high. The small amount 

of additional inoculum that may be produced on these hosts is likely to have little 

or no effect on overall spore numbers. Many of these other foliar hosts tend to 

shed infected leaves readily, which further reduces their ability to produce spores. 

Finally, spores produced on low-growing understory species rarely move more than 

a meter horizontally (Tjosvold and others 2006). 

Figure 1-7—Poison oak (reddish foliage in this image taken in September) growing extensively 

in the canopy of susceptible oaks may serve as a source of Phytophthora ramorum inoculum that 

can initiate trunk cankers on the oak.
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A Reference Manual for Managing Sudden Oak Death in California

Rain washes P. ramorum spores into soil and surface water. Splash dispersal 

from infested soil or watercourses is only likely to transport inoculum up to a meter 

laterally and a half meter vertically. California bay or tanoak foliage on seedlings 

and low hanging branches may be infected by splashing of inoculum from soil or 

streams, but the small amount of spores transported in this way is unlikely to initiate trunk canker infections. Flooding of P. ramorum-infested streams has also been 

associated with the development of foliar infections on inundated hosts. Heavily 

infested soil or water that is directly deposited on oak trunks by human or animal 

vectors may initiate trunk infections, especially if the inoculum is introduced into 

bark wounds. This infection pathway may account for some of the uncommon trunk 

cankers seen in oaks without nearby California bay trees. 

1.3—Factors That Influence Sudden Oak

Death Development

1.3.1—Environmental Conditions

Sporangia and zoospores are the most abundant propagules produced by P. ramorum that can infect susceptible hosts. These propagules can only develop and 

disperse under wet conditions when temperatures are suitable. In laboratory tests, 

California P. ramorum isolates formed sporangia at temperatures between 10 and 

30 °C (50 and 86 °F), with peak production occurring at temperatures between 16 

and 22 °C (61 to 72 °F) (Davidson and others 2005, Englander and others 2006). 

Like spore production, the infection process also requires wet conditions. For 

example, P. ramorum requires 6 to 12 hours of continuously wet conditions to 

infect California bay leaves (Garbelotto and others 2003). Even under wet conditions, infection by P. ramorum can be limited by low temperatures. Under laboratory conditions, wet California bay leaves were much less likely to become infected 

at 12 °C (54 °F) than at 18 °C (65 °F) (Garbelotto and others 2003).

The combination of relatively warm temperatures and extended periods of 

wetness are especially favorable for P. ramorum infection. Infection of oaks and 

tanoaks is greatest in years with frequent late spring rains, and disease potential 

may be especially severe when these conditions occur two or more years in a row 

(Swiecki and Bernhardt 2008a). In contrast, new infections are uncommon after 

a dry spring, especially in inland areas (Swiecki and Bernhardt 2008a). In coastal 

areas, fog drip can wet foliage for extended periods, so new infections can develop 

even in years with low rainfall. 

Greater P. ramorum infection levels and higher spore production develop on 

California bay leaves in shady sites with high humidity and prolonged leaf wetness 

periods. Hence, California bay leaf infections are usually more severe in shaded 
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drainages and in stands on north- and east-facing slopes. High infection levels also 

occur more commonly in the lower, shaded leaves of the inner canopy. Leaves in 

the upper canopy, where lower humidity and more intense sunlight lead to shorter 

leaf wetness periods, develop fewer foliar P. ramorum infections (Swiecki and 

Bernhardt 2007, 2008b). These microclimate effects are more pronounced in drier 

climates away from coastal fog. 

1.3.2—Variation in Host Susceptibility

Sudden oak death canker hosts differ in their relative susceptibility to P. ramorum. 

Tanoak appears to be more susceptible than the oak species (see section 1.5, “StandLevel Sudden Oak Death Impacts”). Among oaks, coast live oak appears to be more 

susceptible than California black oak. 

Effects of tree size and condition— 

Tanoaks are susceptible to trunk cankers, foliar infections, and twig cankers over 

their entire lifespan. In contrast, oaks susceptible to SOD rarely develop twig infections (see section 1.2.2, “Sudden Oak Death in Oak/California Bay Forests”) and 

cankers generally do not develop on trunks less than 10 cm in diameter. 

As oak stem diameter increases, so does bark thickness. The bark of tanoak 

and most susceptible oaks has a thick layer of living phloem tissue. Phloem is the 

primary tissue that P. ramorum invades when it forms trunk cankers (see section 

1.4, “Sudden Oak Death Development on Trunk Canker Hosts”). In coast live oak, 

bark thickness is a better predictor of SOD risk than trunk diameter (Swiecki and 

Bernhardt 2005). Coast live oaks with thicker bark tend to be infected more readily 

and show faster rates of disease progress.

Trees with large canopies may also capture higher amounts of inoculum than 

smaller trees (see “California bay–oak distance and disease risk” in section 1.2.2). 

This may also contribute to the greater incidence of SOD seen in large trees. 

Unlike diseases that preferentially attack stressed trees, P. ramorum canker is 

more likely to affect relatively vigorous, faster growing coast live oaks (Swiecki 

and Bernhardt 2005). Oaks that are growing slowly because of suppression, water 

stress, or severe wood decay are less likely to be infected than more dominant, 

healthier trees. Faster growing coast live oaks develop wide bark fissures with 

unweathered brown bark as a result of rapid bark expansion. New infections are 

often associated with these fissures (fig. 1-3), which may be preferred sites for 

infection by P. ramorum. Slow growing coast live oaks that show weathered 

gray color in almost all bark fissures are less likely to be infected by P. ramorum

(Swiecki and Bernhardt 2004, 2005). 
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A Reference Manual for Managing Sudden Oak Death in California

Genetic resistance— 

Genetic differences between individual host plants appears to influence susceptibility to infections caused by P. ramorum. If sufficient levels of genetic resistance are 

present, it may be possible to select oaks and tanoaks with higher levels of resistance to P. ramorum for restoration of affected forests. Various researchers are actively investigating this area. Results from this research may have implications for 

long-term disease management strategies.

As noted above (“Effects of tree size and condition” in section 1.3.2), the age 

and physiological state of a tree can also influence its resistance to P. ramorum. An 

individual tree located in an infested forest that has no P. ramorum symptoms may 

or may not have genetic resistance to this pathogen. For instance, an individual that 

shows some resistance to P. ramorum infection under low inoculum levels might be 

fully susceptible under very high inoculum levels. 

1.3.3—Pathogen Dispersal

Most of the short-range (mostly less than 5 to 10 m) tree-to-tree movement of inoculum within infested stands is accomplished by rain splash, often assisted by wind 

(Davidson and others 2005). Phytophthora ramorum is spread over larger distances 

by a number of agents, each of which is capable of acting over various spatial 

scales (table 1-5). Most of these long-distance transport methods can deliver only 

low numbers of spores to a given target. Hence, spores transported long distances 

may initiate foliar infections but are unlikely to be present in numbers required to 

initiate trunk cankers.

1.4—Sudden Oak Death Development on Trunk 

Canker Hosts

1.4.1—Symptom Development

Canker location— 

In SOD-susceptible oaks and tanoak, P. ramorum infects and kills phloem tissue 

located in the bark of the lower trunk (fig. 1-8). Infections may kill the vascular 

cambium, the cell layer that produces both phloem and wood (xylem) tissues. The 

pathogen can also invade the outer xylem (Parke and others 2007, Rizzo and others 

2002). Xylem infection appears as dark discoloration that may extend several centimeters into the wood. The pathogen can spread vertically in the xylem and sometimes grows outward from the xylem to form a new canker further up the stem. 

No new xylem and phloem can develop where the cambium has been killed by P. 

ramorum. However, if the pathogen becomes inactive (see below, “Changes in canker appearance over time”), healthy callus tissue can develop from uninfected live 
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Table 1-5—Agents that may be involved in long-range dispersal of 

Phytophthora ramorum

Agent Effective range Notes

Human activitiesa Localb to many 

kilometers to 

global

Many mechanisms are known, ranging from 

transport of infested soil on shoes and 

tires, transport of infested plant materials 

(e.g., California bay leaves) via vehicles, 

and movement of infected nursery stock in 

commerce (local to international). Human 

activities were responsible for the original 

transport of P. ramorum to California.

Wind Local to hundreds 

of meters, rarely 

kilometers

In some topographic situations, water drops 

containing sporangia may be transported tens to 

hundreds of meters (Turner and others 2008). 

Turbulent airflow may occasionally transport 

spores greater distances, up to several kilometers 

(Hansen 2008). Infected leaves or leaf fragments 

sent aloft by strong winds may be transported 

tens to perhaps hundreds of meters.

Watercourses Local to 

kilometers

During the rainy season, inoculum can enter 

watercourses directly from overhead foliage 

or from surface runoff below infected foliage. 

Inoculum is also produced in both rainy and 

dry periods on infected bay leaves that fall into 

watercourses. Phytophthora ramorum has been 

detected in both small streams and in sizeable 

rivers, sometimes kilometers downstream from 

known inoculum sources on land (Davidson and 

others 2005). Inoculum in watercourses may 

be splashed directly onto low hanging foliage, 

moved via animals that visit the water, applied 

to soil or plants when water is pumped for 

irrigation, or deposited on foliage during flood 

events.

Other vectors Local to 

kilometers (?)

Possible movement by animal vectors is very 

difficult to document and may occur only rarely, 

but could explain anomalous spread occasionally 

seen in local and longer range (kilometers) 

situations. Infested soil may be moved on 

feet or fur of large animals. Birds or rodents 

may transport inoculum on their bodies or via 

collected nest construction materials. Insects 

have not been shown to vector P. ramorum, but 

snails feeding on infested debris can excrete 

viable P. ramorum spores (Parke and others, 

2008).

a Transport by humans is well documented. Conclusive data on the other modes of long-distance dispersal are 

currently lacking, although observations and some data suggest that they are possible. 

b

 Local range is considered to be up to about 100 m.
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A Reference Manual for Managing Sudden Oak Death in California

cambium at the edge of the canker. This callus tissue forms new xylem and phloem 

tissue that grows over the nonexpanding canker.

Cankers normally occur on the lower trunk but occasionally occur on low scaffold branches. In tanoak, cankers sometimes develop many meters up on the main 

stems. Cankers do not extend more than a few centimeters below the soil line, but 

can develop on exposed buttress roots. 

Early symptoms: bleeding cankers— 

In SOD-affected oaks and tanoaks, new and older active cankers commonly bleed a 

dark, thick, sticky fluid (fig. 1-9). The fluid, which ranges from dark reddish brown 

to nearly black, oozes from the intact surface of infected bark tissue. The bleeding 

produced by SOD cankers is nearly odorless, but secondary organisms that invade 

the cankers can give rise to alcoholic or fermented odors. The ooze may dry and 

harden over the summer. Rainfall can dissolve the ooze, spreading it out, giving the 

canker and the area below it a “coffee-stained” appearance (fig. 1-10).

The amount of bleeding can vary greatly. Initially, small cankers may bleed 

from a single spot, but most large cankers will bleed from multiple areas. Visible 

bleeding may develop within 6 months of infection or may not develop for 2 years 

Figure 1-8—Cross section through the trunk of a coast live oak with a Phytophthora 

ramorum canker. Discolored areas in phloem and xylem are infected tissues.

Cambium

Phloem/inner bark

Infected phloem

Infected xylem

Phloem/inner bark
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Figure 1-9—Bleeding Phytophthora ramorum canker on coast live 

oak (top); outer bark has been sliced away in center image to reveal the 

canker, which has an irregular dark border. This canker failed to expand 

and became inactive, as shown in bottom photo taken 4 years later.
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A Reference Manual for Managing Sudden Oak Death in California

Figure 1-10—Phytophthora ramorum canker on tanoak. In right image, outer bark has been sliced away to show the irregular canker in 

the inner bark.

or more. Coast live oak, Shreve oak, and California black oak typically develop 

extensive bleeding, but some trees show little bleeding. Tanoak cankers sometimes 

do not bleed, especially in small diameter (<20 cm) stems. Cankers in canyon live 

oak usually show little or no visible bleeding. 

When the outer bark is chipped away, P. ramorum cankers appear as brownish discolored lesions in the phloem. Cankers typically have a distinct edge, often 

delimited by a dark line (fig. 1-10). Phytophthora ramorum cankers are usually 

widest near the outer bark surface. 

Similar symptoms—Bleeding cankers caused by P. ramorum are visually indistinguishable from cankers caused by P. nemorosa and P. pseudosyringae. 

Phytophthora nemorosa primarily affects tanoaks, whereas P. pseudosyringae is 

more commonly found on oaks. Both of these species appear to be less aggressive than P. ramorum; only P. ramorum is associated with extensive tree mortality (Wickland and others 2008). Table 1-6 summarizes symptoms associated with 

agents that cause bleeding in oaks.

Late symptoms: secondary agents— 

Secondary agents, including wood-boring beetles and decay fungi, eventually invade most large P. ramorum cankers (fig. 1-11). These secondary agents destroy

the sapwood, thereby cutting off the water supply to the tree canopy. 

Beetles—Ambrosia and bark beetles (including Monarthrum and Pseudopityophthorus spp.) commonly colonize P. ramorum cankers on oaks and tanoaks. 

These small beetles often attack the stem shortly after bleeding first appears, but
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Table 1-6—Common agents other than Phytophthora ramorum that may cause bleeding on oak trunks

Agent Disease/condition How to distinguish from sudden oak death (SOD) cankers

Phytophthora nemorosa,

P. pseudosyringae

Bole cankers Cankers and bleeding identical to SOD cankers, but generally 

less widespread. Diagnostic tests (culturing or DNA-based) 

are required to differentiate from SOD.

Phytophthora cinnamomi, 

other soil Phytophthora 

species

Root and crown rot Cankers and bleeding identical to SOD cankers, but normally 

low on trunk and extending belowground and associated 

with root decay. More common in irrigated landscapes or in 

areas with high soil moisture.

Armillaria mellea (oak root 

fungus) and other Armillaria

species

Root rot and wood decay 

in lower trunk

Bleeding may be similar but is usually very low on trunk. 

Affected areas have sapwood decay with white fanlike layers 

of fungal tissue (mycelium) between bark and wood and a 

mushroom odor. Decay originates in roots or root crown. 

Inonotus andersonii, I. 

dryophilus, other fungi 

Canker rot, wood decay Ooze is dark but usually thinner. More likely to bleed 

extensively from one or few spots, often associated with 

wounds or cavities. Wood decay is commonly found beneath 

the bleeding area. Cankers are often quite small near the 

bark surface but become wider as they extend into the inner 

bark and wood.

Bacteria and associated 

micro-organisms

Wetwood, alcoholic flux Ooze is usually watery and not dark; may become slimy 

where it accumulates on the bark surface. Usually extensive 

bleeding from a single spot, often associated with wounds 

or cracks. Ooze may have a sour/rancid smell (wetwood) or 

may smell yeasty or alcoholic (alcoholic flux). When area is 

chipped, liquid may stream out under pressure.

Wood-boring insects, 

including gold-spotted oak 

borer Agrilus auroguttatus

Insect boring damage Ooze is typically thin and watery or sometimes foamy and 

light-colored. Bleeding comes from multiple small holes 

made by the insects, which may be localized or scattered 

across the trunk and branches.
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A Reference Manual for Managing Sudden Oak Death in California

Figure 1-11—Beetle boring dust on coast live oak. At right, detail shows small (about 1 mm diameter) exit holes of wood-boring 

beetles on a tree infected by Phytophthora ramorum.

beetle boring may not appear for a year or more after bleeding is seen. Small 

cankers and cankers in small-diameter oaks and tanoaks may not be attacked by 

beetles. 

Ambrosia and bark beetles produce numerous pinhead-sized holes in the 

affected area as they bore through the bark (fig. 1-11). In active invasions, copious 

amounts of fine wood dust are ejected from the boring holes. Boring dust accumulates in bark fissures and at the base of the tree. Accumulated boring dust can 

wash or blow away after beetles leave the tree. At that point, the tiny beetle holes 

can be difficult to see in the bark, but evidence of previous beetle activity can be 

seen in cut or broken stems. Older cankers and branches of SOD-infected trees may 

be invaded by larger wood-boring beetles (commonly flat-headed borers, family 

Buprestidae). 

Fungi—Wood decay fungi, especially Annulohypoxylon (formerly Hypoxylon) 

thouarsianum and Phellinus gilvus also colonize the wood beneath larger cankers. 

Wood decay is often quite advanced before external fruiting bodies develop. 
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Figure 1-12—Annulohypoxylon thouarsianum fruiting bodies on the trunk of a coast 

live oak with late-stage sudden oak death symptoms. 

Annulohypoxylon thouarsianum produces hard, black, hemispheric fruiting bodies 

(fig. 1-12). These may develop within the first year after a canker develops, but 

more commonly appear later, after bark or ambrosia beetle colonization is evident. 

Phellinus gilvus fruiting bodies (fig. 1-13) typically develop in association with

cankers that are at least a year old.

Other fungi can also contribute to the decline of SOD-affected trees. Armillaria species commonly colonize the main roots and lower trunk of trees with 

extensive P. ramorum cankers, causing decay of roots and sapwood in the lower 

trunk. Preexisting infections by wood-decay fungi including species of Inonotus, 

Phellinus, Laetiporus, and Ganoderma may expand more rapidly in trees infected 

by P. ramorum. This accelerated decay hastens tree decline and death, usually by 

contributing to trunk failure.

Similar symptoms—Annulohypoxylon thouarsianum, Phellinus gilvus, ambrosia 

beetles, and bark beetles are opportunistic invaders that also attack trees that have 

been compromised by other agents. The presence of these secondary organisms 

alone does not provide evidence that a tree has been infected by P. ramorum. The 

other wood decay fungi noted above also occur widely in oaks and are not limited 

to SOD-affected trees.
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A Reference Manual for Managing Sudden Oak Death in California

Patterns of canker development— 

Sudden oak death canker development can be grouped into three general categories. 

Canker development is affected by the initial size of the canker and the host tree’s 

reaction to infection.

Large cankers/susceptible reaction—Highly susceptible host trees develop large, 

rapidly expanding cankers. These may result from the merging of multiple cankers. Actively expanding cankers commonly show fresh bleeding around the edges. 

Beetles usually attack these cankers within a year of initial bleeding symptoms. 

Annulohypoxylon thouarsianum is evident within 1 to 2 years after bleeding is seen. 

Figure 1-13—Phellinus gilvus fruiting bodies on the trunk of a coast live oak with 

late-stage sudden oak death symptoms.
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As the cankers expand, beetles and A. thouarsianum may extend beyond the canker 

margins, especially higher on the stem, into areas that are weakened or killed by the 

canker’s girdling effect.

Small cankers/resistant reaction—Many bleeding cankers (28 to 35 percent in 

one long-term study) become inactive within a year or two after bleeding is first 

seen (fig. 1-9). These cankers fail to expand and cease bleeding. They are not colonized by secondary agents and may become undetectable after one to several years. 

Small cankers (usually less than 10 cm across) are most likely to become inactive. 

Although individual cankers can become inactive, trees with inactive cankers can 

develop new cankers on other portions of the trunk if conditions become favorable 

for establishment of new infections.

Intermediate reaction—Many cankers in the moderate to large size class (>10 

cm wide) are invaded by secondary organisms but eventually stop expanding and 

become largely inactive. Bark over the canker may become sunken and healthy 

callus tissue may develop at the edges of the canker (fig. 1-14). The dead bark dries 

out and shrinks, becoming cracked and sunken, and pieces may eventually fall off. 

Wood-boring insects and decay fungi may eventually degrade the wood beneath the 

canker. Old infections of this type are difficult to diagnose as SOD cankers.

1.4.2—Patterns of Disease Progression

Several general patterns of tree decline can be seen in oaks with P. ramorum cankers. Tanoaks most commonly follow the first pattern of decline (“sudden” death), 

but sometimes follow the other patterns. 

“Sudden” oak death— 

Trees develop large cankers and become mostly or entirely girdled within 1 to 2 

years. Water transport to the tree canopy is disrupted by the effects of P. ramorum

infections (Parke and others 2007) and secondary agents on the outer sapwood. 

This eventually induces severe water stress in the tree canopy. The canopy may 

appear dull or off-color just before leaves undergo a rapid change from green to 

brown. These color changes occur within a few weeks to months, especially during 

periods of warm weather. The tree canopy generally shows no obvious thinning before it turns brown and dies. The name “sudden oak death” is based on this disease 

pattern, but trees have typically been infected 6 months to several years before they 

reach this stage.
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A Reference Manual for Managing Sudden Oak Death in California

Slow decline— 

Trees have fewer, less extensive, or slower spreading cankers. After several years, 

the canopy begins to progressively thin and decline (fig. 1-15). Affected trees may 

survive for many years, sometimes maintained by only small areas of healthy callus on a mostly girdled trunk. Death occurs either through a “sudden” drying of the 

remaining canopy as described above or green failure of the trunk (below). Cankers 

are occasionally colonized by fungi such as Stereum species, which primarily decay 

dead wood and are less aggressive than most of the wood decay fungi noted above 

(see “Fungi” on page 25).

Figure 1-14—Old Phytophthora ramorum canker on California black oak 

exhibiting an intermediate disease reaction. Note callus tissue at lower right 

margin of canker and Annulohypoxylon thouarsianum fruiting bodies on dead 

bark.
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Figure 1-15—This coast live oak was almost entirely girdled by Phytophthora ramorum cankers 

in 2000. Only a narrow strip of callus tissue (white arrows, top left) connected the tree's canopy 

to its root system for much of the following decade (top left, 2004; top right, 2009). The canopy 

thinned and showed significant dieback, but foliage remained green (center left, 2004; center 

right, 2009). Sapwood decay was extensive for many years, but the tree remained alive with 

green leaves until it failed in September 2010 (bottom). At the break, the only sound wood was a 

small strip of sapwood associated with the callused area. 
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Green failure— 

Oaks with large trunk cankers that are extensively decayed by A. thouarsianum, 

Phellinus gilvus, or other fungi can fail (break) while the top is green (fig. 1-15). 

Failure may occur in trees with full canopies or in trees with extensive canopy 

thinning. Green failures associated with SOD occur more commonly in coast live 

oak than in tanoak. Major branch and trunk failures typically result in tree death. 

If failure occurs in one of several trunks or main branches and decay is extensive, 

the remaining live portions may decline or fail within a few years. Some failed trees 

resprout vigorously from the stump and may begin to form a new canopy. Factors 

associated with tree failure are discussed further in “Survival and failure potential 

of SOD-affected trees” in section 2.2.3, and section 3.3, “Monitoring Oaks With 

Sudden Oak Death to Assess Survival and Failure Potential.”

1.4.3—Survival of Trees With Sudden Oak Death

The extent to which cankers encircle the trunk can indicate how long a coast 

live oak will survive (see section 3.3.2, “Guidelines for Assessing Mortality and 

Failure Risk in Trees Affected by Sudden Oak Death”). Coast live oaks with early 

symptoms (bleeding cankers only) are unlikely to die unless they are colonized by 

beetles and/or A. thouarsianum. Most coast live oaks with advanced trunk canker 

symptoms (beetle boring and/or A. thouarsianum colonization) that affect more 

than half of the trunk circumference will die within 1 to 3 years. However, coast 

live oaks with extensive SOD cankers colonized by secondary organisms have 

survived for 8 or even 11 years. Trees that live for many years despite extensive 

canker development tend to be relatively large-diameter trees with no preexisting 

heartwood decay in the trunk and relatively well-balanced canopy structure.

Most tanoaks that develop early P. ramorum canker symptoms die within 3 years. 

Tanoaks with extensive P. ramorum cankers sometimes do not develop obvious 

bleeding. Secondary invasion of the trunk by beetle colonization may be the first 

sign of disease. Consequently, the visible extent of trunk girdling by P. ramorum 

cankers is not always a reliable indicator of tanoak survival. 

Both tanoaks and SOD-susceptible oaks commonly occur as multitrunked 

trees. If the individual trunks arise from ground level, each trunk generally functions as a separate tree with respect to infection and disease progress. It may be 

possible to retain healthy trunks of multistemmed trees even if one or more stems 

have been infected or killed by SOD. However, remaining trunk(s) should be evaluated for structural stability. Wood and root decay associated with killed trunks may 

compromise the remaining trunks.
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Box 1-5—Diagnosing Sudden Oak Death

Within the known range of Phytophthora ramorum in California, a preliminary SOD 

diagnosis can be made based on canker symptoms and the distribution of diseased 

trees in the field. Key characteristics of SOD are that it:

• Primarily affects healthy, vigorous trees (see “Effects of tree size and condition” in section 1.3.2).

• Typically causes tree mortality within about 2 to 10 years after infection (see 

section 1.4.2, “Patterns of Disease Progression”).

• Can result in high rates of mortality, especially in tanoak (see section 1.5, 

“Stand-Level Sudden Oak Death Impacts”).

By comparison, most other native mortality agents primarily affect low-vigor 

trees; are slow acting, so that trees decline gradually over decades; and typically are 

associated with lower mortality rates.

Confirmation of SOD in oaks and tanoaks is based on detecting P. ramorum 

in stem cankers. Tissue samples are cut from the edge of the canker below the outer 

bark surface, typically at the border between healthy and symptomatic tissue. The 

tissue sample pieces are placed in petri plates containing a selective medium. If the 

pathogen is still viable in the sampled tissues, it will grow from the pieces into the 

medium. The resulting cultures can be identified based on characteristics seen under 

a microscope. Alternatively, molecular identification methods (e.g., polymerase chain 

reaction (PCR)) can be used to positively identify the DNA of the pathogen in the 

sample. These and related methods are also used to detect P. ramorum in other plant 

tissues, in soil, and in stream water.

Foliar infections caused by P. ramorum on California bay and tanoak may appear 

years before symptoms develop in trunk canker hosts. Confirmation of P. ramorum 

from California bay leaves or tanoak leaf/twig samples using selective media or PCR 

provides positive evidence that an area is infested with the pathogen. Many of the 

initial detections of P. ramorum within infested areas in California were made by 

sampling California bay foliage.

For the purpose of SOD management, you may assume that a site with typical 

SOD symptoms on foliar or canker hosts is infested if P. ramorum presence has been 

confirmed at nearby sites (up to several kilometers away). If your location is more 

than a few kilometers from the confirmed range of SOD, you may want to confirm 

that P. ramorum is present before choosing management actions (Chapter 2). In areas 

well beyond the known range of P. ramorum, especially in counties not currently 

listed as infested, confirmation by California Department of Food and Agriculture 

(CDFA) or University of California (UC) plant pathologists should be obtained. Your 

county agriculture department and UC Cooperative Extension offices can provide 

information about official sampling by CDFA and UC personnel.
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A Reference Manual for Managing Sudden Oak Death in California

1.5—Stand-Level Sudden Oak Death Impacts

Mortality rates have increased among SOD canker hosts since P. ramorum was 

introduced into California forests. Between 2000 and 2012, mortality caused by 

SOD among coast live oaks (1.15 percent per year) was 50 percent higher than mortality caused by all other agents (0.76 percent per year) (fig. 1-16). Among tanoaks, 

the SOD mortality rate (2.4 percent per year) was about 3.4 times that for all nonSOD causes (0.7 percent per year). Mortality rates measured in forests without SOD

(Swiecki and Bernhardt 2008c) are similar to those attributed to non-SOD causes in 

figure 1-16. 

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

2000 2002 2004 2006 2008 2010 2012 2000

2000 2002 2004 2006 2008 2010 2012

Coast live oak plot trees (percent)

Coast live oak

Tanoak plot trees (percent) Tanoak plot trees (percent)

Asymptomatic

Other decline

Other dead

All Pr

Early Pr

Late Pr

Dead Pr

Tanoak

50

45

40

35

30

25

20

15

10

5

0

Coast live oak plot trees (percent)

Year

70

65

60

55

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5

0

2002 2004 2006 2008 2010 2012

2000 2002 2004 2006 2008 2010 2012

50

45

40

35

30

25

20

15

10

5

0

Year

Year Year

Figure 1-16—Changes in disease status of coast live oak (left) and tanoak (right) from September 2000 through September 2012 in 150 

permanent plots in Marin, Sonoma, and Napa Counties. Top graphs: Asymptomatic = no evident symptoms of Phytophthora ramorum

infection or decline resulting from other agents. Other decline = tree in severe decline due to agents other than P. ramorum. Other dead

= tree dead due to agents other than P. ramorum. All Pr = all trees (live or dead) with P. ramorum canker symptoms. Bottom graphs: 

Early Pr = live trees with P. ramorum cankers only. Late Pr = live trees with P. ramorum cankers plus beetle boring or Annulohypoxylon thouarsianum fruiting bodies. Dead Pr = tree dead due to P. ramorum infection.
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The trajectory of SOD incidence and mortality varies from stand to stand. At a 

given time, different stands or portions of a single stand can be at different stages in 

the disease epidemic based on:

• When P. ramorum became established in the area. 

• The number of favorable disease development years that have occurred 

since P. ramorum was introduced.

• The density and spatial arrangement of canker hosts and inoculum sources 

within the area. 

Overall, SOD incidence and mortality tend to increase faster and reach higher 

levels in tanoak stands than in oak stands (fig. 1-16). In oak stands, disease is 

initially very patchy and is strongly associated with California bay. Even after 

SOD is well established, oak mortality is not likely to affect all trees in an area. 

SOD incidence in oak stands tends to increase in pulses over time (fig. 1-16). Large 

increases in SOD incidence typically develop 1 to 2 years after a wet spring that 

is favorable for disease development. Few or no new SOD symptoms may be seen 

following dry years. 

If conditions for SOD development are favorable, many of the most highly 

susceptible trees in a stand may be infected and killed within the first few years 

after P. ramorum becomes established in an area. Subsequent pulses of disease may 

be less dramatic because fewer highly susceptible trees are left.

Some trees that remain healthy after the initial and subsequent waves of disease may have escaped the disease by chance. These susceptible trees can become 

infected later if exposed to enough inoculum during a year in which conditions are 

favorable for disease development. Hence, various disease management actions may 

be justified even in stands that have already experienced substantial disease. 
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A Reference Manual for Managing Sudden Oak Death in California

2.1—Developing a Sudden Oak Death Management Plan

2.1.1—Why is Management Needed?

Most forests affected by sudden oak death are not managed for production of forest 

products. Typical land uses in affected areas include residential development, park 

lands, natural reserves, and watershed lands. The forests in these areas are prized 

for providing wildlife habitat, ecological benefits, recreational uses, or simply a 

desirable living environment (table 2-1). 

Before the introduction of P. ramorum to California, SOD-susceptible oaks and 

tanoaks (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H. Oh) 

typically had low mortality rates (see section 1.5, “Stand-Level Sudden Oak Death 

Impacts”). Little or no active management was necessary to maintain forests 

dominated by these species. However, SOD has greatly changed the stability and 

sustainability of these forests. Landowners can no longer assume that the forest will 

Table 2-1—Some ecosystem services provided by forestsa

General ecological services Carbon fixation and storage

Air quality protection

Erosion protection

Water quality protection

Moderation of stormwater flows/flood protection

Local climate modification

Nutrient cycling

Plant and animal habitat Special status species habitat

Locally rare or uncommon species habitat

Reserves of native biodiversity

Wildlife migration corridors and refuges

Special habitat features

Suppression of invasive species

Human interactions Recreational activities

Health benefits

Historic significance

Cultural, aesthetic, spiritual values

Property value enhancement

Wood products

Energy conservation

Other forest products—edible fungi, spices

Buffering between incompatible land uses

Visual impact—views, screening

Noise reduction

a

 Although most of the general services can be provided by various forest types, the quality of habitatrelated services can differ greatly between forest types.

Chapter 2—Managing Stands Before, During, and 

After Sudden Oak Death
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remain stable under the largely passive management that worked in the past. Stands 

can undergo dramatic changes in canopy cover and vegetation composition within 

10 years once SOD becomes established. Active management is needed in many 

SOD-affected forests to direct forest succession toward desired vegetation types 

and to restore lost ecosystem services. 

Box 2-1—Costs Associated With Sudden Oak Death

The loss of SOD-susceptible species from forests can lead to significant losses in 

ecosystem services, many of which are unique to SOD-affected species. The loss of 

acorns provided by oaks and tanoaks affects the traditional cultures of many Native 

American tribes that utilize this resource. Many wildlife species also depend directly 

or indirectly on the annual acorn crop. In addition, many birds are dependent on the 

uniquely rich diversity of insects associated with oaks. Individual trees or groups 

of trees may also be important for erosion protection, visual screening, or property 

values. Hence, tree mortality can have widely different impacts in different areas. 

Trees killed by SOD can also impose additional costs and risks. Undesirable 

plant species can invade canopy gaps created by tree mortality, increasing the need 

for monitoring and suppression activities. The increased failure potential of SODaffected trees (see section 2.2.3, “After: Dealing with Impacts of Sudden Oak Death”) 

can put lives, property, and other resources at risk. Fuels from SOD-killed trees can 

also increase fire hazard. Tree removal and other actions needed to address these 

issues can be costly and time-consuming. 

Appropriate management actions that prevent or reduce SOD can maintain 

valuable services provided by trees and avoid costs associated with SOD-related 

mortality. Hence, the costs of SOD management activities need to be considered in 

comparison to the costs and losses that would be incurred if no actions were taken. 

2.1.2—Why Develop a Management Plan?

Resource management plans facilitate the long-term management of forest 

resources. There are several reasons to consider SOD management actions within 

the broader context of a resource management plan.

1. Landowners typically manage forest lands for multiple purposes. Planning helps 

mesh management efforts related to SOD with those for other purposes, thereby 

minimizing conflicts.

2. Forest ecosystems are complex and dynamic systems that change over time and 

can vary widely across the landscape. A management tactic that is effective at 

one time and place may be ineffective under a different scenario. Planning allows 

consideration of all major ecosystem components and how they are likely to change 

over time.
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A Reference Manual for Managing Sudden Oak Death in California

3. Management actions are often constrained by economic considerations. Planning allows limited resources to be allocated based on priorities and allows for cost 

comparisons among alternative management scenarios. 

4. Taking no action is a management decision with consequences. Over the short 

term, a landowner may not incur any costs by taking no action. However, impacts 

and costs associated with inaction (e.g., allowing SOD to progress without intervention) could be more expensive over the long term than taking simple, well-timed 

management actions (see box 2-1). A plan can help you assess costs and benefits of 

management.

5. Trees and forests are long-lived resources whose values develop over a long timeframe, but can be damaged quickly by inappropriate management actions. Developing an adequate plan before executing actions minimizes unintended consequences 

that may be difficult or costly to correct. 

Developing and implementing a resource management plan that addresses SOD 

can benefit owners of either large or small properties.

2.1.3—Elements of a Management Plan

The process of developing a management plan can be summarized as shown in 

figure 2-1. The process begins by addressing two basic questions with respect to the 

resources being considered: “What do you have?” (asses the resources and identify 

trends) and “What do you want?” (identify needs and set goals). These questions 

are interdependent and not necessarily answered in order. Landowners’ overall 

goals will help shape the assessment process. For example, landowners who are not 

interested in timber harvest would not need specific information related to wood 

volume. 

In the next step (“How do you get what you want?”), specific strategies are 

selected to achieve goals and objectives. Financial and regulatory constraints on 

possible strategies also need to be considered. Many management actions need to 

be carried out over long time periods, and the order and timing of various actions 

may be critical. Develop a multiyear schedule to show how elements of the plan will 

be implemented over time. 

The final step of this adaptive management process (“Are you getting what you 

want?”) involves monitoring and analysis. The outcome of various management 

actions may differ with site conditions or other factors. Because of information 

gaps, it may be necessary to test management actions on an experimental basis. 

By tracking management inputs and outcomes over time, you can determine what 

works and what does not, and alter future plans accordingly. 
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What do 

 you have? 

Biological resources

Physical/environmental factors

Existing management

Regulatory constraints

Trends

What do 

 you want? 

Are you getting

 what you want? 

How do you get

 what you want? 

Monitor

Analyze

Revise as needed

Select management strategies

Develop time/action schedule

Implement plan

Identify needs

Set goals and objectives

Figure 2-1—Schematic of an adaptive management process for managing forest resources.

Box 2-2—Disease, Damage, and Mortality Related to 

Other Agents

Since its initial appearance in the 1990s, sudden oak death has become the most 

important cause of mortality of tanoak and susceptible oaks in coastal California 

forests. However, other agents and environmental factors can cause tanoaks and oaks 

in these forests to decline and die. Root diseases and wood decay fungi, especially 

canker rot fungi, can cause relatively high levels of tree decline and mortality in 

forests whether SOD is present or not. 

Mortality resulting from other agents may also affect trees that are not at risk 

from Phytophthora ramorum. Oaks in the white oak group (table 1-1) are not affected 

by SOD but are susceptible to most of the same root diseases and decay fungi that 

affect the SOD-susceptible red oaks. The exotic root rot pathogen P. cinnamomi

causes decline and death of Pacific madrone (Arbutus menziesii Pursh) and California 

bay (Umbellularia californica (Hook. & Arn.) Nutt.) and can kill or cause root disease 

in a wide variety of other native species, including oaks. 

A comprehensive management plan should also consider other invasive insects 

or diseases that are not currently present at a site but could affect forest health if 

introduced (e.g., the goldspotted oak borer). For example, management practices that 

prevent the movement of potentially contaminated materials can reduce the risk that 

the forest will be affected by a variety of invasive agents.
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A Reference Manual for Managing Sudden Oak Death in California

2.2—Management Strategies for Different Stages of the 

Sudden Oak Death Epidemic

Stand conditions change after P. ramorum invades. Over time, SOD affects progressively higher percentages of the canker hosts in the stand. As a result, SOD 

management objectives change with the SOD disease stage (table 2-2). The SOD 

disease stage of a given stand can be considered under the broad categories of 

Before, During, and After (table 2-2). Specific considerations for managing SOD 

at these epidemic stages are discussed briefly in the respective sections below.

Table 2-2—Stand characteristics, information needs, and management objectives for forests at different 

stages of a sudden oak death (SOD) epidemic

Stage in SOD epidemic

Before: No Phytophthora 

ramorum in foliar or 

canker hosts

During: SOD disease 

levels increasing

After: most or all susceptible trees infected or dead

Stand characteristics:

Percentage of canker hosts with SOD 0 Variable, 0 to over 

50 percent

High—50 to 100 percent 

of at-risk hosts

P. ramorum present in stand — + +

P. ramorum symptoms in foliar 

hosts such as bay

— + +

Primary management information 

needs: 

Proximity to known P. ramorum + — —

Likely impact if P. ramorum is 

introduced

+ — —

Treatment priority (see table 2-7) + + +

Current distribution of SOD 

across site and location of 

noninfected canker hosts

— + +

SOD-related failure hazard potential — + +

Fuel loading from SOD-killed trees — + +

Restoration objectives — — +

Site capabilities for restoration — — +

Existing regeneration — — +

Factors constraining restoration — — +

Main management objectives:a

Preventing P. ramorum introduction +++ + (other strains) + (other strains)

Protection of noninfected canker 

hosts

++ +++ — (higher if noninfected 

hosts still present)

Minimize P. ramorum inoculum 

production

++ +++ — (higher if noninfected 

hosts still present)

Minimize hazards associated with 

SOD-killed trees

— +++ +++

Restoration of affected areas — + +++

a

 More “+” symbols indicate higher relevance/importance. “—” indicates low priority or not applicable.
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The severity of SOD impacts often varies over short distances. It may be useful 

to divide management units into areas in which the predominant SOD disease stage 

is relatively uniform. Management in these different areas needs to reflect the current disease impact stage and may need to be adjusted over time as disease impacts 

change.

Management options differ for stands without SOD and stands that have 

already been affected by SOD (table 2-3). Detailed technical descriptions of the 

methods are in Chapter 3. The management options in table 2-3 are based on 

a considerable body of scientific data and practical experience. They represent 

the best management practices currently available. However, research into these 

management practices is ongoing. Check the California Oak Mortality Task Force 

(COMTF) Web site (http://www.suddenoakdeath.org/) for updates on research and 

management recommendations. 

Box 2-3—Time Considerations and Sudden Oak Death

When planning or evaluating SOD management activities, you will need to consider 

if weather conditions over the previous several years have been favorable for SOD 

development. As discussed in Chapter 1, the likelihood that P. ramorum will initiate 

many new cankers or spread into new areas varies from year to year based on amount 

and timing of spring rainfall. 

Sudden oak death prevention treatments are most successful when applied to 

trees that are not infected. Within 1 to 2 years of favorable SOD infection conditions, 

many infected trees will not yet have external symptoms and will appear healthy. In 

contrast, after several dry years with low inoculum production, most trees without 

symptoms are likely to be noninfected. 

To evaluate the effectiveness of SOD prevention treatments, you need to monitor 

both treated areas and similar untreated control areas. During periods of low disease 

pressure, you will not be able to tell if SOD prevention treatments are working 

because even untreated trees are unlikely to develop symptoms. Trees need to be 

monitored for at least several years after one or more seasons of high disease pressure 

to determine whether treated trees show less disease than untreated controls.

2.2.1—Before: Reducing Sudden Oak Death Potential in Areas 

Not Currently Infested

In areas that are not currently infested with P. ramorum, the main management 

objective is to prevent introduction of the pathogen. The first question to address 

for such areas is whether P. ramorum could become established, given the climate 

and vegetation type. If it is likely that P. ramorum could invade the area, the next 
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Table 2-3—Management methods to prevent or reduce sudden oak death (SOD) disease incidence and 

severity before, during, and after invasion by Phytophthora ramorum

Methods Targets/variants Management objectivea Notes

Exclusion of infested 

plant material

Nursery stock Prevent Also important for exclusion of other 

soil-borne Phytophthora spp. such as 

P. cinnamomi.

Leaves, plant debris Prevent Includes both intentional movement (e.g. 

green waste transport) and inadvertent transport (e.g., debris in equipment such as 

chippers).

Firewood Prevent Freshly cut wood with attached bark poses the 

greatest risk.

Exclusion/sanitation Soil Prevent P. ramorum and soil-borne Phytophthora 

species can be moved on tires, shoes, and 

equipment, mainly during wet conditions.

Water Prevent P. ramorum spores are common in streams 

that drain infested areas.

California bay 

removal

Localized removal Prevent 

Minimize 

Protect

Reduces SOD risk of individual oaks by increasing distance from infected California 

bay foliage. 

Area-wide removal Prevent 

Minimize 

Protect

More efficient than localized removal if

California bay are relatively small and total 

bay cover is low.

California bay canopy 

thinning/raising

Minimize 

Protect

Reduce California bay foliar disease potential 

and inoculum production where complete 

bay removal is not practical.

Targeted removal of 

highly susceptible 

California bayb

Minimize Aim is to slow the seasonal epidemic by 

removing trees that show most disease carryover and high susceptibility. Removed 

California bay trees may not be near oaks.

Remove climbing 

poison oak

Minimize 

Protect

Primarily done in combination with other 

disease management treatments.

Avoid pruning or 

wounding prior to 

wet season

Protect Wounds have increased risk of infection for 

4 months.

Fungicide application Individual tree 

treatment

Protect Primarily used on selected individual high 

value trees. For tanoak stands, block treatment may be more effective. Practical levels 

of efficacy not yet documented in controlled 

field tests.

Block treatmentc Minimize 

Protect

Preferred approach for tanoak stands to protect trees and reduce inoculum production in 

treated block. Practical levels of efficacy not 

yet documented in controlled field tests.

Removal of understory 

tanoak

Minimize Primarily used in conjunction with block treatment of tanoak with phosphite fungicides. 

a

 Management objectives related to treatment methods: 

Prevent P. ramorum introduction. For “during” and “after” SOD impact stages, this refers to preventing introduction of additional P. ramorum strains.

Minimize potential for P. ramorum inoculum production.

Protect noninfected SOD canker hosts. b

 Only possible for “during” and “after” SOD impact stages.

c

 Block treatment refers to treating all trees within a stand or in a contiguous area of forest.
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question is how soon this could happen. Answers to these questions will indicate 

the degree of threat posed by P. ramorum. As the threat level increases, more 

intensive precautionary measures may be justified. 

Information needs— 

Is the area currently free of Phytophthora ramorum?—Phytophthora ramorum

may be present as a foliar disease on California bay, tanoak, and possibly other 

hosts (see section 1.2, “Sudden Oak Death Disease Cycle in California Forests”) for 

years before SOD cankers appear. Inspection, sampling, and testing of California 

bay leaves or tanoak twigs can be used to help determine whether P. ramorum is 

already established on foliar hosts in the area (see box 1-5 in chapter 1). If infections 

on foliar hosts are present, proceed to section 2.2.2, “During: Managing Disease in 

Areas Currently Affected by Sudden Oak Death.”

Phytophthora nemorosa and P. pseudosyringae can produce symptoms on 

California bay leaves and canker hosts that are nearly identical to those caused by 

P. ramorum. To determine whether such symptoms are caused by P. ramorum, 

lab testing is needed to identify the pathogen (see box 1-5 in chapter 1). Sampling 

should be conducted by persons that have been trained in SOD recognition and 

sampling protocols. The COMTF Web site (http://suddenoakdeath.org) provides a 

list of individuals who have recently completed a training session.

How far away are known Phytophthora ramorum infestations?—If a noninfested 

management unit is less than about 5 km from an infested area, it has a moderate to 

high risk of becoming infested in the short term (1 to 5 years, depending on weather). Spread of the pathogen via wind and water will be favored if prevailing storm 

winds blow, or watercourses drain, from infested areas toward the noninfested area. 

Stands that include large patches of California bay or tanoak have a higher risk of 

becoming infested than stands where these species are uncommon or lacking. More 

intensive management actions are needed to counter the disease threat in stands that 

have a higher risk of P. ramorum infestation (table 2-4).

If the noninfested unit is 5 to 10 km or more from an infested area, it has a 

low to moderate risk of becoming infested in the short term via natural processes. 

Watercourses that flow from an infested area may carry inoculum into the unit, 

where it could be splashed onto susceptible foliar hosts to initiate an infestation. 

However, human activities pose a greater risk of introducing P. ramorum through 

movement of infested plant material or soil. High risk areas include areas near 

roads, trails, and buildings. 

Maps of P. ramorum distribution are available from the OakMapper (Kelly and 

others 2012) and SODMAP (University of California–Berkeley 2012) Web sites. As 

shown in figure 1-1, the two sites primarily display different sets of data, although 
43

A Reference Manual for Managing Sudden Oak Death in California

some records occur in both Web sites. At the OakMapper site, only the officially 

confirmed SOD records (from California Department of Food and Agriculture and 

other research laboratory records) should be used as indicators of P. ramorum distribution. Other information about the distribution of P. ramorum in your local area 

may be available from your local county agricultural commissioner and University 

of California Cooperative Extension offices. 

What is the risk of severe disease if Phytophthora ramorum is introduced?—

Introduction of P. ramorum to an area does not guarantee that it will become established and cause disease. Disease will not develop if suitable hosts are not present 

or if the environmental conditions are unfavorable for pathogen survival, reproduction, and infection (see section 1.3, “Factors that Influence Sudden Oak Death 

Development”).

The density and distribution of California bay and tanoak are good indicators of 

SOD disease risk. Areas that lack both of these species are at low risk for SOD even 

if susceptible oak species are present and climate conditions are favorable. In oak 

stands, SOD risk increases as the amount of California bay in the stand increases 

(see section 1.2.2, “Sudden Oak Death in Oak/California Bay Forests”). Risk is also 

higher in stands where California bay is distributed throughout the area so that the 

pathogen can easily spread from tree to tree. Similarly, SOD risk in tanoak stands 

is greater where tanoak stands are denser and more continuous. California bay 

presence within tanoak stands increases overall SOD risk and is commonly associated with earlier and more rapid disease progress. However, SOD risk can be high 

in tanoak stands with little or no California bay.

Table 2-4—Relative importance/usefulness of management actions to prevent 

new sudden oak death (SOD) infections in areas where Phytophthora ramorum

does not currently occur

 Relative importance of management actionsa

Risk Proximity to known Bay Chemical

of SODb P. ramorum Exclusion removal control

Low Distant (5 to 10+ km) +++ — —

Midrange +++ + —

Close (<1 km) ++ +++ ++

Moderate Distant (5 to 10+ km) +++ + —

Midrange +++ ++ +

Close (<1 km) + +++ ++

High Distant (5 to 10+ km) +++ ++ —

Midrange +++ +++ ++

Close (<1 km) + +++ +++

a

 Relative importance of management actions: +++ = high, ++ = moderate, + = low, — = very low. b

 Risk of SOD is affected by species mix and climate conditions.
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For a given stand composition, overall SOD risk increases as the climate and 

microclimate become more favorable for P. ramorum (see section 1.3.1, “Environmental Conditions”). Disease risk is greater in areas that receive higher levels of 

spring rainfall and have longer wet periods because of high humidity, fog, shading, 

slope, or aspect. Relative disease risk is lower in drier areas, or where wet conditions occur more sporadically during a given wet season or from year to year. 

Maps that predict the risk of P. ramorum infection based primarily on hosts and 

climate variables are shown in Kliejunas (2010). These models generally indicate 

that the greatest risk of SOD establishment is in the coastal counties from Monterey 

to Del Norte County. Sudden oak death risk extends eastward into the coast ranges 

in adjacent counties where suitable hosts are found. These models are best used to 

predict SOD risk at a regional scale. They may drastically over- or underpredict 

disease risk at the scale of a specific stand or parcel because the vegetation maps 

used in the models are generally not detailed enough to reflect the local scale. 

Management actions— 

The relative importance and usefulness of management actions in noninfested areas 

vary with the overall SOD threat (table 2-4). In noninfested stands close to an area 

where P. ramorum is present, monitor canker and foliar hosts at least annually for 

evidence of SOD. Foliar symptoms on California bay or tanoak provide an earlier 

indication of P. ramorum presence than stem cankers or SOD-killed trees. Detailed 

descriptions of specific management actions are discussed in Chapter 3.

2.2.2—During: Managing Disease in Areas Currently Affected by 

Sudden Oak Death

Once P. ramorum has become established in an area, the main focus of disease 

management changes from preventing pathogen introduction to limiting disease 

impacts. In P. ramorum-infested areas where SOD impacts are likely to continue to 

increase over time (the During phase), typical management goals are:

1. Minimize P. ramorum infection and mortality of SOD canker hosts. 

2. Minimize hazards associated with SOD-affected trees.

Management considerations related to the first goal are described in this 

section; those related to the second goal are described in the After section below. 

Management emphasis may shift from the first to the second goal as the number of 

SOD-infected trees increases. 

Information needs—

What is the incidence of SOD in the affected stand?—More SOD management options are available when disease incidence is still low than when most at-risk trees 
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A Reference Manual for Managing Sudden Oak Death in California

have become infected. Only some of the at-risk trees typically become infected 

during each favorable infection period; little or no new disease may develop in dry 

years (fig. 1-16). Management actions implemented during lulls in the epidemic can 

protect asymptomatic trees from becoming infected during later disease pulses. 

When assessing SOD impact within stands, note both the presence of cankers 

and the severity of the infection (percentage of the trunk circumference that is 

affected, presence of secondary organisms). Sudden oak death may go into remission (see section 1.4.1, “Patterns of Canker Development”) in some oaks that only 

have one to several small cankers. Such trees may be candidates for future disease 

suppression activities. The amount of trunk girdling and the presence of secondary invaders (beetles or Annulohypoxylon thouarsianum) also affects the tree’s 

likelihood of dying or failing. Also, remember to take past weather conditions into 

account when making SOD assessments (see Box 2-3) to avoid underestimating 

infection levels.

Management actions— 

The main methods for minimizing P. ramorum infection are listed in table 2-4 and 

are described in Chapter 3. Many of these management methods work best when 

combined in an integrated fashion. For example, chemicals used to protect trees 

from infection will be more effective if measures are also taken to reduce local inoculum levels.

Selecting individual trees for treatment—Some SOD management treatments

are implemented on a tree-by-tree basis. To minimize costs, you may want to limit 

the most intensive treatments to high value trees that have a high infection risk 

(table 2-5). 

Table 2-5—Factors associated with increased sudden oak death (SOD) risk for 

individual trees

Category Factors associated with higher risk

Favorable microclimate conditions 

for infection

North- or east-facing slopes, shaded drainages, 

high canopy cover, fog drip

Amount of Phytophthora ramorum 

spores to which the tree is exposed

Little or no clearance from bay, high local bay or 

tanoak density, extensive poison oak climbing 

through canopya

Host tree size and condition Larger stem diameter, greater bark thicknessa

, 

high tree vigora, actively expanding (brown) 

bark fissuresa, low water stressa

a

 Correlated with SOD risk for coast live oak.
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Monitoring treatment efficacy—By assessing disease symptoms in treated and 

nontreated areas on an annual basis, you can determine how disease is progressing at a location, how well treatments are working, and whether additional actions 

may be needed. Select individual trees to monitor so that trends over time can be 

tracked. Trees can be identified by using permanent tree tags in combination with 

geographic coordinates or other spatial information.

2.2.3—After: Dealing With Impacts of Sudden Oak Death

As long as noninfected P. ramorum canker hosts are present, disease prevention 

activities of the “During” phase (table 2-3) may still be appropriate. However, once 

a high proportion of canker hosts have died, management emphasis shifts toward 

dealing with the aftermath of SOD. This section discusses three main management 

issues: 

1. Survival and failure potential of SOD-affected oaks and tanoaks.

2. Fire hazard and fuel loading due to SOD-induced tree mortality.

3. Restoration of SOD-affected forests.

Survival and failure potential of SOD-affected trees— 

Removing SOD-affected oaks has no effect on disease spread in the stand because 

the pathogen does not sporulate on oaks; they are a “dead end” for the disease. 

Determining whether or when to remove SOD-affected oaks should be based on 

management criteria such as potential hazard, cost, and wildlife habitat value. 

Information needs—How long will SOD-affected trees survive? Trees with P. 

ramorum canker can follow a wide range of survival trajectories over time (see section 1.4.2, “Patterns of Disease Progression” and section 1.4.3, “Survival of Trees 

With Sudden Oak Death”). This variation in tolerance can make it more difficult to 

decide if and when to remove symptomatic trees. Factors correlated with survival 

are summarized in table 3-3.

What is the failure potential of SOD-affected trees? Oaks and tanoaks with 

late symptoms of P. ramorum canker have an increased risk of failure (breakage) 

compared to healthy, noninfected trees (table 2-6). Because SOD cankers typically 

affect the lower trunk, decay associated with SOD cankers often leads to trunk 

failure. Trees infected by P. ramorum often die as the result of large scaffold or 

trunk failures before the SOD canker actually causes death of the top (fig. 1-15).
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A Reference Manual for Managing Sudden Oak Death in California

Trees with P. ramorum canker have an increased risk of failure because of two 

main factors: 

• Death of the entire tree or large portions of it.

• Increased decay, often in combination with extensive beetle boring. 

Dead trees and dead parts of live trees have an elevated likelihood of failure 

whether or not they are affected by SOD (table 2-6). In SOD-killed trees, small 

diameter branches and trunks typically decay fairly rapidly and often become heavily colonized by wood-boring insects. If trunk failure does not occur first, branches 

and scaffolds commonly begin to fail within 1 to 2 years after a tree is killed by 

SOD (fig. 2-2). Remove dead trees or branches that pose a significant risk to people 

or property as soon as possible.

Many oaks and tanoaks with P. ramorum trunk cankers develop an extensive 

white rot of the sapwood caused by the decay fungus A. thouarsianum. Cankered 

areas and killed portions of the canopy are typically affected first. Other decay 

fungi, including Phellinus gilvus, may also be present and cause similar decay. In 

small-diameter trees (less than about 30 cm), sapwood decay associated with these 

secondary decay fungi can be extensive enough to cause trunk failure (fig. 2-2). 

Such failures typically occur within one to several years after the top has died. 

In larger trees, trunk failure commonly results from a combination of SODrelated sapwood decay and preexisting heartwood decay (fig. 2-3). Many large oaks 

have extensive internal decay associated with canker rot fungi (especially Inonotus 

andersonii, I. dryophilus, and Phellinus robustus) or other aggressive wood decay 

fungi such as Laetiporus gilbertsonii. A small amount of SOD-related sapwood 

decay may trigger failure if it extends to wood already affected by heartwood decay 

Table 2-6—Average annual failure rates (percentage of trees in disease category 

that failed each year) by disease category among 630 coast live oaks observed 

over a 10-year period in Marin and Napa County forests affected by sudden oak 

death (SOD) 

Disease category Average annual failure rate (percent)

Dead from SOD 42

Dead from other causes 20

Late SOD symptoms (beetles or 

Annulohypoxylon thouarsianum present) 11

Early SOD (bleeding cankers only) 0.4

Healthy; not SOD-infected 0.4
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Figure 2-2—Possible paths of sudden oak death progress and eventual tree degradation in coast live oaks that have little or no preexisting 

wood decay in the trunk. Such trees typically do not fail before the top is killed, and snags may stand for a number of years before the 

trunk fails. Green tree at upper left represents a newly infected coast live oak with only bleeding cankers. 

Extensive girdling,

susceptible reaction;

1- to 2 year delay,

color change in 1 to 

several months 

Moderate to severe

 girdling; several years

Few small cankers, resistant reaction; 

1 to several years

Moderate girdling, large cankers

sound with callused edges;

several to 10+ years

Severe girdling, large cankers,

become decayed;

several to 10+ years

Subsequent decay as

shown in figure 2-4

Trees with small trunk diameter or some

preexisting decay and/or defects

Trees with larger trunk diameter or little

or no preexisting decay or defects

1 to several 

years delay, color

change in 1 to

several months

1 year

1 year 1 year

1 to

several years

1 to

several years

1 to several years

1 to several years

1 to several years

1 to several years
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A Reference Manual for Managing Sudden Oak Death in California

Figure 2-3—Failure of a large coast live oak killed by Phytophthora ramorum. Trunk failure was 

associated with preexisting heartwood decay and a cavity in the center of the trunk. Fruiting bodies of Annulohypoxylon thouarsianum and Phellinus gilvus seen on the bark are associated with 

sapwood decay that developed after Phytophthora ramorum infection.

(fig. 2-3). Failures of live SOD-infected trees (fig. 2-4) commonly develop where 

sapwood decay approaches or merges with existing heartwood decay. In contrast, 

large-diameter oaks that are killed by SOD but lack preexisting heartwood decay 

may stand for 5 to 10 years or more before trunk failure occurs (fig. 2-2).

Management actions—If SOD-affected trees are located where the failed stems 

may impact persons or property, periodic monitoring is needed to identify and 

mitigate potential hazards well before failure is imminent. Hazards may be reduced 

by pruning or removing trees or by keeping people and property outside of the 

potential impact area (see section 3.3, “Monitoring Oaks With Sudden Oak Death 

to Assess Survival and Failure Potential”). In areas with many potentially hazardous trees, permanent or temporary closures and posting may be used to reduce the 

chance that likely impact areas will be occupied when tree failures occur. Trees that 

have not been affected by SOD can also have a high risk of failure because of decay 

or structural problems (fig. 2-5), so hazard assessments need to consider all potentially hazardous trees, not only those with SOD.
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Figure 2-4—Possible paths of sudden oak death disease progress and eventual tree degradation in coast live oaks that fail while 

the top is still live (green failure) or within a short time after rapid death of the canopy. Trees following this pattern typically have 

large amounts of preexisting wood decay in the trunk or other severe structural defects, and are extensively colonized by secondary decay fungi and beetles. Green tree at upper left represents a newly infected tree with bleeding cankers only. 
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A Reference Manual for Managing Sudden Oak Death in California

Figure 2-5—The trunk of this coast live oak failed because of extensive internal decay. Although 

the tree was in a location where sudden oak death is common, this tree was not infected with 

Phytophthora ramorum. 

In areas where many trees are affected by SOD, it may be necessary to prioritize and phase removals. Trees that are likely to impact persons, structures, utility 

lines, or access roads typically have a high priority for removal or mitigation. Trees 

that are likely to fail before the next inspection period should be targeted as the 

highest priorities. The criteria in tables 3-2, 3-3, and 3-4 (in section 3.3) can be used 

to help identify trees that are more likely to die or fail in the near future. 

SOD-related mortality and fire— 

Tree mortality resulting from SOD has the potential to affect fire hazard and behavior by changing the types and distribution of fuels in the forest. Many other factors 

unrelated to SOD can also influence fuel distribution and fire hazard in forests. 

In many stands, the change in fuel distribution from a few scattered SOD-killed 

trees will not significantly alter fire behavior in the stand. At the other end of the 

spectrum, high levels of SOD-related mortality seen in some tanoak stands can 

greatly alter fire hazard for a number of years. Impacts of SOD need to be considered within the context of the entire landscape to determine how much they change 

fire hazard and which, if any, management actions may be needed.
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The presence of SOD-killed trees in a stand can affect fuel amount and distribution as follows:

• During the dry season, dead oaks and tanoaks have low moisture content 

and are more likely to burn than living trees.

• Recently killed trees have an abundance of dead leaves and twigs. These 

dry fine fuels can ignite readily. During a fire, they can generate or receive 

embers that cause spotting (ignitions beyond the fire front).

• Dead leaves and branches from SOD-killed trees can increase the total 

amount and depth of surface fuels. 

• If mortality is common and widespread, dead leaves and branches can form 

areas of continuous surface fuels in stands that would otherwise have a 

much more patchy surface fuel distribution. 

• Piles of woody debris from dead trees and leaning failed trees can form fuel 

ladders between surface and aerial fuels.

• SOD-killed trees can contribute a large amount of large-diameter dead 

woody material to the surface fuel layer. Firefighting operations are complicated when large logs ignite because they burn for extended periods.

Because live trees with late SOD symptoms and standing dead trees have an 

increased likelihood of failure, these trees can pose a hazard to firefighting personnel during fire suppression and mop-up operations. 

Information needs—What kinds of fuels have SOD-killed trees generated?

Fuels generated by SOD-killed trees differ depending on tree species, size, and the 

length of time between infection and tree death (figs. 2-2 and 2-4). Furthermore, the 

fuels from killed trees are not static but degrade over time. Once woody materials 

and leaves decay to the point that they have lost their structural integrity, they typically do not affect fire potential. 

Trees killed by SOD begin to degrade before they are dead as they are attacked 

by beetles and wood decay fungi. Decay and degradation of dead materials proceed 

faster in wetter sites (high rainfall, frequent fog, deep shade). Degradation patterns 

and timelines differ between standing dead trees and material that has fallen to the 

ground because materials on the ground remain moist longer (figs. 2-2 and 2-4). A 

photo series showing typical fuels associated with decaying SOD-killed coast live 

oaks is shown in section 3.4, “Assessing Fire Hazards Related to Trees Killed by 

Sudden Oak Death,” tables 3-5, 3-6, and 3-7. The series is based on data collected 

primarily in Marin County from 2000 through 2011.
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How much additional fuel is present and how is it distributed? Tree mortality caused by SOD affects forests in ways that can alter fire behavior. Sudden oak 

death can alter the fuel loading and distribution in a forest. Fire intensity and rate 

of spread depend on the total amount of fuels present, typically expressed as tons 

of fuel per acre. Higher fuel loads increase the amount of heat that can be produced 

during a fire. The horizontal and vertical distribution of fuel also affects fire 

behavior. Fire will spread more uniformly and reliably if a continuous fuel layer is 

present. Spread will be slower and more unpredictable if fuels are patchy in distribution. Flame length is affected by the height of the fuel bed. Tall piles of debris 

and dead material that lean up against standing trees can serve as ladder fuels, 

allowing ground fires to move into the canopy. 

Canopy gaps created by SOD-killed overstory trees can open the forest floor to 

more wind and light. This creates hotter and drier conditions that can increase the 

rate at which fuels dry as the season progresses. In addition, higher windspeeds at 

the forest floor can increase the rate of fire spread.

Light penetration into canopy gaps promotes the growth of understory vegetation. Gaps may be invaded by annual grasses and forbs that dry out to form fine 

fuels that ignite easily. Canopy gaps also support greater growth of native or invasive shrubs, which can further change fuel types and fuel height distribution. Tree 

regeneration, especially Douglas-fir seedlings and saplings, can also become dense 

in canopy gaps, forming ladder fuels. Eventually, competition causes dense Douglas-fir clumps to develop extensive lower canopy dieback and mortality, resulting in 

a more hazardous fuel distribution than existed in the stand prior to SOD. 

Management actions—Dead trees: cut or leave? Defensible space typically extends 30.5 m (100 feet) from buildings. Within this zone, trees, shrubs, and other 

fuel sources should be managed to make it difficult for fire to transfer from one 

fuel source to another (State Board of Forestry and Fire Protection, California 

Department of Forestry and Fire Protection, 2006). Within defensible space fuel 

management areas, SOD-affected trees should be removed as soon as the foliage 

starts to turn brown. Data from several studies indicate that the water content of 

foliage of SOD-affected trees does not change appreciably until the tree changes 

color. 

In more extensive forest areas beyond defensible space envelopes, scattered 

tree mortality occurring over many years will probably not significantly alter fuel 

distributions at the landscape level. In such sites, leaving trees to degrade and decay 

in place is often the best option. Where large patches of trees have died within a 

relatively short period (1 to 3 years), problematic levels of readily ignitable fuels 

will be present on the ground and in the canopy for at least several years. Canopy 
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fire hazard can be reduced by felling standing dead trees that still have many dead 

leaves and fine twigs. Once standing dead trees have lost their leaves and fine 

twigs, they do not serve as significant aerial fuels. Cutting dead trees at this stage 

will reduce the potential for tree failure and reduce the fuel ladders that develop 

when failed trees hang up on adjacent standing trees.

If dead trees are cut to manage fuels, it will be necessary to treat the biomass 

from these trees to avoid creating excessive amounts of surface fuel. Treatment 

options include:

• Removal to an acceptable disposal area in accordance with state and federal 

regulations.

• Piling and burning in accordance with local fire and air quality regulations.

• Chipping, with chips preferably spread out in a thin layer and left onsite.

• Lopping and scattering debris to hasten degradation.

Wood from SOD-killed trees may be used as firewood on the same property 

(see below, section 3.1.3, “Firewood”), but should not be moved beyond the local 

area. State and federal regulations generally prohibit the movement of wood from 

susceptible oaks and tanoak from SOD-infested counties to noninfested counties.

Managing vegetation in canopy gaps. You may need to actively manage 

vegetation that recruits in canopy gaps to keep hazardous fuel conditions from 

developing, especially within defensible space. Annual and perennial exotic species 

that invade canopy gaps should be eradicated or suppressed. It may also be necessary to prune, thin, or selectively remove native species, such as dense Douglas-fir 

regeneration.

Restoration in SOD-affected forests— 

Many of the forests now affected by SOD were previously well stocked with little 

mortality, so restoration activities were largely unneeded. As SOD-related mortality 

increases, a landowner or manager may wonder what, if any, actions are needed to 

restore forest canopy cover. You can identify appropriate options for forest restoration based on stand conditions and management goals by using the adaptive management process discussed above (fig. 2-1). 

Information needs—Is stand restoration needed based on the current or projected levels of mortality from SOD and other factors? Although tree mortality 

from SOD will cause a loss in tree density (trees per unit area), mortality does not 

always reduce overall canopy cover. If a forest has overlapping tree canopies, scattered tree mortality may not create large canopy gaps. Small canopy gaps may be 

quickly erased by growth of overstory or understory trees.
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A Reference Manual for Managing Sudden Oak Death in California

Some tree mortality in a dense stand may not be detrimental. Reduced tree 

density may reduce competition for moisture or light within the stand and improve 

growing conditions for remaining trees. In contrast, mortality of dominant trees 

in relatively open stands will typically cause significant changes in canopy cover 

and forest structure. As canopy cover loss from SOD increases, valuable ecosystem 

services (table 2-1) are likely to be reduced or lost. 

Even small losses of canopy cover can have disproportionately large impacts if 

they affect critical areas or involve unique resources. For example, acorns produced 

by oaks and tanoaks can contribute significantly to wildlife habitat value, even if 

these trees make up a small percentage of the stand. When evaluating the need for 

restoration in SOD-affected forests, consider both effects on overall canopy cover 

and ecosystem services that are not strictly related to canopy cover loss.

Assess the size and distribution of canopy gaps created by tree mortality and 

failure. Canopy gaps represent opportunities for natural or assisted regeneration. 

Canopy gaps can also provide habitat elements that are absent in closed-canopy 

forests, which can benefit some wildlife species and increase overall biodiversity.

As noted above (see box 2-2) P. ramorum canker is not the only mortality agent 

that needs to be considered when assessing restoration needs. If other mortality 

agents are active in SOD-affected forests, mortality can affect a wider range of species and a greater percentage of the total forest canopy. This can further complicate 

prospects for restoring a functional forest. 

Which species are regenerating at the site? Most SOD-affected forests 

contain understory regeneration (seedlings and saplings) of various tree species. 

Established understory seedlings and saplings in SOD-caused canopy gaps can be 

an important source of regeneration. When released from the suppressive effects 

of overstory competition, these persistent seedlings and saplings commonly show 

rapid growth (fig. 2-6). Understory regeneration may include both current canopy 

species and trees not found in the overstory.

In contrast, seedlings of canopy species may be rare or lacking in the understory. For example, California black oak seedlings are often rare under their parent 

trees (Swiecki and Bernhardt 2008d). Seedlings of coast live oak differ widely in 

number from year to year and between locations (Swiecki and Bernhardt 2008a). 

Gaps may also be colonized by species that do not establish well under canopy. 

Both shade-intolerant native species and many weedy exotic species may be favored 

by canopy gaps. Weedy species may suppress natural regeneration of preferred 

native species by competing for soil moisture and other resources. If aggressive 

exotics such as broom species (Cytisus, Genista, Spartium spp.) are present in or 
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near the site (fig. 2-7), action may be needed to suppress exotics and favor desired 

native species.

Management actions—After reviewing your information about tree mortality 

and natural regeneration, your next steps are setting goals and selecting actions for 

restoring forest structure and function. If SOD impacts are limited, you may not 

need to take any actions to maintain overall stand conditions. In other situations, 

you may need to undertake modest interventions in a timely fashion to achieve 

desired stand conditions. In the most severely affected stands, significant management inputs may be needed to restore lost ecosystem services or prevent undesirable 

changes in the stand. 

Specific management actions needed to achieve restoration goals will vary 

widely depending on forest types, site conditions, landowner resources and capabilities, and other factors (see below, section 3.5, “Restoring Sudden Oak DeathAffected Forests” and box 2-4). The key to cost-effective restoration is determining 

what factors are likely to constrain regeneration at the site. Restoration inputs 

can then be selected to specifically address these site constraints. Monitoring the 

Figure 2-6—Dense populations of Douglas-fir seedlings have established in a canopy gap that 

developed when overstory coast live oaks were killed by sudden oak death in this Marin County 

site. Douglas-fir overstory was not previously a component of this stand. Douglas-fir can easily 

outcompete slower growing hardwood species in canopy gaps. 
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A Reference Manual for Managing Sudden Oak Death in California

Figure 2-7—French broom (Genista monspessulana), an invasive exotic shrub, has become well 

established in the understory of this coast live oak stand. Canopy thinning (seen in coast live oak, 

center) and tree mortality caused by sudden oak death has favored French broom invasion by 

reducing competition for light and soil moisture.
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results of your restoration efforts will provide additional site-specific data needed to 

improve and optimize your methods (see below, section 3.5.5, “Monitoring Restoration Outcomes”).

A mosaic of restoration approaches may be applied in different areas and 

implementation may be phased over a period of many years. Risks associated 

with unfavorable weather (e.g., drought) can be minimized if the implementation 

schedule is flexible. Timelines for projects with third-party funding may have less 

flexibility in the implementation schedule. 

Box 2-4—Restoration in Developed Versus Wildland Settings

Developed areas and wildland settings provide distinctly different opportunities 

and constraints with respect to restoration. In general, more intensive inputs such as 

irrigation and attention to individual trees may be warranted in developed areas (e.g., 

near home sites) but are often impractical or unnecessary for wildlands. 

Developed areas at the edge of wildlands (the wildland-urban interface) are 

especially complex. Although it is possible to manage these areas intensively, tree 

management has the potential to significantly affect ecological processes in the 

adjacent wildlands. For example, planting nursery stock can introduce soil pathogens 

into native stands. Planting can also introduce nonlocal genotypes that may alter the 

genetics of the locally adapted gene pool (see “Genetic considerations” in section 

3.5.4). The surrounding forest also has the potential to influence plantings in the 

interface area. For example, browsing by deer or other wildlife species may damage 

plantings. In general, plantings in the interface area should follow guidelines established for restoration in wildlands to minimize potential adverse impacts.

2.3 Prioritizing Areas for Sudden Oak Death 

Management Activities

On small properties with relatively few trees, it may be feasible to reduce disease 

risk for every tree. On large parcels, economic or logistic constraints will limit the 

extent to which management actions can be applied. Hence, a key element of the 

planning process involves setting priorities. Limited resources can be used most 

effectively by determining:

• Where: Which areas, if any, should be treated?

• How: What types of treatments should be used? 

• When: Which areas should be treated first? 

A matrix for prioritizing areas for SOD management activities is shown in table 

2-7. The matrix provides a framework for prioritizing candidate areas based on the 

following main categories.
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1. Management opportunities: Which areas can be treated most easily and 

effectively?

2. Disease risk: What is the likelihood of severe SOD impacts in the area? 

3. Loss of resource benefits: Which resource benefits, services, and values will be 

affected by SOD-related tree mortality?

4. Hazards: What detrimental or undesirable conditions will develop if the stand is 

affected by SOD?

You will obtain the greatest benefit per unit cost by focusing on the most 

important resources that are likely to be affected and are relatively easy to treat.

2.3.1—Prioritization Categories

Management opportunities— 

You can readily identify low-priority sites by determining which areas are not feasible to treat. These include areas that are so steep or inaccessible that treatments 

would be excessively difficult and expensive. Areas that are not practical to treat 

can be removed from further consideration.

Among sites that could be treated, the ease of treatment and likelihood of 

obtaining acceptable levels of disease control will be affected by the factors shown 

in table 2-7. By assessing these factors, you should be able to rank potential sites 

based on overall SOD management opportunities.

Disease risk— 

Another way to prioritize sites for treatment is to assess factors that influence SOD 

risk. If the risk that SOD will develop in a given stand is small, that stand will have 

a very low priority for SOD management activities. Disease risk may change over 

time as conditions change, so priority rankings based on disease risk need to be 

reevaluated periodically. 

Loss of resource benefits— 

Sites with at-risk SOD canker hosts can be prioritized by considering the various 

ecosystem services they provide. Sites that provide the most important benefits and 

values have the highest priority. Table 2-7 lists a number of potential factors to consider. To develop a list that matches your management objectives and interests, add 

or delete items from table 2-7 and adjust the definitions of the high- and low-priority 

categories. Weights applied to various factors can also be shifted. 
60

general technical report psw-gtr-242 Table 2-7—Prioritization matrix: factors to consider when prioritizing stands for sudden oak death (SOD) management activities Category Factors High priority Low priority Management opportunities Ease of access for treatment (slope, roads, etc.) Near level Adjacent to roads Excessively steep No road access

Position of treatment site Site is discrete unit; little or no likely contribution 

of inoculum from adjacent areas beyond treated zone 

Site is adjacent to areas that cannot be treated

SOD impact stage No or low percentage of trees with cankers or 

SOD mortality

High percentage of trees dead or with severe SOD 

symptoms

Relative cost of treatment Low cost per unit treated High cost per unit treated

Overall likelihood of success Likely to be successful because of favorable site 

or logistical issues

Low probability of success because of difficult site 

or logistical issues

California bay presence/density/size class California bays mostly understory saplings and 

small trees

California bays are large overstory trees

California bays are low percentage of total tree 

cover

California bays are majority of tree cover

California bay continuity California bay distribution is discontinuous in 

local area or could be made discontinuous with limited tree removal 

Continuous California bay distribution over large 

area, little or no possibility of creating significant gaps in distribution

Disease risk California bay density Low to moderate density in overstory or 

understory

None present within stand

Proximity to known Phytophthora 

ramorum infestations

P. ramorum present in stand Nearest known P. ramorum at least 10 to 20 km 

(6 to 12 mi) away

Risk of pathogen introduction (if not 

present)

High number of visitors and vehicles Infrequently used site

Climate/ 

microclimate conditions

Wet—high rainfall, foggy, north-facing slope, 

dense tree canopy

Dry—low rainfall, fog uncommon, south-facing 

slope, open canopy

Tree variables Large, dominant, vigorous trees with thick, 

expanding bark (bark fissures with brown,unweathered tissue)

Small, overtopped/suppressed, or water-stressed 

trees with thin bark, slow bark expansion (bark fissures weathered and grayish)
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Table 2-7—Prioritization matrix: factors to consider when prioritizing stands for sudden oak death (SOD) management activities (continued)Category Factors High priority Low priorityResource benefits Stand condition without SOD Sustainable stand; trees mostly fair to good 

condition

Stand declining due to other factors

Stand uniqueness Stand unusual because of species composition, 

age structure

Widespread and common stand type

Historical/cultural value Significant trees or stand No special significance

Aesthetic/amenity/visual values Significant scenic resource Stand not within a viewshed

Strongly contributes to property value Low or no specific contribution to property valueServes as visual buffer (e.g., screening, privacy) No visual buffer effects

Habitat value Essential or preferred habitat of rare or special 

status species

Unsuitable habitat for rare or special status species; 

none present 

Important habitat for other locally significant or 

desirable species

Poor habitat quality for species of interest

Special habitat features/areas present (e.g., 

riparian, wetland, old growth, unique soils)

No special habitat features 

High native species biodiversity Low native biodiversity (e.g., highly disturbed site)Important corridor or connection between 

different habitats

Does not function as corridor (e.g., isolated forest 

fragment) 

Soil stability and water quality SOD-susceptible species in critical location 

providing slope stability/erosion protection

SOD-susceptible species not important contributors to soil stability at site

SOD-susceptible species present along 

watercourse and providing important bank stabilization 

SOD-susceptible species not near watercourse or 

do not contribute to bank stability

SOD-related 

hazards 

Hazards posed by infected and dead trees 

to persons and properties

Failures would affect occupied structures Not near structuresFailures would affect heavily used roads or trails Not near roads or trailsFailures would affect other built infrastructure 

(utility lines, drainage structures, etc.)

Not near infrastructure

Contribution to fire hazard Defensible space fire hazard significantly increased by fuels associated with dead standing 

or down trees 

SOD-related mortality poses little or no change to 

overall fire hazard within or near defensible space 

Conversion to exotic-dominated or other 

undesirable vegetation type

Highly susceptible to invasion by undesirable 

exotics

Invasion by exotics unlikely

Gaps likely to be converted to undesirable 

vegetation type

No undesirable changes likely because of species 

mix and small or no gaps
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Hazards— 

The presence of many SOD-killed trees on a landscape may pose a number of hazards. Consider the degree to which you can avoid these hazards by preventing or 

minimizing SOD-related mortality when prioritizing sites for treatment. 

2.3.2—Developing Overall Priority Rankings

Sites can be prioritized by comparing their potential management opportunities, 

loss of resource benefits, SOD risk, and hazards. If you have a limited number of 

sites, ranking them may only require a quick mental review of these factors. If you 

have many sites, it may be helpful to use a numeric rating system to develop priority scores for each site. You can then compare sites based on their priority scores. 

Priority scores for areas can also be displayed on a map for further consideration. It 

may be more efficient to treat several adjacent sites rather than scattered sites with 

similar priority scores. A method for creating numeric ratings from the matrix in 

table 2-7 is described in box 2-5. 

Box 2-5—Developing a Numeric Priority Ranking Scale

The prioritization matrix (table 2-7) can be used to develop numeric rankings to aid 

in comparing a large number of sites. Start by assigning a number to each factor 

based on the priority rating (e.g., high priority = 3, moderate priority = 2, low priority 

= 1, not applicable = 0). If you want to weight some factors more than others, multiply 

the priority rating by a weight factor. For example, a weight factor of 2 doubles the 

importance of a factor; weighted scores will range from 0 to 6 (table 2-8). 

If you simply sum the ranks for each category (with or without weighting), the 

categories with more factors to consider (e.g., resource values) will have a greater 

influence on the total score. You can correct for this by calculating the percentage of 

the maximum score for each category. First, determine the maximum score possible 

for each category. If you use weighted ratings, sum the maximum weighted scores. 

Then, divide the sum of ranks for each site by the maximum score for the category. 

You can multiply this number by 100 to convert it to a percentage.

The final step involves combining the four category scores to obtain an overall 

priority score. Averaging the four scores is the simplest method. Alternatively, the 

four scores (expressed as decimals) can be multiplied. This latter method yields a 

wider spread of values, which may be useful if many sites have similar ratings. Either 

method yields values that can be expressed as percentages of the maximum possible 

rating (table 2-9).

When you convert your observations and evaluations to numbers, you typically 

lose some information. Numeric priority rankings can readily separate the most and 

least promising sites, but more detailed review of candidate sites may be needed to 

rank sites with similar numeric ratings.

continues
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Table 2-8—Example showing use of prioritization matrix to create numeric priority rankings 

 Priority Weighted

 rating rating Maximum 

 weighted

Factor Subfactor Weight Site 1 Site 2 Site 1 Site 2 score

Management

opportunities:

Ease of access Slope conditions 2 2 3 4 6 6 

 for treatment

Adjacent to roads 2 3 3 6 6 6

Position of 1 2 3 2 3 3

 treatment area

Relative cost of 1 2 2 2 2 3 

 treatment

Overall likelihood 1 3 3 3 3 3

 of success

Existing sudden oak 1 3 3 3 3 3 

 death incidence

California bay Bay mostly 2 3 3 6 6 6 

 presence/density/ understory

 size classes saplings and 

small trees

Bays sparse 2 3 3 6 6 6

Note: Scoring for part of the “Management opportunities” category is shown for two example sites.

Table 2-9—Category totals and overall scores calculated using the averaging 

method for this example

Unweighted Maximum Percentage of 

ratings Weighted ratings weighted maximum

Site 1 Site 2 Site 1 Site 2 score Site 1 Site 2

Management 21 23 32 35 36 89 97

opportunities

Disease risk 10 10 10 10 12 83 83

Resource 7 23 7 29 42.5 23 58

benefits

Hazards 6 8 8.5 10 25.5 33 40

Totals/average 44 64 57.5 84 116 50 72

percentage of 

maximum

Note: Total scores using the multiplication method would be 6 percent for site 1 and 19 percent for site 2. In this 

example, site 2 has a higher overall priority because of higher ratings in the management opportunity, resource 

benefits, and hazards categories.

Box 2-5—Developing a Numeric Priority Ranking Scale (continued)
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When considering large tracts of at-risk forests, land managers may feel that 

nothing can be done that will have a significant impact and that SOD should just be 

allowed to “run its course.” However, costs that will be incurred from SOD-related 

mortality could outweigh the costs of targeted SOD management activities. Prioritization helps you break down a seemingly impossible task—protecting extensive 

forest resources from SOD—into viable projects that can be addressed in a logical 

order. 
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This chapter provides technical information about (1) the sudden oak death (SOD) 

disease management methods listed in table 2-3; (2) monitoring and assessing hazards of SOD-affected trees; and (3) restoration of SOD-affected forests. Additional 

information related to SOD management, including Best Management Practices 

for various user groups developed by the California Oak Mortality Task Force, is 

available at http://www.suddenoakdeath.org. 

3.1—Excluding Phytophthora ramorum From 

Noninfested Areas

Exclusion is the preferred management tactic in susceptible forest types where SOD 

has not been detected. Exclusion is most likely to be effective if the site is relatively 

distant from existing infestations and if exclusion can be implemented over a large 

area, such as an entire watershed or isolated stand. Exclusion is a worthwhile strategy for small parcels, but may be less effective if the land manager cannot exclude 

P. ramorum from adjacent parcels.

Practices related to sanitation and exclusion can also have an important role in 

P. ramorum-infested stands. The distribution of P. ramorum can be spotty in the 

early stages of a local SOD outbreak. Practices that avoid transporting the pathogen 

throughout the site may slow the rate of P. ramorum spread and provide a longer 

time for implementing control treatments. Sanitation practices also help prevent the 

spread of P. ramorum from an infested location to other noninfested locations.

Exclusion and sanitation measures are also important for preventing the 

introduction of additional strains of P. ramorum. Additional strains introduced via 

nursery stock or from other forested locations may be more aggressive or better 

adapted to the site than the existing strain (see box 1-4 in chapter 1). In addition, 

exclusion and sanitation can reduce the chances that additional exotic pests and 

pathogens would be introduced into the stand (see box 2-2 in chapter 2). 

Phytophthora ramorum may be inadvertently introduced into forests in a variety of ways. Methods for preventing P. ramorum introductions are discussed below. 

3.1.1—Nursery Stock

Phytophthora ramorum was originally introduced into California forests through 

movement of contaminated nursery stock. Since that time, federal and state regulations and inspections have been implemented to prevent the movement of 

P. ramorum-infested nursery stock. Nonetheless, contaminated nursery stock 

remains a pathway through which P. ramorum and other exotic Phytophthora

species can be introduced to a site. 

Chapter 3—Technical Management Guidelines
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Nursery detections have been made in California and other states in every year 

since monitoring for P. ramorum was initiated. It is difficult to consistently detect 

nursery stock that is infested with P. ramorum, in large part, because:

• A wide variety of species are potential hosts.

• Symptoms in most hosts (small leaf spots) are not distinctive.

• Asymptomatic infections occur in some hosts.

• Fungicide treatment in the nursery can suppress symptom development 

without eliminating the pathogen. 

Nursery-grown plants that are most commonly infested with P. ramorum 

include species and varieties of Camellia, Rhododendron, Pieris, and Viburnum. 

These species in particular should not be planted adjacent to forests containing 

SOD-susceptible native species. Also avoid using species that are moderately to 

highly susceptible to P. ramorum and support moderate to high levels of sporulation (table 1-3).

Given the potential risks posed by P. ramorum and other pathogenic Phytophthora species, planting nursery stock in or near native forests should be avoided in 

general. Nursery-grown plants should not be used where spores produced on the 

plants could be dispersed into native stands or where contaminated soil or water 

runoff from the planted area can reach native stands. 

3.1.2—Plant Material—Pruning Waste, Wood

Phytophthora ramorum can be transported by moving infected leaves from California bays (Umbellularia californica (Hook. & Arn.) Nutt.). Infected leaves can 

fall into vehicles (truck beds, the area below the windshield) and be moved to new 

locations. Infected California bay leaves dropped on the ground may produce P. 

ramorum spores that could be splashed to low leaves of susceptible species and 

initiate a new infestation.

Collect and safely dispose of any hitchhiking leaves discovered in a vehicle. 

Dispose of potentially contaminated leaves by burning or by placing the leaves in a 

sealed bag that will be sent to a landfill. Do not add them to a green waste recycling 

container.

Chippers and trucks used in pruning operations involving SOD hosts, especially California bay or tanoak (Notholithocarpus densiflorus (Hook. & Arn.) 

Manos, Cannon & S.H.Oh), should not be moved between infested and noninfested 

areas without being thoroughly cleaned. Clean trucks, chippers, chain saws, and 

any tools that may contain infested plant fragments before leaving the job site so 

infested material is not moved to the next work location (see box 3-1).
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3.1.3—Firewood

Although cankers on intact oak and tanoak trees do not appear to produce sporangia under normal field conditions, chlamydospores can be formed within infested 

wood (Parke and others 2007). Under wet conditions, sporangia can be produced on 

the cut surfaces of wood from trees with P. ramorum cankers (Davidson and Shaw 

2003, Davidson and others 2008). Therefore, movement of firewood cut from SODinfested trees poses a risk of transporting P. ramorum to noninfested areas. State 

and federal regulations prohibit movement of firewood cut from SOD hosts out of 

quarantined infested areas.

Wood from living or very recently killed trees has the greatest risk of being 

a source of P. ramorum spores. Wood from SOD-killed trees that have been dead 

for many years is unlikely to be a source of viable P. ramorum spores. For tanoak, 

wood of any size has the potential to be infested. For oaks, only the lower portion of 

the trunk of such trees are likely to have P. ramorum cankers. Small-diameter wood 

from the upper part of the oak canopy is rarely infested. 

Storing wood under dry conditions will prevent P. ramorum from producing 

sporangia on firewood. Rapid drying of cut wood also reduces reproduction of 

various wood-boring beetles. Split and stack firewood in well-aerated piles in a dry, 

sunny location away from other host trees until it is thoroughly dry. 

Wood can also be solarized for several months to speed wood drying and 

limit beetle reproduction. Cover woodpiles with a clear (not black) UV-resistant 

polyethylene tarp, 0.15 to 0.25 mm (6 to 10 mil) thick, that is sealed to the ground 

at the edges. The greenhouse effect created under the clear tarp heats the wood and 

speeds drying. Heating will be greatest in sunny locations under the long days from 

late spring through fall. Stacks should be no more than 0.6 to 0.9 m (2 to 3 ft) tall to 

achieve uniform heating. Angle the sides of the tarp outward so that condensation 

that forms on the inside of the tarp will drain onto the soil away from the wood. 

Burning wood from SOD-affected trees is an effective way to dispose of 

infested or potentially infested wood because burning destroys P. ramorum in the 

wood. There is no evidence that smoke or embers from infested wood can transport 

viable P. ramorum inoculum. 

If firewood is cut from either host or non-host trees within an infested area, P. 

ramorum can also be transported via infested California bay leaves, tanoak leaves 

and twigs, and soil that may be mixed in with the wood. Do not allow leaves and 

soil to be moved in or on firewood, equipment, or truck beds. Soil movement can be 

minimized by cutting and loading firewood when soils are dry. Leaves and twigs of 

California bay and tanoak, soil, and other potentially contaminated materials that 
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are accidently moved to a new location should be collected. This material should be 

burned or sealed in a bag and sent to a landfill for disposal. 

3.1.4—Soil

Moist soil on hiking boots and bicycle tires has been shown to spread SOD. 

Vehicles driven on dirt roads that pass through lands infested with P. ramorum may 

also spread contaminated soil, especially when conditions are muddy. Footwear and 

vehicles should be cleaned before leaving infested areas and before entering new 

areas (see box 3-1). Dry soil poses a low risk for spreading SOD because dry soil is 

less apt to stick to surfaces and the amount of viable P. ramorum inoculum on the 

surface of dry soil is very low. 

3.1.5—Water

Phytophthora ramorum can survive, and appears to reproduce, in streams and 

rivers. Watercourses that drain SOD-affected watersheds can contain spores of P. 

ramorum. More spores are typically present in watercourses during the wet season, but spores may be present in some streams year round. Untreated water from 

potentially infested streams should not be used for irrigation, dust control on roads, 

or similar purposes. Water can be treated with ultrafiltration, chemicals (chlorine, 

ozone), or UV radiation to eliminate Phytophthora spores.
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Disinfecting with diluted bleach. Standard commercial bleach (5.25 percent 

sodium hypochlorite) diluted to 10 percent strength (one part bleach, nine parts 

water) may be used as a disinfectant for hard surfaces. For concentrated (“ultra”) 

bleach (6.15 percent sodium hypochlorite, EPA Reg. No. 67619-8), the corresponding dilution ratio is about 1 part bleach to 10.5 parts water. Equipment to be 

disinfected needs to be maintained in contact with the bleach solution for at least 

a minute to kill P. ramorum propagules. Longer contact times and more thorough 

soaking will be needed if large pieces of debris are present. Use freshly made 

bleach solutions and change them once they become contaminated. Bleach solutions 

lose potency over time and are deactivated with use.

The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.

Box 3-1—Cleaning Contaminated Tools and Equipment

If possible, clean tools, equipment, shoes, vehicles, and other items that may have 

become contaminated with P. ramorum inoculum in soil or debris before leaving the 

infested location. If cleaning is done at a site that is already infested, inoculum can be 

brushed or washed from the contaminated surfaces; the contaminated material does 

not need to be collected and destroyed. Cleaning should be done on a surface that is 

unlikely to allow cleaned materials to become recontaminated, such as pavement, a 

plastic tarp, or a continuous layer of gravel. 

Simply spraying disinfectants on contaminated surfaces is not effective for 

killing P. ramorum inoculum that is present in large pieces of plant tissue or thick 

layers of soil. Thorough cleaning of debris and soil from equipment, tires, and shoes 

is necessary to remove P. ramorum propagules (spores or mycelium). Compressed 

air can be used to help blow debris and soil out of tools and equipment such as chain 

saws and chippers. Debris and soil can also be brushed or swept from surfaces. Surfaces with clinging mud or damp debris may need to be washed off. Use a brush and 

clean, uncontaminated water to thoroughly remove soil from surfaces. An appropriate 

cleaning agent can be used after removing large particles of debris and soil to provide 

a higher level of protection. Detergent solutions, diluted bleach, and ethyl or isopropyl 

alcohol (70 percent or higher) can facilitate cleaning of contaminated surfaces and 

may directly kill exposed spores. Some formulated products, such as Lysol® Disinfectant Spray (79 percent ethyl alcohol), can be used for this purpose. Allow wet surfaces 

to dry thoroughly to further reduce the chance that any P. ramorum propagules will 

survive. 

Because wash water from vehicles may contain heavy metals, hydrocarbons, and 

other materials, vehicle wash stations should be configured to prevent these residues 

from contaminating surface or ground water. Contaminated wash water should 

be contained so it can be treated or disposed of in compliance with local and state 

regulations.

If it is not possible to completely clean items such as shoes or hand tools at the 

infested area, these items may be placed in plastic bags for later cleaning at an indoor 

site such as a utility sink. It is important to make sure that all potentially contaminated material is washed down into the sanitary sewer system (not into stormwater 

drains) or is sealed in bags that will be sent to a landfill.
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Concentrated bleach and hypochlorite solutions are corrosive and need to be 

handled with care, using both eye and skin protection. Bleach solutions can also 

react with various chemicals to release toxic chlorine gas and other hazardous 

chlorinated gases. Bleach solutions can corrode steel, and are not advisable for 

equipment such as chain saws.

3.2—Reducing Disease Risk in Susceptible Stands

Once P. ramorum has become established in an area, only a few management 

practices can reduce disease risk to susceptible oaks and tanoak. These practices 

are primarily preventative, and can also be used in at-risk stands in which the 

pathogen has not yet been detected. Large-scale field testing of these techniques is 

still ongoing, so the level of protection provided by these practices under various 

field conditions is still unknown.

3.2.1—Timing of Pruning or Other Wounding

Although P. ramorum readily attacks intact bark, large pruning wounds have an 

elevated risk of infection for about 4 months (Lee and others 2010). Other wounds 

that expose inner bark or wood would likely show a similar pattern. 

These results suggest that SOD risk may be increased if major pruning or other 

operations that cause bark wounds are conducted when P. ramorum spores are 

present. To minimize this risk factor, avoid these activities during the rainy season. 

Complete pruning or activities that cause wounding by the end of summer or early 

fall if possible. 

Many native canker rot and other wood decay fungi that colonize pruning 

wounds also sporulate during the rainy season. Pruning during the dry season 

minimizes the risk that pruning wounds will be infected by wood decay fungi.

3.2.2—Removing California Bay

Oak forests— 

Removing small understory California bays to reduce SOD risk to canker hosts 

can be justified in most stands. However, a thorough analysis of stand conditions 

and management objectives is needed before contemplating extensive removal of 

overstory California bays. For example, if oaks are already infected but have not yet 

developed visible symptoms (see box 2-3 in chapter 2), oak mortality could develop 

after California bay overstory trees are removed. If California bay is the only other 

canopy tree in the area, the combined canopy loss from SOD-related mortality and 

California bay removal could be excessive. 
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Many cities and counties have ordinances that regulate the removal of trees 

above a given diameter, especially native species such as California bay. Check 

with your local planning department before considering removal of large-diameter 

California bay trees.

Before stage—In areas that are not yet infested with P. ramorum, reducing or 

eliminating California bay canopy near oaks can reduce the overall likelihood that 

the stand will become infested with P. ramorum. If the uninfested stand is far from 

known P. ramorum sources, focus initial California bay removal efforts on understory seedlings and saplings and low branches of larger trees. California bay foliage 

near the ground has the highest risk of being infected by P. ramorum spores that 

may have been introduced via infested soil, plant debris, or water. If SOD-infested 

areas are close enough that P. ramorum spores may reach the area via long-range 

spore dispersal (table 2-4), more intensive localized or area-wide California bay 

removal efforts may be justified. 

During stage—Once P. ramorum is established in an area, reducing or eliminating California bay canopy near susceptible oaks can reduce the amount of inoculum 

that will be splashed onto the oaks, reducing disease risk. California bay removal 

can be used alone or in combination with preventive chemical applications to further reduce SOD risk. 

Tanoak forests— 

Tanoaks near California bays are commonly the first in a stand to become infected 

and killed by SOD. Hence, California bay removal may be beneficial for delaying 

SOD establishment in noninfested stands. Once P. ramorum is introduced into a 

tanoak stand, removing California bay near tanoaks may slow the local epidemic. 

However, California bay removal alone will not eliminate risk in tanoak stands. 

California bay removal in tanoak stands will help most in the following situations:

• Early in the local SOD outbreak. 

• Stands where California bay is a relatively minor component.

• When used in combination with other management activities, such as 

chemical application. 

California bay removal strategies— 

California bay removal may be used across a range of spatial scales, from large 

blocks to small localized patches. Even in areas where extensive removal to protect 

oaks may not be practical, it may be helpful to remove highly susceptible California 

bays and those that tend to retain many infected leaves for extended periods. Across 

a property or landscape, one or more of these strategies may be applied in different 
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areas, depending on the size and density of California bays and oaks present as well 

as management goals and priorities.

Methods discussed here are based primarily on relationships that have been 

observed between SOD risk and California bay in nonmanipulated coast live oak 

(Quercus agrifolia Née) stands. Long-term controlled studies involving California 

bay removal are under way, but experimental data are not yet available to show to 

what degree SOD can be prevented by using California bay removal.

Area-wide California bay removal—Area-wide removal involves eliminating all 

California bay from a relatively large contiguous area containing SOD-susceptible 

oaks. This method provides the greatest reduction in SOD risk and is preferable 

where it is practical and consistent with other management objectives. Removing 

all California bays within an oak stand can be more efficient than localized clearing 

around individual trees because larger California bays often increase the SOD risk 

for several nearby oaks. Candidate sites for area-wide California bay removal may 

encompass an entire stand or only portions of a stand. If the treated area is smaller 

than about 0.25 ha (0.5 acre), refer to the discussion of localized California bay removal below.

Area-wide removal of California bay is most likely to be feasible and cost effective in the following types of stands:

• Stands in which California bay is a minor component or is limited to localized areas.

• Stands in which most or all of the California bay present consists of seedlings, saplings, or small trees.

• Stands in which California bay removal is consistent with other management goals (e.g., eliminating ladder fuels for fire risk management).

Area-wide removal will be difficult, impractical, or undesirable in stands in 

which California bay is the dominant canopy species or where much of the California bay canopy consists of large trees.

Remove all California bay size classes, from seedlings to mature trees, from 

within the treated area if possible. Over the short term, small California bay seedlings (less than 0.5 m tall) that are more than 3 m from a susceptible oak trunk can 

be left in place. Remove these seedlings before they grow taller than about 1 m. 

Periodic inspection of the cleared area will be needed to remove California bays 

that were left in place, have resprouted from cut stems, or have grown from seed. 

Removing all seed-producing California bays in the treated area will help reduce 

seedling populations and the need for frequent followup inspections and treatments. 
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Localized California bay removal—Where California bay makes up a high percentage of the stand or large California bay trees are distributed throughout the 

stand, area-wide removal may be prohibitively expensive or inconsistent with other 

management objectives. Removal becomes more difficult and expensive as the 

size of the tree increases. In addition, it may be difficult to justify removing large 

healthy California bays in an effort to protect oaks whose disease status is uncertain. In such stands, it may be possible to reduce SOD risk for individual oaks or 

groups of oaks by increasing oak-California bay clearance in localized areas (fig. 

3-1). Increased clearance is achieved through a combination of selective California 

bay removal and canopy pruning. 

In some situations, clearance can be increased by pruning branches or stems 

from a California bay tree rather than removing the entire tree. As noted above 

(section 1.3.1, “Environmental Conditions”), low, shaded California bay foliage in 

the inner canopy typically has higher levels of P. ramorum infection than the sunexposed leaves at the top. Removing low branches (via canopy raising and inner 

canopy thinning) can reduce the amount of inoculum that will reach a nearby oak 

even if the clearance from the oak to California bay foliage is less than optimal. 

Compared with area-wide removal, localized removal zones may require more 

frequent inspection and followup treatment to maintain the target oak-California 

bay clearances. Because fewer California bay trees are removed, localized removal 

may be less costly and time consuming than area-wide removal, but fewer oaks 

within the stand will be protected. 

For high-value oaks, chemical treatment may be needed in combination with 

localized California bay removal to reduce the risk of SOD to an acceptable level.

Figure 3-1—Site before (left) and after (right) localized California bay removal around two coast live oaks. California bay foliage 

that could serve as nearby sources of Phytophthora ramorum spores was eliminated by removing one small California bay tree 

(left side of left image; stump in right image) and several California bay seedlings.
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Oak-California bay clearance distance. California bay removal reduces SOD 

risk by increasing the distance between the spore source (California bay leaves) and 

the target (a susceptible oak trunk). The number of P. ramorum spores that land on 

an oak depends on:

• The California bay foliage-oak trunk clearance.

• The overall amount of California bay canopy (overstory and understory) 

that is close enough to serve as a source of inoculum (see sections 1.2.2, 

“California bay–oak distance and disease risk” and 1.3.1 “Environmental 

Conditions”). 

Increased California bay-oak clearance should be developed only by removal or 

pruning of California bay. Pruning off oak branches or stems to increase California 

bay-oak clearance is not recommended. No single prescription has been verified 

as effective for localized California bay removal. Consider the following general 

relationships and recommendations when implementing localized California bay 

removal:

• The risk of SOD decreases as the oak-California bay clearance increases. 

Removing California bay closest to an oak is associated with the largest 

reduction in SOD risk. The minimum horizontal clearance from California 

bay foliage to the oak trunk (fig. 1-6) that is likely to have an effect is about 

2.5 m; clearances of 5 m or more are preferable.

• Larger California bay-oak distances are needed as the overall amount of 

California bay canopy in the oak neighborhood increases. If possible, provide additional clearance in the direction of prevailing storm winds, especially if tall California bay canopy is present in that direction.

• In climates that are more favorable for P. ramorum spore production, larger 

oak-California bay clearance may be needed to achieve adequate risk 

reduction. 

• Removing low inner-canopy California bay branches close to oak trunks 

may help reduce risk even if foliage in the upper portion of the California 

bay canopy cannot be cleared to the minimum target distance (2.5 to 5 m). 

• California bay seedlings less than 1 m tall should be removed if located 

within about 3 m from an oak trunk. 

When using localized California bay removal, it is often necessary to strike 

a balance between the maximum risk reduction (greatest clearance possible) and 

minimizing cost and other impacts (smallest clearance that will provide a substantial risk reduction). Each doubling of the desired clearance distance translates to 

a quadrupling of the area that needs to be cleared, with a corresponding increase 
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in effort (fig. 3-2). Furthermore, California bay canopy within the target area may 

arise from a trunk located outside the clearance distance. Hence, California bays 

rooted outside of the target clearance distance may also need to be removed or 

pruned. 

Eliminating highly susceptible California bay trees—Individual California bay 

trees vary in their susceptibility to foliar P. ramorum infections (Anacker and

others 2008, Meshriy and others 2006). California bay trees also vary with respect 

to how many infected leaves they retain over the dry season (Swiecki and Bernhardt 

Figure 3-2—Bay removal area (blue circles) for four California bay foliage-oak trunk clearance distances 

(orange arrows). Green outline represents a large oak canopy (about 22 m canopy diameter) with trunk 

shown as the central dot. The smallest clearance likely to provide a significant reduction in sudden oak 

death risk is about 2.5 m. Greater clearances provide more risk reduction, but the area of bay removal 

quadruples with each doubling of the clearance distance. 
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2007, 2008a). Because P. ramorum does not appear to affect the survival of 

infected California bays, these more disease-prone trees are not eliminated from 

the population by natural selection. If California bays that consistently show high 

levels of infection or retain high numbers of infected leaves throughout the dry 

season are identified, they can be targeted for removal. Reducing the density of 

these California bays in the landscape should help slow the buildup of P. ramorum 

spores on leaves during the wet season, reducing SOD risk to oaks in the area.

In low rainfall years that are unfavorable for P. ramorum sporulation, few 

California bays develop severe foliar symptoms. California bays that have significant numbers of infected leaves by late summer in such years are logical candidates 

for removal. Some of these California bays may have high persistent infection levels 

because of microclimate effects (e.g., being located in shaded drainages) rather 

than high genetic susceptibility. Removing such trees may not help select for more 

resistant California bay genotypes, but should still help slow the seasonal buildup of 

P. ramorum in the stand. 

In years that are favorable for disease, most California bays may be heavily 

infected and highly susceptible genotypes cannot be identified. It may be possible 

to identify California bays that are somewhat resistant to P. ramorum infection in 

favorable disease years. Resistant trees should show especially low infection levels 

when surrounded by California bays with a high level of foliar disease.

Selective elimination of highly susceptible California bays is more likely to be 

effective in areas where conditions are less favorable for P. ramorum sporulation, 

such as drier, more inland areas near the edge of the current P. ramorum range. 

This tactic is also more likely to be useful in areas with relatively low California 

bay density. 

Bay removal procedures—

Timing of removal operations—In areas where P. ramorum is present, avoid cutting down or pruning California bay during wet conditions, especially in the spring 

rainy season, because:

• Phytophthora ramorum spores may be splashed directly onto susceptible 

oak stems during felling operations if foliage is wet.

• Phytophthora ramorum is more likely to be spread around and beyond the 

area in wet soil and California bay litter.

• Fresh wounds on oak stems made during clearing operations may provide 

favorable sites for infection. 

• Cut California bay foliage on the ground will dry slowly and may serve as a 

potential source of inoculum for an extended period.
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If you need to remove California bay in winter or spring, do it after an extended 

period of dry weather and observe stringent sanitation procedures. 

The optimal time for California bay removal operations is between late spring, 

after rains have stopped and the surface soil has dried out, and late fall, before the 

soil has become wet. During these periods, it is unlikely that viable spores would be 

dispersed and cut foliage will dry out relatively quickly. 

In areas where P. ramorum is not present, it is still preferable to avoid California bay removal during wet conditions or when soil is muddy. This will reduce the 

risk of introducing P. ramorum or other Phytophthora species to the site. Follow 

strict sanitation protocols (see section 3.1, “Excluding Phytophthora ramorum From 

Noninfested Areas”) to ensure that all vehicles and equipment are free of contamination before entering the site. 

Mechanical removal—Small California bays in the understory are relatively easy 

to remove with hand tools. Seedlings up to about 2.5 cm (about 1 inch) diameter 

at breast height can be pulled out using a brush/seedling removal tool (e.g., Weed 

WrenchTM, fig. 3-3). Small seedlings (<1 cm basal diameter) can often be pulled by 

hand or can be chopped out below the soil surface with a mattock. Removal is easier if the soil is somewhat moist. Small seedlings may also be cut off with loppers or 

a brush cutter. Cut seedlings may resprout, so repeated cutting may be necessary. 

California bay wood is relatively light, so small stems are relatively easy to 

cut and handle using small chain saws or hand saws. In contrast, removing large 

California bay trees requires much greater effort and is more hazardous. Felling 

large California bay trees requires trained and experienced personnel with proper 

equipment and safety gear. Crews will need chain saws, powered or manual pole 

pruners, and ropes. In dense stands, cut California bays may hang up in the canopies of adjacent trees. Care must be taken to avoid breaking oak branches when 

felling California bays. Oak branches or stems with substantial decay can fail if 

they are hit by felled branches or trees. In addition, decayed oak branches or stems 

may fail during or after removal of a supporting California bay. 

Dealing with cut California bay—Leave cut California bay onsite if possible to 

minimize the risk of spreading P. ramorum. The main options for disposing of cut 

California bay are noted below.

Lop and scatter. This is the most practical method in locations well away 

from roads or structures, and may be used in other areas if the amount of cut material is not excessive. The main stem and branches are cut into sections that lie low 

to the ground and are spread out to avoid stacking. California bay debris treated 

in this manner dries out quickly and decays substantially within 1 to 2 years. As a 
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Figure 3-3—Brush pullers (Weed WrenchTM shown) can be used to extract bay seedlings that are 

too large to pull by hand. 

precaution, keep cut California bay foliage at least 2 m (6.5 ft) away from trunks of 

SOD-susceptible oaks. This is especially important if damp weather will prevent 

drying and allow foliage to serve as a potential inoculum source for many days or 

weeks. 

Chipping. In areas where it is undesirable to leave cut material on the ground, 

branches can be chipped. Preferably, both chipping and disposal should occur on 

site to avoid possible spread of inoculum to other areas. Set the chipper so that 

chipped material is not deposited on trunks of susceptible oaks. Clean all potentially infested debris out of the chipper and truck before they are moved from an 

infested area (see box 3-1). 

Pile and burn. In some sites, it may be possible to pile cut California bay 

branches for later burning. Local fire and air quality regulations should be checked 

before contemplating this option. Set burn piles well away from retained trees to 

avoid scorching tree canopies or damaging shallow roots that are present beneath 

the duff layer. Burn piles can cause excessive heating of the soil, which can have 

long-term negative effects on soil properties, soil microbes, and the soil seed bank. 

Keep burn piles relatively small and include only small-diameter branches to help 

minimize soil heating and associated negative impacts. 
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Sprout regrowth/retreatment intervals—California bay typically resprouts from 

cut stumps. The amount of regrowth that occurs within a season varies with climate 

and stump size. In Humboldt County, California bay resprouts can grow to a height 

of 2 to 3 m (6 to 10 ft) in a year, whereas regrowth in drier sites in Sonoma, Solano, 

and San Mateo Counties has been less than 1 m per year. Where sprout growth 

rates are relatively low, browsing by deer or other animals can be sufficient to completely suppress sprout regrowth (fig. 3-4). 

Monitor sprout regrowth after cutting to determine when sprouts should be cut. 

As long as sprouts remain short (1 m or less), spores produced on infected leaves 

are unlikely to be dispersed more than about 2 m. In addition, sprouts that develop 

during the dry season will normally remain free of P. ramorum lesions at least until 

the next substantial infection period. Thus, California bay sprouts that are well 

away from oak stems may not serve as significant sources of inoculum for at least 

several years. 

If stump sprout removal is done while shoots are still small, sprouts can be 

removed using various hand tools and the amount of material to dispose of will be 

minimal. Repeatedly removing sprouts will eventually deplete stored carbohydrates 

in the root system, leading to weaker sprout regrowth and eventual death of the 

stump. This depletion will occur faster if sprouts are not allowed to regrow substantially before recutting.

As discussed below, sprout regrowth can be suppressed by treating freshly cut 

stems with appropriate herbicides. In sites where the survival of oaks is questionable and where California bay would be acceptable as a replacement canopy species, it may be preferable to suppress resprouting by cutting rather than herbicides. 

This allows the possibility of recruiting California bay resprouts as replacement 

canopy if needed. 

Herbicides—Trees that are too difficult to cut can be killed using herbicides, although this method may not be appropriate in many situations. Herbicide-treated 

California bays may not die for an extended period of time (months to over a year) 

and may continue to produce P. ramorum spores during this time. Also, because 

trees are left to die in place, management concerns related to visual appearance, fire 

hazard, and tree failure can limit where herbicide treatments can be used. Direct 

removal is preferable if an herbicide-treated tree will eventually need to be cut because of hazard issues.

The most common method for applying herbicides to kill large standing trees is 

the “hack and squirt” or frill method. A hatchet is used to make downward angled 

cuts through the bark around the circumference of the tree. Herbicide (usually 

full-strength concentrate to quarter-strength dilution depending on the product; 
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Figure 3-4—Bay stump sprouts 1 year after tree felling. (A) Strong sprout growth from a nonbrowsed, untreated bay stump. 

(B) Untreated stump with sprout growth kept in check by deer browsing. (C) Glyphosate-treated stump showing no sprouts. 
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follow label directions) is immediately applied into the open wounds. Alternatively, a specialized hatchet- or lance-like herbicide injection device can be used to 

simultaneously cut the bark and apply herbicide. Some herbicides (e.g., glyphosate, 

triclopyr) will not be effective on large California bay trees unless they are applied 

to overlapping cuts around the entire trunk circumference. Herbicide combinations 

(e.g., imazapyr plus glyphosate) may be more effective than individual herbicides.

Timing of herbicide application can make a large difference in efficacy. In 

a San Mateo County study, California bays treated with 20.5 percent glyphosate 

(hack and squirt method) in March died within a year, but trees in the same stand 

treated in July showed almost no effect of the application. Additional observations 

suggest that glyphosate applications made during the wet season from late fall 

through early spring are most effective, at least in the San Francisco Bay area.

Stump sprouts from cut California bays may also be suppressed through the 

use of systemic herbicides such as glyphosate. Apply the herbicide to the cambial 

area (bark/wood junction) of freshly cut stumps. Appropriate herbicides may also 

be applied to sprouts after they develop on cut stumps, but such applications will 

typically require greater amounts of spray solution and pose a greater hazard to 

nontarget plants from overspray and drift. In San Mateo County research plots, 

applications of 20.5 percent glyphosate to freshly cut stumps significantly suppressed resprouting (fig. 3-4). 

In one site, glyphosate applied to freshly cut stumps in November translocated 

through root grafts and killed California bays many meters away. At another site 

treated in September, well before the start of the rainy season, no evidence of 

glyphosate translocation from treated stumps to nearby California bay was seen. 

3.2.3—Removing Other Sporulating Hosts

Climbing poison oak— 

Observations indicate that poison oak vines that climb into oak canopies may sometimes serve as an important source of P. ramorum spores (see “Other sources of 

inoculum” in section 1.2.2). Kill climbing poison oak vines when SOD management 

treatments are being used to protect susceptible trees. Climbing poison oak vines 

can be killed by cutting their stems near the soil line. Cut stems may immediately 

be treated with an herbicide to prevent regrowth. Once stems are severed, vines in 

the canopy die quickly and can be left in place. Poison oak can cause a highly irritating rash on many people, so wear protective clothing and take precautions to 

avoid contact with the plant. 
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Other species— 

No California native plants species other than California bay, tanoak, and poison 

oak are clearly associated with SOD risk in susceptible oaks. However, it is possible 

that some other native foliar host species could be significant sources of P. ramorum 

spores in some situations. If further research shows that other native hosts can serve 

as sources of P. ramorum inoculum, localized removal, as discussed for California 

bay above, should help reduce SOD risk. Check http://www.suddenoakdeath.org for 

current information. 

In the United Kingdom, other species have been shown to be associated with 

infection of canker hosts in the field. Infected rhododendrons, particularly Rhododendron ponticum L. and Japanese larch (Larix kaempferi (Lam.) Carrière) produce 

enough P. ramorum spores to initiate trunk infections. Based on this information, 

SOD risk may be minimized by keeping horticultural plants that can support 

moderate to high levels of P. ramorum sporulation (table 1-3) at least 5 to 10 m from 

susceptible oaks. As a general rule, keep the area within 2.5 to 5 m of an oak trunk 

free of both plants and irrigation to minimize the risk of other oak diseases, such as 

Armillaria root disease.

3.2.4—Use of Chemical Protectants to Prevent Infection

Although Phytophthora species are no longer considered to be fungi, chemicals that 

are used against diseases caused by Phytophthora are still classified as fungicides. 

Most fungicides do not kill fungi directly; they inhibit fungal growth (see box 3.2). 

Hence, fungicides generally work best when disease pressure has also been reduced 

to the maximum degree possible by using other management methods. 

Some fungicides have shown promise against SOD in greenhouse tests and 

assays on mature trees. However, field studies to determine if fungicides can reduce 

SOD incidence or severity in mature trees are still ongoing. Results available 

to date have been mixed and no studies have shown that oaks or tanoaks can be 

completely protected from SOD by fungicides alone.

Systemic fungicides (box 3-2) have the highest potential to be useful for SOD 

management because infections may occur over a long time period through the 

winter and spring. To be effective, systemic fungicides need to be absorbed by living plant tissues and accumulate in areas that are subject to infection. Areas already 

affected by cankers will not effectively absorb or translocate fungicides, so applications will be most effective when applied before infection occurs. Even if applied 

well before infection occurs, systemic fungicides may not provide useful prevention 

of SOD if poor uptake or translocation patterns result in low fungicide concentrations in the bark of the lower trunk.
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Chemical application is relatively expensive and needs to be repeated on an 

ongoing basis to provide continued protection against SOD. Therefore, fungicides 

are best suited for use on limited numbers of high-value trees in areas where SOD 

risk is high. Consider the overall disease potential at the site, including the abundance of California bay, and the significance of at-risk trees when deciding whether 

chemical applications may be appropriate.

Fungicide use considerations differ for tanoak, which can produce P. ramorum 

spores, and oaks, which are normally infected by spores produced on California 

bay.

Fungicide treatment of tanoak— 

Systemic fungicide treatments may have the potential to inhibit both P. ramorum 

trunk cankers and twig infections. If fungicide-treated tanoaks develop fewer or 

smaller twig infections, spore production will be reduced, lessening the likelihood 

of trunk infections. To benefit from the synergistic effects of reduced spore production and increased resistance to trunk infections, contiguous blocks of tanoaks need 

to be treated. If untreated tanoaks are left within a stand, spores produced on these 

trees could splash onto treated trees, which may impede disease control.

In well-stocked tanoak stands, it may be more efficient and cost-effective to 

remove small understory tanoak seedlings and saplings instead of treating them 

with fungicides. Because spores from small (<1 m) understory tanoak regeneration 

or stump sprouts are unlikely to splash more than about 2 m, these small shoots 

only need to be removed if they are within about 2 to 3 m of treated tanoaks. 

Fungicide treatment of oaks— 

Because California bay is the primary source of P. ramorum inoculum in oak 

stands, fungicide treatment of all oaks within a block does not provide any advantage over selective treatment of individual oaks. For susceptible oaks, use of fungicides should always be combined with California bay removal, if possible, to reduce 

disease pressure. Where California bays are too large or significant to remove, 

fungicide use may be warranted to help protect high-value oaks that are closest to 

California bay trees. However, fungicides may not be sufficient to prevent SOD in 

trees that are exposed to large numbers of P. ramorum spores. 

Use of phosphites (phosphonates) to manage SOD— 

Phosphites (also known as phosphonates) are a group of fungicides which have 

potential for use in managing SOD. Phosphites are selective, systemic fungicides 

that have been used to manage Phytophthora diseases for a number of years. They 

have a high level of environmental safety and very low nontarget toxicity. The most 

widely used material in this fungicide group is potassium phosphite (also known as 
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Box 3-2—Selecting and Using Fungicides

Controlled studies are needed to determine how well any fungicide will work for a given host/pathogen combination 

and what dosage is required. To date, only phosphite fungicides have been studied extensively for their ability to 

protect trees from sudden oak death (SOD). It is not yet clear whether phosphites can provide useful levels of protection against Phytophthora ramorum in forests. Other fungicides that are active against Phytophthora species may 

be evaluated for potential use in SOD management in the future. The following factors should be considered when 

selecting and using fungicides as a SOD management tool.

Type of activity: Most fungicides work best when applied as protectants (before infection has occurred), rather than 

as eradicants or curatives (after the plant has been infected). Using fungicides as eradicants increases the chance of 

selecting for fungicide resistance in the pathogen population. 

Contact or surface fungicides are not absorbed by plants and act mainly to inhibit germination and growth of spores 

on plant surfaces. Contact fungicides need to be applied as a complete coating over the susceptible plant and must be 

reapplied as the fungicide residues break down, are washed away, or spread out as the plant parts expand. 

Systemic fungicides are absorbed by plant tissues and are translocated within the plant. Fungicides transported in 

the water-conducting xylem tissues can move upward only, whereas those transported in the phloem (stem tissue that 

transports carbohydrates) can move downward as well. Because systemic fungicides move within the plant, it is less 

critical to obtain complete plant coverage. Systemic fungicides typically have longer residual activity than contact 

fungicides and are reapplied less frequently. Materials that need to be reapplied more than once each year may not be 

practical for protecting large numbers of trees from SOD.

Spectrum of activity: Most systemic fungicides are selective. Many fungicides that are effective against true fungi 

have little or no activity against Phytophthora species, and vice versa. Many contact fungicides (e.g., copper salts) are 

relatively nonselective and are active against a wide variety of fungi as well as Phytophthora species.

Application methods: Fungicides are applied by a variety of methods (e.g., spray, injection, soil application), which 

are specific to the fungicide/disease/plant combination. The application method can affect cost and practicality of 

treatment, absorption and distribution of the chemical in and on the plant and the risk of nontarget exposure. Application methods that may be practical for treating small numbers of trees may be too expensive to use for entire forest 

stands.

Phytotoxicity: Some fungicides can cause damage to plant tissues (phytotoxicity) under certain conditions. Phytotoxicity can be influenced by factors such as plant species and variety, plant maturity, temperature, application method, 

or interactions with other applied materials. Fungicides need to be effective at a dose that is well below levels that 

cause serious phytotoxicity. 

Effects on nontarget organisms: Most fungicides and other pesticides work by interfering with vital metabolic 

processes in the target pathogen. Fungicides also have the potential to interfere with metabolic processes of other 

nontarget microorganisms, invertebrates, and vertebrates. Fungicides used in settings such as forests need to have 

little or no negative effects on the wide variety of nontarget species found in these environments.

Toxicology: Humans are very important nontarget organisms to consider when selecting a fungicide. Potential effects 

of fungicides on human health are related to the toxicity of the material (its potential to cause acute or chronic health 

effects) as well as the amount and routes of exposure to the material. Persons who mix and apply fungicides to trees 

have the greatest likelihood of exposure to these chemicals. Dietary exposure to fungicide residues used for SOD 

prevention may occur if acorns are used for food or if animals that consume acorns (e.g., livestock, deer, wild turkey) 

are eaten.

Applicator safety: Proper safety procedures and personal protective equipment (PPE) should be followed to minimize 

exposure to applied chemicals. Appropriate PPE for applicators, such as gloves, face shields, respirators, and protective clothing, depends upon both the type of chemical used and the application method. The fungicide label indicates 

types of required safety equipment and other necessary precautions. 

continued on next page
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potassium phosphonate, or mono- and di-potassium salts of phosphorous acid). This 

material is sold under a number of trade names. 

At least two general modes of action have been identified for phosphites (Guest 

and Grant 1991). Potassium phosphite moves systemically, in both the xylem and 

phloem. At high concentrations in the plant, phosphite is directly toxic to Phytophthora species. At lower concentrations, phosphites act indirectly by increasing 

a plant’s natural resistance response to Phytophthora infection. At these lower 

concentrations, phosphite appears to interfere with Phytophthora metabolism, 

causing the release of compounds that trigger host defense responses. Phosphites 

are primarily effective against Phytophthora species and have little or no activity 

against most diseases caused by true fungi. 

Phosphite is most likely to be effective when applied as a preventative, before 

trees are infected. In field tests, phosphite has not been effective against cankers in 

previously-infected tanoaks, even if external cankers symptoms were not visible at 

time of application. 

Most studies indicate that phosphite does not prevent infection by P. ramorum, 

but may reduce the rate at which P. ramorum cankers expand. If disease pressure 

is high, phosphite may not suppress disease progress enough to provide practical 

levels of disease control. In a recent controlled study in San Mateo County, involving over 470 mature tanoaks, 3 years of preventative annual phosphite bark spray 

applications did not reduce disease incidence or mortality when SOD subsequently 

spread into the stand. Phosphite applications were started well before P. ramorum 

was detected in or near this stand.

Box 3-2—Selecting and Using Fungicides (continued)

Persons applying pesticides for hire in California are required to have a Qualified 

Applicator’s License or Certificate issued by the California Department of Pesticide 

Regulation (CDPR). Qualified Applicators must pass tests related to pesticide application and safety and take continuing education classes to maintain certification.

Regulatory issues: Chemicals or natural products that are sold expressly for the purpose of controlling a plant disease, such as SOD, need to be registered as pesticides 

by the U.S. Environmental Protection Agency (EPA). In addition, pesticides used 

in California must be registered by CDPR. Registered pesticides that are classified 

as general use pesticides may be applied by general users, including homeowners. 

More hazardous pesticides are classified as restricted use pesticides and may only be 

applied by CDPR-certified applicators. Fungicides or other products that are used to 

manage SOD must be registered for that use and applied in a manner consistent with 

the label, observing all label precautions.
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Phosphite solutions can be phytotoxic to some species if applied at sufficiently 

high concentrations. Leaves of many species, including tanoak, may develop extensive browning (necrosis) if they are sprayed with the highly-concentrated phosphite 

solution used in trunk-spray applications. Mosses, which are common on oak and 

tanoak trunks in some locations, are also killed by the spray. Injected phosphite 

solutions can also cause necrosis in the bark and sapwood. Phosphite should be 

tested for potential to cause phytotoxicity whenever it is used on a new plant or if a 

modified-application method is used.

Environmental and user safety—Phosphites are considered by the EPA to be 

biopesticides because these salts are closely related to common, widely occurring 

substances. However, phosphonate salts are not naturally occurring mineral substances. Information on toxicity and environmental fate of this material was summarized in an EPA fact sheet (USEPA 1998):

ECOLOGICAL RISK ASSESSMENT

A potential for exposure exists to nontarget insects, fish, and other wildlife 

with foliar spray applications. However, test results indicate that the compound is practically nontoxic to birds and freshwater fish, and, at most, 

slightly toxic to aquatic invertebrates. Low toxicity, the proposed rate of 

application, and mitigating label language present minimal to nonexistent 

risk to wildlife.

Phosphorous acid and its ammonium, sodium, and potassium salts are also 

exempt from food tolerance, for both crop and postharvest uses (USEPA 2006). In 

exempting phosphites from food tolerance, the EPA cited both the low toxicity of 

phosphites and the fact that phosphites had already been used widely as fertilizers. 

In a preliminary study, acorns from tanoak trees treated with phosphite showed no 

substantial differences in nutritional quality when compared with nontreated trees 

(Meyers and others 2007). 

Although phosphites are relatively safe materials, phosphite solutions, particularly in concentrated form, may be harmful if inhaled, ingested, or absorbed 

through the skin and must be handled safely to avoid exposing applicators or others.. Formulations of potassium phosphite typically have the “Caution” signal word 

on the label. Applicators and others handling the material for mixing and loading 

must wear protective eyewear, long pants and a long-sleeved shirt, waterproof 

gloves, shoes, and socks. Depending on the application method, additional personal 

protective equipment may be necessary.
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Potassium phosphite is corrosive to many metals, especially brass, which is 

commonly used in sprayers and injection equipment. Sprayer components that are 

exposed to potassium phosphite solution should be made of plastic or stainless steel. 

To minimize the potential for corrosion, thoroughly rinse application equipment 

with water immediately after use. 

Comparison of application methods—Phosphite can be applied to trees via stem 

injection, bark or foliar spray, and soil drench. In tests conducted to date, soil 

drench applications have not been effective against P. ramorum canker (Garbelotto 

and others 2007). Foliar applications, which are widely used to control root-rotting 

Phytophthora species, have shown relatively short-lived activity against P. ramorum canker in greenhouse tests on oaks. Foliar applications are also difficult to 

execute on large trees and may require high volumes of spray. Low volume aerial 

application of phosphite by helicopter to mature tanoaks has been tested in Oregon 

(Kanaskie and others 2010). Although these tests showed that aerial phosphite application suppressed P. ramorum in treated trees to some degree, infections were 

not completely prevented. This study did not provide conclusive results on aerial 

phosphite application for controlling SOD, but results were not overly promising. 

Phosphite applied via stem injection or bark spray application reduced P. 

ramorum canker size on potted coast live oaks that were artificially inoculated 

(Garbelotto and others 2007, Garbelotto and Schmidt 2009). The effect appeared 

to last at least 18 months after treatment. In addition, branches cut from coast live 

oaks after treatment with a bark spray application developed smaller lesions than 

branches from nontreated trees when inoculated in the laboratory with P. ramorum

(Garbelotto and Schmidt 2009). Based on these results, injection and bark spray 

application are currently the preferred methods for applying phosphite to oaks and 

tanoaks. Each method has advantages and disadvantages (table 3-1). In general, 

injection is more suited for treating a small numbers of trees. 

Injection—Stem injection involves introducing a chemical into the water-conducting tissues of a tree (xylem) via holes drilled into the outer sapwood. To speed 

uptake, the solution is applied under slight to moderate pressure. Some injectors 

apply the chemical to the cambial region, which may allow some of the chemical to 

be directly absorbed by the phloem. Current recommendations for injecting with 

potassium phosphite products are discussed below. These practices are subject to 

change based on the outcome of ongoing studies.

Timing: Trees should be injected at least 4 to 6 weeks before the onset of conditions favorable for disease because of the time lag between injection and subsequent 
88

general technical report psw-gtr-242 Table 3-1—Comparison of injection and bark spray methods for phosphite application Stem injection Bark spray application Equipment Need injection equipment. Most injector types also require a drill motor and bit. Cost varies, depending on type of injection equipment used. Having more injectors can reduce application time, but increases equipment cost. Need sprayer, wand, and nozzles. Cost varies depending on equipment used (backpack vs. tank sprayer). Material required Lower amount of phosphite required—minimum of about 1.75 ml formulated product (45.8 percent potassium phosphite) per inch stem diameter. Higher amount of phosphite required—minimum of 31 ml of formu- lated product per centimeter stem diameter. Surfactant (Pentrabark®) also required. Efficiency of uptake Except for leakage from injectors, all of applied product is introduced into tree. Significant losses of material via overspray and runoff during applica- tion process. A relatively small percentage of the material deposited on the trunk is absorbed by the tree. Translocation Most or all of the chemical introduced into the tree’s xylem. 

Phosphite moves up to the tree canopy before it is translocated back to the bark of the lower trunk. Some injectors apply chemical to the cambial zone, where direct uptake into the bark is possible.

Material is absorbed directly through bark into the phloem and can 

move upward and downward.

Application speed Varies with injection equipment and tree size, but slower than 

spraying, especially if many trees are being treated. More suitable for small numbers of trees.

Relatively fast. Because of fixed setup and cleanup time, generally 

more efficient to treat multiple trees at a time. Suitable for treating large numbers of trees.

Frequency of application Current recommendation is a 1.5- to 2-year retreatment 

interval.

Current recommendation is two treatments in first year and annually 

thereafter.

Accidental inoculation via 

contaminated equipment

Pathogen could be accidentally introduced into trees via drilling and injectors (depending on type used) when moving 

from asymptomatic infected trees to noninfected trees unless equipment is cleaned and disinfested. 

Negligible risk of cross-contaminating trees via spraying process.

Nontarget effects Wounds created on trunk may provide entry point for insects 

and pathogens, including Phytophthora ramorum. Multiple treatments will result in a large number of injection wounds. Injected solutions can kill phloem and xylem tissues if too concentrated. Injection sites may ooze for up to several years.

Spray solution kills mosses on bark surface. Overspray can cause 

browning (necrosis) of wetted foliage of some species.
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translocation of phosphite throughout the plant. Repeat injections at 1.5- to 2-year 

intervals, depending on the timing of the previous treatment. For instance, if the 

end of the 1.5-year interval falls in the summer, injection may be delayed until fall. 

For low pressure injectors, trees must be actively transpiring in order to take up 

injected phosphite solution. Deciduous trees such as California black oak should be 

injected when expanded leaves are present. The fastest uptake of injected solution 

occurs under sunny conditions. Uptake may be slow to negligible during rainy, 

foggy, or heavily overcast conditions. Uptake may also be slow late in the day, 

especially in dense, shaded stands. In very dry sites, uptake may be better with light 

cloud cover and cooler temperatures than under very hot, dry conditions. 

Injection holes in the bark have an elevated susceptibility to P. ramorum infection, so trees should not be injected in late winter or spring. Injection is not recommended for trees under severe water stress because phytotoxicity may develop in 

the canopy. Foliar phytotoxicity may also develop if trees are injected when temperatures are above 32 °C (90 °F).

Dose: Approximately 0.71 ml of nondiluted potassium phosphite formulation 

(45.8 percent potassium phosphite) per cm of stem diameter is a common label rate 

for tree injection. This dosage may not be adequate for large diameter trees. Different methods of calculating the applied dose provided on some labels can generate 

dosages up to about 10 times this level.

Concentrated potassium phosphite solutions (45.8 percent) must be diluted with 

water before injection, although dilution ratios have differed with the injector type. 

Dilutions of concentrated phosphite solution and water ranging from 1:2 to 1:5 (15.3 

to 7.6 percent potassium phosphite) have caused significant phytotoxicity in injected 

tanoaks and coast live oaks. Injection holes exhibited prolonged bleeding (up to 

several years) and necrosis in the phloem and xylem (fig. 3-5). Studies are underway to determine the dilution rate needed to avoid phytotoxicity. The dose needed 

to provide efficacy may also need to be adjusted for more dilute injection solutions.

Injection points: To ensure that injected material is evenly distributed throughout the tree canopy, injection points need to be spaced at relatively close intervals. 

Injectors are typically spaced 15 to 20 cm (6 to 8 inches) apart, but some injector 

manufacturers recommend wider spacing, up to 23 to 25 cm (9 to 10 inches). If 

injectors are spaced too widely, phosphite will not be evenly distributed throughout 

the canopy. The optimum injector spacing needed to ensure even distribution of 

phosphite in the lower trunk is currently unknown.

Injection holes should be at least 0.3 m above ground level to minimize contamination of the holes with soil, but the actual height of the injection holes is not 

critical. If bark is too thick to allow proper placement of the injectors (see below), 
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Figure 3-5—Necrosis in phloem and xylem (dark brown discoloration) associated with injections of 

15.3 percent potassium phosphite (10 ml per injection). Injections were made 2 and 4 years before the 

tree was cut. Outer bark has been chipped away around injection points.

insert injectors higher on the trunk where the bark is thinner. Drill holes level 

horizontally or angled with the opening pointing slightly downward so that water 

running down the stem will not collect in the holes.

Follow the injector manufacturer’s instructions to select the appropriate diameter of the drill bit. Use a sharp drill bit to obtain a cleanly cut hole and use the 

bit to clear shavings from the hole before injection. For most injector types, holes 

should extend through the bark to the outer sapwood. If possible, clean the drill bit 

with isopropyl or ethyl alcohol (70 percent or higher) between each hole to prevent 

cross-contamination. At a minimum, clean the drill bit when moving from tree to 

tree. Clean injectors with 70 percent alcohol prior to use in another tree. 

Ensure that the injector tip fits tightly into the hole to prevent leakage. For 

injectors that are designed to apply material into the sapwood, holes should extend 

about 1 cm into the sapwood. Some injectors allow phosphite solution to be injected 

into the inner bark and cambium area, but this has been associated with phloem 

necrosis at the dilutions noted above. Some injection systems employ a plastic plug 

with a septum that is inserted into the drill hole and remains in the tree. These 

plugs must be seated at the depth specified by the injector manufacturer. 
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Drill holes into healthy areas of the trunk. Avoid areas with dead bark, decayed 

wood, cracks, sunken areas, branch stubs, or callus tissue. Stagger the injection 

height, include a vertical offset of about 10 to 20 cm (4 to 8 inches) for adjacent 

holes to avoid placing all injection points in a single line around the tree’s circumference. Drill new holes for injectors when retreating trees; do not reuse holes made 

for a previous injection. Offset new holes horizontally as far as possible from the 

previous set(s) of holes and use different vertical heights.

Applicator safety: Some injectors contain solution under pressure after filling. 

Take precautions while carrying or handling injectors and while making injections 

to ensure the solution is not unintentionally squirted at the applicator. A full face 

shield is preferable to goggles for persons handling injectors.

Bark spray application (basal bark spray)—Standard spray application equipment 

can be used to apply phosphite to the bark. A high rate of an organosilicate surfactant (trade name Pentrabark®

) is mixed with the phosphite solution to enhance absorption through the bark. The efficacy of the bark spray applications are still being 

tested in the field. In several field trials, preventative bark spray applications made 

as specified below have failed to lower SOD incidence and mortality rates in treated 

tanoak trees compared to nontreated trees.

Timing: Spray trees at least 4 to 6 weeks before the onset of favorable disease 

conditions to allow for uptake and translocation. Two applications are recommended in the first year (fall-spring or spring-fall), followed by annual applications 

thereafter. Based on results of field studies noted above, this regime may not be 

adequate to protect large tanoaks.

If the bark is saturated by rainfall at the time of application, spray will run off 

sooner, which may lead to reduced absorption. Otherwise, phosphite uptake is not 

adversely affected by rainfall. Light to moderate rainfall after application can rewet 

residues on the bark surface and may favor additional bark uptake. Phosphite that 

washes off the trunk into the soil is inactivated by soil micro-organisms within a 

few weeks.

Dose: For the bark spray application, potassium phosphite (45.8 percent potassium phosphite) is diluted 1:1 with water. Pentrabark® surfactant is added to attain 

a final surfactant concentration of 2.3 percent by volume in the spray mixture. 

This surfactant concentration can cause excessive foaming and may be higher than 

is necessary to obtain uptake. The resulting diluted spray solution contains 22.4 

percent potassium phosphite or 302 g potassium phosphite/L solution (2.52 lb/gal). 

Various stem characteristics and the spray equipment used can affect the point 

where the spray begins to run off the stem. These include:
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• Bark roughness: rougher bark will retain more spray than smooth bark.

• Stem angle: spray will begin to drip off strongly sloping stems or branches 

sooner than it will from a nearly vertical stem.

• Spray application rate: a high spray flow rate may flood the bark surface so 

quickly that little adsorption occurs and runoff begins almost immediately. 

Because of these variables, it is not possible to precisely calculate the amount 

of spray needed to treat any tree from the label instructions alone. Figure 3-6 shows 

the approximate amount of diluted spray needed to treat trees of varying diameters.

When phosphite is applied following label instructions, the amount of spray 

applied per tree will increase as the trunk diameter (and bark surface area) 

increases. In addition, larger trunks receive more spray solution per diameter unit 

because of their rougher bark. For stems less than about 30 cm (12 in) diameter, 

about 31 ml of spray solution is applied per cm of trunk diameter when the lower 

stem is sprayed to runoff. For a 75 cm (30 in) diameter stem, the amount increases 

to 62 ml per cm of trunk diameter. It is necessary to increase the amount of spray 

per unit stem diameter (ml/cm) as trunk diameter increases to maintain a more 

constant spray amount per unit trunk volume (ml/cm3

).

0 10 20 30 40 50 60 70

Diameter at breast height (inches)

0

5

10

15

20

Total spray volume (liters)

0

1

2

3

4

5

Total spray volume (gallons)

0 50 100 150

Diameter at breast height (centimeters)

Figure 3-6—Approximate amount of diluted potassium phosphite spray solution (1:1 dilution of 

45.8 percent concentrate with water plus Pentrabark® surfactant) used to treat oak and tanoak 

trunks. The applied amount varies with trunk diameter at breast height (DBH) = diameter 

measured 137 cm (4.5 ft) above the soil grade. For trees less than 30 cm (12 in) DBH, a linear 

function is used to calculate spray volume from trunk diameter. For stems 30 cm DBH or larger, 

the function includes a quadratic term, which causes the line to curve upward. 
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Application procedures: The applied phosphite/surfactant solution must 

penetrate dead outer bark tissues and be absorbed by living cells in the bark to 

be effective. To facilitate uptake, direct the spray to areas where the outer bark is 

thinner, such as bark fissures. Spray is less likely to be absorbed if applied to thick 

areas of dead outer bark.

Dense layers of moss on the bark surface can also decrease phosphite absorption. Target spray away from dense moss patches. Where this is not possible, use a 

larger spray droplet size and hold the spray nozzle close to the bark surface to spray 

beneath leafy mosses that extend outward from the bark. For appressed mosses, 

apply enough spray to wet the bark under the moss. Moss on the bark surface is 

killed by the phosphite/surfactant spray solution, so moss presents less of a problem 

after the initial phosphite application.

Brushes with stiff bristles can be used to remove moss from the portion of the 

stem that will be sprayed. This is very labor-intensive and may not be practical 

for treating large numbers of trees. Vigorous brushing can cause small wounds by 

chipping off bark fragments, so brushing or other mechanical moss removal should 

be completed several months before the rainy season. 

Phosphite uptake may also be reduced if the bark surface is dusty. For dustcovered stems, apply after early rains have rinsed off the dust or use water to hose 

off excess dust before application. Allow the bark surface to dry after washing so 

that the applied spray solution will not run off immediately.

Select nozzle type and spray pressure to maximize spray deposition and minimize spray drift. Drift can be minimized by using nozzles that produce uniformly 

large spray droplet sizes, such as air-induction nozzles. Low spray pressure (near 

the nozzle’s minimum pressure rating) will help minimize the production of fine 

spray droplets that drift and will also reduce the tendency of larger spray drops to 

bounce off the bark surface. 

Spray is commonly applied to the lower 2 m (6 ft) of the trunk to just before the 

point of runoff. The spray volumes shown in figure 3-6 can be used to determine 

the amount of spray to be applied if the treated zone is shifted upward. Applying 

the spray higher on the stem provides several potential advantages:

• Material is applied where the outer dead bark is thinner and more 

permeable.

• Higher portions of the trunk may have less moss cover. 

• When spray residues high on the trunk are rewetted by rain and washed 

farther down the stem, additional uptake of phosphite is possible on lower 

portions of the stem. Phosphite applied low on the stem is more quickly 

washed into the soil where it becomes inactivated. 
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Spray can be applied high on the trunk (up to about 6 m [20 ft]) by attaching 

spray nozzles to a telescoping pole (fig. 3-7). Compared with basal spraying (to 

about 2 m in height), higher spraying requires some specialized equipment and 

more effort. High bole application also has greater potential for overspray, so the 

applicator should wear appropriate PPE to avoid exposure. The relative efficacy of 

high bole application versus basal application is currently under study.

Applicator safety: Most spray equipment generates fine droplets that can be 

inhaled, so appropriate respiratory protection is required. All sides of the stem need 

to be sprayed, so it may not be possible for the applicator to rely on wind to avoid 

spray mist. When applying at heights at or above head level, wear a waterproof hat 

Figure 3-7—A telescoping pole with nozzles mounted at the end can be used 

to spray potassium phosphite solution high on the trunk.
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and full face shield to prevent exposure to spray mist and solution that bounces or 

drips off the stem. Coveralls or waterproof outer clothing are also recommended, 

especially if large numbers of trees are to be treated. 

3.2.5—Nonrecommended Treatments

Cultural techniques for managing disease are not recommended unless they have 

been scientifically tested to show both efficacy and a lack of adverse effects. Many 

oaks and tanoaks infected by P. ramorum have symptoms that fail to progress for 

multiple years or may go into complete remission (see “Patterns of canker development” in section 1.4.1). Because symptom remission can occur with or without 

treatment, anecdotal or case-study reports of treated trees do not provide proof 

that SOD treatments are effective. To document a real effect, treatments need to 

be tested under experimental protocols that include an adequate number of appropriately matched untreated controls. Because of the slow progress of SOD in many 

trees, and years during which few or no new infections occur, studies typically need 

to last at least 5 years to produce meaningful results.

Enhancing tree vigor— 

Arborists recommend various cultural practices to enhance or maintain oak tree 

vigor as a general prescription to maintain tree health or counteract decline. These 

practices may include:

• Root zone mulching.

• Fertilization.

• Light irrigation in the outer portion of the root zone.

• Radial trenching or other manipulations to reduce soil compaction.

• Application of mycorrhizal inoculants or other soil amendments.

• Light pruning.

• Removal of competing vegetation under the canopy.

If appropriately applied, some of these practices have the potential to stimulate 

new shoot growth and improve tree appearance, at least over the short term. However, there is no evidence that any of these practices provide any protection against 

SOD or enhance survival in infected trees. 

Furthermore, high tree vigor is associated with an increased risk of developing SOD in coast live oak (see “Effects of tree size and condition” in section 1.3.2, 

and table 2-5). Because measures intended to increase tree vigor have the potential 

to increase disease risk, they are not recommended for managing trees at risk of 

developing SOD.
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Soil pH modification— 

One marketed tree vigor prescription involves the use of a liming material 

(Azomite® Soil Sweetener—40 to 50 percent limestone [calcium carbonate]) applied to the soil around affected trees to raise soil pH. The method also calls for 

coating the lower portion of the trunk with a lime suspension (Arboright® limewash—26.8 percent calcium hydroxide), a material “intended for topical use to prevent sunburn,” according to its label. No epidemiological or biological data suggests 

that there is any relationship between soil pH and P. ramorum canker. Furthermore, 

in controlled studies with both potted and field grown coast live oaks, this treatment 

provided no measurable protection against P. ramorum, whereas phosphite significantly reduced disease susceptibility (Garbelotto and Schmidt 2009).

3.3—Monitoring Oaks With Sudden Oak Death to 

Assess Survival and Failure Potential

3.3.1—Overview 

The hazard posed by a tree that may fail (break or topple over) is related to three 

factors:

• The likelihood that the tree or a part of it will fail in a given time period.

• The ability of the failing part to cause damage (because of its size and the 

distance that it may fall).

• The presence of a valuable target within the failure’s impact zone. 

The USDA Forest Service, International Society of Arboriculture, and other 

organizations have developed standardized numerical tree hazard rating systems 

based on evaluation of these factors (see below “Sources of information on tree 

hazard assessment methods”).

Although many trees on a given property may have an elevated likelihood of 

failure, only those trees that have the potential to cause personal injury, death, or 

property damage need to be included in most tree hazard surveys. If a target zone 

is rarely occupied (for instance, a low-use trail), it is not likely to be occupied at 

the moment of tree failure. Hence, the risk of damage can be low even if the tree 

is likely to fail. In contrast, if the target area is continuously occupied, any failure 

is likely to cause injury or loss. In such sites, even trees with a moderate failure 

potential need to be considered for possible hazard reduction. 

As discussed in Chapter 2 (see “What is the failure potential of SOD-affected 

trees?” in section 2.2.3), trees with late-stage SOD symptoms have an elevated likelihood of failure overall compared to similar trees lacking these symptoms. Specific 

factors related to failure risk in SOD-affected trees are shown in tables 3-2 and 3-3. 
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Table 3-2—Guidelines for assessing mortality risk and hazard potential in wildland coast live oaks based on 

Phytophthora ramorum canker symptoms (see also table 3-3)

Risk factor Factor level

Effect on 

mortality risk

Contribution to 

failure potential 

within 1 year

Additional considerations and 

interactions

Early P. ramorum

canker symptoms 

(bleeding cankers 

only)

Low risk of 

mortality 

within the next 

year

None Symptom remission is most likely 

in trees with small cankers (<20 

percent of circumference girdled). 

Trees with extensive girdling (>80 

percent of circumference) are 

likely to die within 2 to 4 years, but 

cankers normally become invaded 

by secondary organisms before 

mortality or failure. Mortality risk 

may be underestimated if cankers 

are larger than indicated by bleeding. 

Monitor trees at least annually for 

symptom progression.

Late P. ramorum canker 

symptoms (cankers 

plus beetle boring and/

or Annulohypoxylon

thouarsianum

sporulation)

Varies with 

amount of 

trunk affected

Moderate to high Mortality risk and failure potential 

increase with increasing colonization 

by secondary organisms. Failure 

potential is also influenced by the 

presence of other types of decay and 

defects present (table 3-3).

Fruiting of A. 

thouarsianum or 

Phellinus gilvus

2.5 to 50 

percent 

of trunk 

circumference 

with 

sporulation 

Moderate risk 

of mortality 

within 

1 to 2 years 

Moderate Mortality and failure risk of live trees 

increase with percentage of trunk 

circumference affected. Failure risk 

interacts in an additive fashion with 

other decay columns present in tree.

Decay occurs before fruiting bodies 

develop, so sapwood decay may be 

extensive even if fruiting bodies are 

not seen. Also, old fruiting bodies 

can fall off or degrade.

50 percent or 

more of trunk 

circumference 

with 

sporulation

High risk of 

mortality 

within 

1 to 2 years

High

Beetle boring 2.5 to 50 

percent 

of trunk 

circumference 

with exit 

holes

50 percent or 

more of trunk 

circumference 

with exit 

holes

Moderate risk 

of mortality 

within 1 to 2 

years

High risk of 

mortality 

within 1 to 2 

years 

Moderate

High

Mortality and failure risk of live 

trees increases with the percentage 

of trunk circumference affected. 

Beetle boring is usually associated 

with decay. Beetle boring seldom 

penetrates more than 10 cm into the 

wood, so beetles damage without 

extensive decay poses less risk 

to large diameter trees. Inactive 

galleries may be difficult to detect if 

boring dust has washed off.
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general technical report psw-gtr-242 Table 3-3—Additional factors related to hazard potential in trees affected by sudden oak death (SOD)a Risk factor Factor level Contribution to failure potential Additional considerations and interactions Decay and related factors: Tree dead or partly dead Present High Small-diameter stems tend to fail sooner than larger stems. Decay <25 percent of the stem cross section affected Low Failure potential increases with increasing decay. Decay in structurally critical areas (e.g., branch junctions) can greatly increase failure potential. Decay assessment methods (drilling, etc.) can be used to assess amounts of decay in standing trees. In the absence of direct 

assessments, use other indicators of decay (fruiting bodies, canker rot symptoms, decline symptoms, beetles) to estimate decay extent.

25 to 50 percent of the stem cross section affected Moderate Presence of fruiting bodies indicates decay is present. This 

decay interacts in an additive fashion with decay caused by A. thouarsianum. Primary decay fungi raise failure risk more than secondary decay fungi, with some differences between species in each group. In the absence of a positive identification, consider any fruiting body emerging from or through bark to be important. 

More than 50 percent of the stem cross section 

affected

High

Fruiting of wood 

decay fungi otherthan Annulohypoxylonthouarsianum

Inonotus spp., Phellinus spp., Laetiporus

gilbertsonii and other primary decay fungi 

Stereum spp., Trametes spp. and other secondary 

decay fungi

HighModerate

Cavities >50 percent of stem cross sectional area affected Moderate 

to high

Risk increases with the percentage of cross sectional area 

affected. Cavities increase failure potential primarily if decay and other risk factors are also present. 

Canker rot symptoms Symptoms present, but no fruiting bodies Moderate In the absence of fruiting bodies, canker rot symptoms 

provide an indication that decay columns are present. Decay from canker rot fungi interacts in an additive fashion with decay caused by A. thouarsianum and Phellinus 

gilvus. 

Decline resulting from

other agents

Tree in severe decline, but no fruiting bodies Moderate In the absence of fruiting bodies, symptoms of severe 

decline may indicate that extensive decay columns or root disease are present.

Old failures, large

decayed stubs

Present Low to 

moderate

Can serve as point of weakness where subsequent failure 

is likely to occur. May also serve as indicators of internal decay.
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Table 3-3—Additional factors related to hazard potential in trees affected by sudden oak death (SOD)a (continued)Risk factor Factor level

Contribution to failure potential Additional considerations and interactions

Tree structure factors:

Number of stems from 

ground

Multiple stems from ground Low to 

moderate

Mainly increases failure potential in trees with dead stems, 

decay, and other substantial risk factors.

Multiple branches 

from one point

Present (especially if crowded) Low Mainly increases branch failure potential in trees with dead 

stems, decay and other substantial risk factors.

One-sided canopy Present Low Mainly increases bole failure potential in trees with dead 

stems, decay and other substantial risk factors. Severe trunk lean can have a similar effect.

Stand factors: 

Sky-exposed canopy rating <50 percent of canopy exposed to overhead 

sunlight

Low Failure potential increases as sky exposure decreases (i.e., 

higher in overtopped trees). Trees with dead stems, decay, and other substantial risk factors are most likely to show increased failure risk due to this factor.

Tree neighborhood 

altered

Other dead or failed trees present within 2 to 3 

canopy widths of tree

Low This factor may primarily serve as an indicator of a 

Phytophthora ramorum disease cluster. Increased exposure 

to wind may increase failure risk in trees with dead stems, decay and other substantial risk factors.

a Most of these factors are not strongly related to mortality risk in the near term, see also table 3-2.
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These risk factors should be incorporated into the tree hazard evaluation process 

in areas where SOD is present and should be considered in combination with other 

tree failure risk factors. 

Monitoring. As SOD symptoms develop over time, the likelihood of failure 

also changes. Monitor SOD-infected trees so they can be removed in a timely manner to minimize hazards. 

A 1-year reinspection interval is a useful starting point in many situations. Of 

all the factors that contribute to failure risk (tables 3-2, 3-3), factors related to P. 

ramorum infection tend to change the most rapidly over time. If you collect detailed 

observations on individual trees in an initial baseline survey, changes in condition 

should be apparent when follow-up inspections are made (table 3-4).

Sources of information on tree hazard assessment methods—

Johnson, D.W. 1981. Tree hazards: recognition and reduction in recreation sites. 

Tech. Rep. R2-1. U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Region, State and Private Forestry. http://www.na.fs.fed.us/spfo/pubs/

hazardtrees/treehazards/thazards.pdf. (27 August 2012).

Pokorney, J.D.; O’Brien, J.; Hauer, R.; Johnson, G.; Albers, J.; Bedker, P.; Mielke, 

M. 2003. Urban tree risk management: a community guide to program 

design and implementation. NA-TP-03-03. St.Paul, MN: U.S. Department of 

Agriculture, Forest Service, Northeastern Area, State and Private Forestry. 

194 p. http://www.na.fs.fed.us/spfo/pubs/uf/utrmm/. (27 August 2012).

National Park Service hazard tree guidelines. http://na.fs.fed.us/fhp/hazard_tree/

pubs/misc/nps.htm. (27 August 2012).

Other publications on tree risk assessment are available for purchase from the 

International Society of Arboriculture (http://secure.isa-arbor.com/webstore).

3.3.2—Guidelines for Assessing Mortality and Failure Risk in 

Trees Affected by Sudden Oak Death

The guidelines in tables 3-2 and 3-3 can be used to augment existing tree hazard 

rating systems to account for additional failure risks associated with SOD. The 

likelihood that a tree will be killed by SOD needs to be considered because SODkilled trees have a high risk of failure. Table 3-4 lists specific assessment methods 

to monitor the progression of SOD symptoms that predict survival and hazard 

potential. 

The guidelines were developed from data on coast live oak branch failures (≥20 

cm in diameter) and bole failures (≥15 cm in diameter) in SOD-affected stands in 
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Table 3-4—Methods for assessing factors related to mortality and failure potential 

in trees affected by sudden oak death (SOD)

Factor Assessment methods Notes

Phytophthora ramorum

canker activity/expansion

• Note presence of current-year 

bleeding 

• Estimate percentage of trunk 

circumference affected

• Note which sides of trunk are 

affected using compass directions (N, NE, E, etc.)

• Mark edges of canker using 

paint or lumber crayon

• Document with digital photos

• Note if callus is developing 

along edge of older cankers

Comparisons between 

annual assessments will 

indicate whether cankers are 

expanding or inactive.

Trunk decay • Types of fruiting bodies 

present (photograph if not 

identified)

• Estimate percentage of 

trunk circumference with 

Annulohypoxylon thouarsianum or other fruiting bodies

• Note location and extent of 

cavities, decay zones

Decay can be extensive before 

fruiting bodies appear. 

Intensive decay-detection 

methods (drilling, imaging) 

can provide more detail, 

but are expensive and timeconsuming. Decay fungi 

present can be identified 

via DNA testing of wood 

shavings from drilled holes.

Beetle colonization of 

trunk 

• Estimate percentage of trunk 

circumference affected

• Note which sides of trunk are 

affected using compass directions (N, NE, E, etc.)

• Note presence of fresh boring 

dust

Bark and ambrosia beetle 

boring is easiest to rate 

when fresh boring dust is 

present. Beetle holes may be 

difficult to see when dust has 

washed away, so the extent 

of colonization may appear 

to decline over time.

Marin and Napa Counties (Swiecki and Bernhardt 2003, 2010; Swiecki and others 

2006). These data are from a limited range of forest types. The importance of some 

factors may vary geographically. Additional local observations may be needed to 

determine if other factors are important. 

Additional risk factors are important for trees in urban areas that are subject to 

root disturbances, altered moisture regimes, or other conditions. For example, oaks 

in developed areas commonly show a much higher incidence of root failure than 

those in undisturbed forests. Factors associated with root failure potential should 

be given greater attention in areas where roots may have been compromised by 

excavation, construction activities, or summer irrigation.
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3.4—Assessing Fire Hazard Related to Trees Killed by 

Sudden Oak Death

The types of fuels that develop from SOD-killed trees are influenced by multiple 

factors. These include how rapidly a tree has died, whether failures occurred before 

or after tree death, and the length of time since tree death (see also figs. 2-2 and 

2-4, and “SOD-related mortality and fire” in section 2.2.3) as well as local environmental conditions. Tables 3-5 and 3-6 show examples of ground and aerial fuels that 

form over time in coast live oaks killed by SOD. 

Table 3-7 shows example time sequences for degradation of three different trees 

that were killed by SOD. Degradation of trees that are dead and defoliated before 

failure is about 1 to 2 years advanced compared with green failures. Degradation 

rates are similar if the date of tree death is used as the starting time. In tables 3-5, 

3-6, and 3-7, icons representing tree condition are the same as used in figures 2-2 

and 2-4. 

Note that all standing dead trees have an elevated risk of failure and in particular can pose a hazard to firefighting personnel.

3.5—Restoring Forests Affected by Sudden Oak Death

When developing a restoration plan, start by assessing your planting needs and 

existing regeneration (see “Restoration in SOD-affected forests” in section 2.2.3). 

Although SOD can significantly affect forests within a few years, site restoration 

may take many decades. First, address critical needs such as soil stabilization. Next, 

plan for a transition to a forest structure that provides a wider set of services and 

resource values.

In many sites, specific restoration goals can be achieved using various methods. 

Consider the pros and cons of various alternatives before deciding on restoration 

methods. Some factors to consider include:

Short- and long-term costs: High cost alternatives do not necessarily provide 

the greatest benefit or highest likelihood of success. Using existing natural regeneration can cost little and is often superior to planting. Consider costs of followup 

maintenance (e.g., for thinning or weed suppression) when comparing alternatives.

Long-term sustainability: Areas that will not be actively managed are best 

suited to techniques that do not require frequent attention. In areas that will be 

managed more intensively (near structures, roads, etc.), methods that require 

periodic inputs may be viable. 

Timeframe: Critical needs (e.g., erosion protection) that need to be addressed 

in the short term may require the use of more intensive methods. Less intensive 

methods that take longer to achieve results may be preferable if conditions are fairly 

stable. 
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Table 3-5—Changes in aerial fuels characteristics of standing dead coast live oaks killed by sudden 

oak death

Factor Level Icon Photo

Fuel stage Dead1. Standing dead with leaves

Time period 1 to 2 years after tree death

Primary fuel type(s) Aerial

Fire effects Canopy readily ignited, will serve 

as source of embers

Fuel stage Dead2. Standing dead without 

leaves with fine branches

Time period 2 to 3+ years after tree death

Primary fuel type(s) Aerial

Fire effects Canopy may ignite if sufficiently low 

and dense. Failure risk is high

Fuel stage Dead3. Standing dead with branch 

failures, some fine branches present

Time period 2 to 4+ years after tree death

Primary fuel type(s) Surface, aerial

Fire effects Piles of failed branches (failed2 to 

failed3) increase depth of ground 

fuel layer, can form fuel ladders to 

connect ground and aerial fuels. 

Aerial fuels may be too sparse to 

ignite readily but are highly prone 

to failure and in or near duff layer
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Table 3-5—Changes in aerial fuels characteristics of standing dead coast live oaks killed by sudden 

oak death (continued)

Factor Level Icon Photo

Fuel stage Dead4. Snag, large-diameter 

stems failed

Time period 4 to 6+ years after tree death

Primary fuel type(s) Surface

Fire effects Large standing stems do not easily 

ignite, but are prone to fail. Debris 

piles are fuel stage B3 to B4, 

dominated by large diameter 

branches that may be mostly 

decayed. Fine fuels are decayed 

and in or near duff layer
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Table 3-6—Changes in ground fuel characteristics of failed coast live oaks killed by sudden oak death

Factor Level Icon Photo

Fuel stage Failed1. Failed tree or branch 

with leaves

Time period 1 to 2 years after tree death or 

green failure

Primary fuel type(s) Surface

Fire effects Very readily ignited, can serve as 

fuel ladder to carry surface fire 

to canopy

Fuel stage Failed2. Large-diameter stems, 

fine branches, few to no leaves

Time period 2 to 4 years after tree death or 

green failure

Primary fuel type(s) Surface

Fire effects Readily ignited, can serve as 

ground-to-canopy fuel ladder

Fuel stage Failed3. Primarily large-diameter 

branches, fine fuels mostly rotted 

and in or near duff layer

Time period 3 to 8+ years after tree death or 

green failure

Primary fuel type(s) Surface

Fire effects Increases total ground fuels, but 

debris piles are generally low, 

typically no more than 1 m tall

Fuel stage Failed4. Decayed large-diameter 

stems, all small branches decayed

Time period 6 to 10+ years after tree death or 

green failure

Primary fuel type(s) Surface

Fire effects Minimal contribution to ground fuels
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Table 3-7—Time sequences of degradation of aerial and ground fuels in three coast live oaks killed by 

sudden oak death 

Icon Photo Description

Tree crown in year of tree death. Fuel stage Dead1. 

One year later. Fuel stage is intermediate between 

Dead1 and Dead2. Several large branches have 

failed.

Two years later. By 2 years after death, this tree has 

progressed to fuel stage Dead3. (Marin County)
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Table 3-7—Time sequences of degradation of aerial and ground fuels in three coast live oaks killed by 

sudden oak death (continued)

Icon Photo Description

Three years later. Fuel stage 

Failed3. (Marin County)

One year later. Fuel stage 

intermediate between 

Failed2 and Failed3

Tree died and was defoliated 

before the trunk failed 

2 years later. Fuel stage 

Failed2. 
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Table 3-7—Time sequences of degradation of aerial and ground fuels in three coast live oaks killed by 

sudden oak death (continued)

Icon Photo Description

Large tree about 1 to 2 

months after live

failure. Fuel stage Failed1.

Three years after failure. 

Fuel stage Failed3. 

Note sprout growth from 

stump. 

Eight years after failure. 

Fuel stage still Failed3. 

(Marin County)
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Potential for nontarget effects: When more intensive methods are used, 

unintended effects may be possible. For example, soil disturbance associated with 

clearing or grading may increase erosion potential and promote weed establishment. 

3.5.1—Species Selection 

The term “restoration” implies a return to a previous condition. Because forest 

composition changes over time, various forest types may have occupied a site in 

the past. Forest stands in most of California have been substantially altered by 

human activity over the past two centuries. Because of tree cutting, changes in 

fire frequency, grazing, and other cultural practices, current stand composition 

and structure may differ from conditions that existed prior to European/American 

settlement. For example, tanoak-dominated stands in some areas developed after 

clearcutting of redwood/Douglas-fir forests in the mid-20th century. 

Reliable information about the former forest cover can tell you what once grew 

in an area. Also consider how site conditions may have changed since that forest 

type was established. In addition to the presence of P. ramorum, sites may be 

substantially changed in other ways, including:

• Increased soil erosion from clearing and burning.

• Litter and debris accumulation from tree mortality.

• Increased growth of competitive, nonnative weedy species.

• Hotter, drier microclimate resulting from the loss of overstory species.

• Increased browsing by herbivores (deer, rodents, livestock).

Such changes may inhibit the establishment of species that once occupied the 

site. Some of these factors can be addressed by management actions. For instance, 

surface litter can be removed or exclosures can be used to reduce herbivory. These 

interventions typically increase costs. Other constraints that cannot be eliminated 

need to be dealt with through appropriate species selection. For example, warmer, 

drier conditions associated with microclimate change may render sites unsuitable 

for species that were only marginally suited to the area in the past. 

Use of SOD canker hosts for restoration— 

Under some conditions, it may be feasible to replace SOD-killed oaks with the same 

species. If the stand will be managed to eliminate California bay and any other significant sources of P. ramorum spores, SOD-killed oaks may be replaced with susceptible species such as coast live oak, California black oak, and canyon live oak. 

Although tanoaks vary in their susceptibility to P. ramorum, highly SODresistant tanoaks have not yet been identified. Until tanoaks with useful levels of 

SOD resistance can be identified, regenerated tanoak stands will continue to be at 
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risk. Consider replacing tanoaks lost to SOD with other species. It may be possible 

to include tanoak in regenerated stands as widely separated trees, which have a 

reduced risk of sustaining a lethal P. ramorum infection.

Choosing replacement species— 

In SOD-affected stands where California bay is abundant, killed SOD canker hosts 

(tanoak and susceptible oaks) need to be replaced with species that are not affected 

by P. ramorum. Replacement species that will dominate the stand must be well 

adapted to the site and stocked at an overall density that is sustainable over time.

Both canopy cover and tree species characteristics need to be considered 

when designing restoration projects to specific restoration goals (table 3-8). Visual 

considerations may be especially important when choosing trees to plant around 

homes. For example, replacing shorter oaks with tall conifers may obstruct views or 

sunlight. Where visual screening is desired, evergreen species will be more effective than deciduous trees. Deciduous blue or valley oaks provide many of the same 

ecosystem services as SOD-susceptible coast live oaks, but do not provide the same 

level of visual screening during the winter. Various combinations of species and 

canopy cover levels can be used to meet multiple goals, but a single forest structure 

is not likely to fulfill all goals.

Growing conditions can change over relatively short distances because of 

factors such as soil type and depth, topographic position, slope, and aspect. Deep 

soil at the base of a slope near a seasonal creek provides a different growing environment than an adjacent west-facing slope with shallow soils. Some SOD canker 

hosts, such as coast live oak, tolerate a broad range of site conditions. Multiple 

replacement species may be needed to fill the sites previously occupied by a single 

SOD-killed species. 

Integrating restoration with other land management goals and practices— 

Organizations that manage large areas of forest and other natural habitats (e.g., park 

and open space districts, land trusts, water districts) typically have overall land 

management goals that apply across their holdings. When setting goals for restoration of SOD-affected sites, resource managers need to consider how the restored 

sites will function in the context of broader land management goals.

Individual private landowners commonly manage smaller parcels. These 

parcels occur within the context of a wider landscape that can influence restoration 

project outcomes. For example, invasive exotic plants may be more difficult to suppress if adjoining lands are heavily infested. Where possible, coordinate management efforts with adjacent landowners.
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Table 3-8—Levels of canopy cover and tree characteristics needed to achieve various restoration goals

Restoration goal Canopy cover Tree characteristicsa

Soil stabilization High Fast establishment, wide fibrous root system, 

evergreen

Moderation of stormwater f﻿lows High Evergreen or deciduous with high leaf surface area 

during rainy season

Maintenance or improvement of 

wildlife habitat and biodiversity

Varies, diversity of cover 

levels may be important

Provides food (e.g., acorns, insects), cover, nesting 

sites

Maintenance or improvement 

of native plant habitat and 

biodiversity

Varies, diversity of canopy 

cover densities may be 

important

More open canopies allow for better development of 

many understory plants 

Suppression of undesirable 

invasive species

High Dense canopy or allelopathic (suppressive) properties

Visual screening Varies Evergreen trees with high canopy density at desired 

height range

Aesthetics/property value 

enhancement

Varies Varies

Shading for energy conservation High in areas near structures Deciduous trees with relatively early leaf drop and late 

leaf emergence

Minimize fire hazard in defensible 

space 

Low in firebreak area; low 

to moderate in reduced fuel 

zone

Relatively low flammability, high canopy with clear 

lower trunk, low litter production

a

 Understory species (shrubs, herbaceous plants) may also be consistent with specific goals, but will not restore forest canopy cover. 

Most small-scale restoration projects do not involve activities that are addressed 

by various federal, state, and local environmental and land use regulations. However, larger projects that involve intensive inputs such as land clearing, grading, or 

burning may require agency permits or approvals. Landowners should check with 

appropriate local and state agencies before undertaking these activities. To comply 

with state and local regulations and guidelines related to fire hazard, vegetation in 

defensible space areas must be managed appropriately. Proper plant selection, placement, and maintenance will help minimize fire hazard in these areas. 

3.5.2—Factors That Constrain Regeneration

Various factors can constrain regeneration of oaks and other species in SODaffected forests (table 3-9). Restoration inputs are designed to overcome these 

constraints. You can minimize cost and effort by using only those management 

inputs needed to address the site’s constraints. 
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general technical report psw-gtr-242 Table 3-9—Factors that limit regeneration of tree species and possible remedies Factor Constraints on regeneration Possible remedies Seed production • Seed production varies widely between individual trees and years. • Oaks and other outcrossing, wind-pollinated species may not set seed reliably if trees are widely separated. • Oak and tanoak acorns do not store well and must be planted within a few months of collection. • Plan to collect and plant seed in multiple years. • Expand area of seed collection as needed to find adequate numbers of seed trees. Seed dispersal • For heavy seeds such as acorns, dispersal is primarily by gravity and most seed is deposited near the parent tree. Many species with small seeds have adaptations that allow seed to be dispersed longer distances via wind. • Animals can move seeds relatively long distances, but resulting dispersal patterns are nonuniform. • Plant if needed to obtain desired species mix. Seedbed 

conditions 

• Acorns lose viability upon drying, so lack of organic debris on 

the soil surface may greatly reduce germination and seedling establishment.

• Some species with small seeds are adapted to germinate on bare 

mineral soils and will not tolerate deep duff layers.

• Soil compaction can reduce root growth, leading to poor 

establishment in many species.

• Adjust seedbed (e.g., increase or decrease litter layer) as needed to favor 

target species.

• Till soil in areas where compaction is excessive.

Damage from 

herbivores 

• Livestock, deer, rodents, and insects can damage seeds and 

seedlings. 

• In areas with high herbivore pressure, regeneration of 

preferred palatable species can be completely suppressed while reproduction of less palatable species is favored. 

• Herbivore damage varies seasonally and from year to year 

because of changes in herbivore populations, alternative food availability, and plant maturity. 

• Impacts of herbivory are most serious on small seedlings and 

saplings. 

• Use fencing or individual plant protection (wire cages, tree shelters, etc.) 

to exclude herbivores through the plant establishment period.

• Modify or suspend livestock access to restoration site until tops of plants 

are well above browse line (the height that grazing animals can reach).

• Modify habitat to disfavor damaging agents and increase populations 

of natural enemies (e.g., mowing to increase predation of rodents by raptors).

Diseases • Phytophthora ramorum has potential to reduce seedling growth 

and survival for species that are subject to foliar and twig blighting (e.g., tanoak). 

• Soilborne plant pathogens and some foliar diseases have the 

potential to reduce seedling growth and survival. 

• Avoid using species that are susceptible to diseases present on site.

• Use only nursery stock that is certified or tested to be free of 

Phytophthora spp. and other significant pathogens.

• Increase species diversity to minimize overall losses caused by a single 

pathogen.
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Soil moisture • Levels of soil moisture, especially in late summer, limit species 

survival.

• Species that are relatively tolerant of low soil moisture when 

mature (e.g., many oaks) may be killed by drought during the first few seasons of establishment.

• Drought cycles or successive years of heavy precipitation may 

strongly influence plant establishment.

• High plant densities increase competition for limited amounts of 

soil moisture.

• The total amount of plant-available water in soils varies by soil 

type. Available water is lowest in coarse or sandy soils, greatest in 

loam and clay loam soils.

• Available water increases with increasing soil depth (to bedrock or 

impermeable layers) and increasing organic matter content.

• Shallow water tables or surface flows can increase plant available 

water.

• Organic mulch on the soil surface helps retard evaporative water 

loss.

• Match species’ moisture needs to the local soil properties.

• Plan to restock over multiple years to take advantage of wetter seasons 

and avoid drought periods.

• Control weeds to maximize available soil moisture.• Thin to conserve soil moisture.

• If needed, maintain an organic mulch/litter layer to conserve soil 

moisture.

Soil erosion • Soils prone to erosion may be too unstable to establish slowgrowing species. 

• Excessive topsoil loss reduces soil water-holding capacity and 

available plant nutrients. Severe erosion may render the site unsuitable for species that previously occupied the site.

• Use temporary vegetation, mulch, and other soil stabilization methods to 

minimize erosion while target species are becoming established.

• Modify species selection to match species tolerance to current site 

conditions.

Solar radiation • The amount of light needed for seedling establishment and growth 

differ by species. Some species establish better under partial to complete canopy cover, others require full sunlight.

• Species’ shade tolerance can vary with age. Many species tolerate 

less shade as they mature.

• Shade reduces water use, and may partly compensate for limited 

soil moisture.

• Solar radiation is lowest on north-facing slopes and greatest on 

south- and west-facing slopes.

• Select species with light tolerance that matches site conditions.

• Plan for successional changes in vegetation as tree growth over time 

changes available light.

• Start restoration activities before all tree canopies are killed to provide 

shade for seedling establishment.

• Thin or prune as needed to maintain appropriate light levels for target 

species.

Factor Constraints on regeneration Possible remedies

Table 3-9—Factors that limit regeneration of tree species and possible remedies (continued) 
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3.5.3—Natural Regeneration

Most sites support natural regeneration of various herbaceous and woody species. 

Natural regeneration may be so plentiful that thinning is needed to achieve target 

plant densities. If natural regeneration of preferred species is sparse, you may need 

to favor it through cultural inputs, such as protection from browsing animals or 

reducing competition from fast-growing exotic or native plants (fig. 3-8). 

Potential regeneration may be present as dormant seed in the soil or as incoming seed transported to the site by wind, water, or animals. Canopy gaps caused by 

SOD-related mortality can have an altered environment (light, temperature, soil 

moisture) that favors germination of dormant or incoming seeds. Consequently, 

species that recruit in gaps from these seeds may have been absent before the gaps 

developed.

Existing seedlings and saplings are a common source of natural regeneration. 

Oaks and many other forest trees have short-lived seeds. These seeds germinate 

within a year, and seedlings develop to a limited degree. Seedlings remain small 

because overstory trees limit their access to light and soil moisture. These persistent seedlings (also known as advance regeneration or a seedling bank) may grow 

rapidly when overstory trees die or are removed.

Stump sprouts are another potential source of natural regeneration. Many species found in SOD-affected forests, including most oaks, tanoaks, California bay, 

and coast redwood, commonly resprout from stumps. SOD-killed oaks and tanoaks 

may produce basal sprouts that can give rise to a new tree. However, such sprouts 

are clones of the susceptible parent and therefore SOD-susceptible. The resulting 

trees are likely to develop SOD cankers eventually if a source of P. ramorum spores 

remains in the area. Sprout-origin oaks can also have poor structural properties. 

Decay associated with the old stump increases failure potential and reduces the useful life of these trees. However, stump sprouts grow fast because of their established 

root systems. In some situations, stump sprouts can provide fast, if temporary, 

replacement tree canopy.

3.5.4—Planting

Where existing regeneration is insufficient or does not include the preferred species 

to meet restoration goals, planting may be necessary. The optimum type of planting material varies with species and site conditions. The USDA Forest Service and 

other organizations have developed a wide variety of materials related to forest tree 

planting and silviculture (see below “Sources of information on tree planting and 

silviculture”). For California forest species, more information is available for commercially utilized conifers than for noncommercial hardwoods such as native oaks. 
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Figure 3-8—Madrone and Douglas-fir regeneration in a canopy gap caused by sudden oak death 

(SOD) shown in September 2008 (top) and October 2010 (bottom). The gap formed when a large 

coast live was killed by SOD in 2002 and failed 2 years later. Without management, the faster 

growing Douglas-fir regeneration is likely to outcompete the madrone.
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Direct seeding— 

For large-seeded species such as oaks, direct planting of seed at the restoration site 

can be effective and cost-efficient. Direct seeding avoids various problems that may 

be associated with nursery stock (table 3-10). Timing of seed collection, storage 

conditions, and planting methods differ by species. 

Transplanting natural seedlings— 

For small-scale projects, it may be possible to locate and transplant naturally occurring seedlings during the late fall or winter. This requires a plentiful supply of 

nearby seedlings growing in soil that is not excessively rocky. Seedlings must be 

small enough that the root system can be easily dug up and moved. Species differ in 

their ability to survive transplanting. It is possible to transplant large saplings using 

a tree spade (a truck-mounted device that can move a sapling tree or shrub along 

with a relatively large volume of soil), but this method is expensive and typically 

limited to use in developed areas. 

Locally collected natural seedling transplants have the advantage of being from 

the local plant population. They are normally colonized by local species of mycorrhizal fungi, which are important for plant growth and survival. However, locating 

and transplanting natural seedlings can be time consuming and will disturb the 

seedling source site. Deleterious organisms, such as root disease pathogens, may 

also be moved with transplants. If the source site is very close to the restoration site, 

the risk of introducing new pathogens may be low. Nonetheless, seedling source 

sites should be inspected for evidence of disease problems before they are selected. 

Testing of plants or soil may be appropriate to determine if soilborne Phytophthora 

species or other pathogens are present before transplanting natural seedlings.

Nursery-grown transplant stock— 

Nursery-grown planting stock is commonly used in forestry and environmental restoration. However, nursery stock can pose a high risk of introducing exotic pathogens, especially Phytophthora species, into forested areas. 

• The initial introduction and spread of P. ramorum in California was associated with movement of contaminated nursery stock (Mascheretti and others 

2008). 

• The exotic pathogen Phytophthora lateralis, which causes a lethal root rot 

of Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murray bis) Parl.), was 

apparently introduced into forests of Oregon and California via contaminated nursery stock (Hansen and others 2000).
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Table 3-10—Comparisons between direct seeding and nursery-grown container transplants for restockingFactor Seed TransplantsRelative cost Low. Local seed reduces transportation and handling costs. High. Production requires seed collection and seedling maintenance 

until outplanting.

Storage Low space requirements. Short-lived seed such as acorns can typically 

be stored for only a few months. Other seed may remain viable for many years under proper storage conditions.

High space requirement because of the size of the containers. Plants 

may be held for multiple years under good growing conditions, but more space and larger containers will be needed as plants grow.

Planting Relatively fast and easy to plant large number of sites. Greater effort required for planting, especially if many sites are 

involved or larger stock is used.

Postplanting success Varies by species and site conditions. Overplanting can be used to 

obtain adequate survival. For species such as oaks, high rates of germination and establishment can be obtained. 

Varies with stock quality and site conditions. Transplants are often 

highly susceptible to dry conditions after planting and irrigation may be needed to ensure establishment. High rates of success can be obtained with many commercial forest species with proper site preparation.

Root structure Root structure adapts to soil conditions as seed germinates and grows. 

Taproot development typically good.

Root system is modified strongly by container and is more 

restricted, requiring adaptation after transplanting. Taproot structure of species with strong taproots (e.g., many oaks) is distorted or eliminated. Root form problems may develop in the container or as a result of poor planting technique.

Shoot structure Shoots typically develop in proper balance with roots and show 

adaptations to site conditions.

Shoot-to-root ratio may be artificially high, leading to water stress 

after planting. Shoots may be thin and mechanically weak if plants are crowded in the nursery. 

Local germplasm Can be obtained by collecting seed in and near restoration site. Seed source is unlikely to be local unless custom propagation is 

done, requiring adequate lead time for growing stock.

Introduction of pathogens 

or insect pests

Typically not an issue for locally collected seed. Can pose high risk for introduction of soil diseases such as 

Phytophthora spp. and movement of some insect pests.
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• In Bavaria (Germany), nursery stock used for reforestation was the source 

of a now-widespread Phytophthora root and crown rot of alders in reforested stands (Jung and Blaschke 2004).

• The soil-borne root pathogen Phytophthora cinnamomi is a serious forest 

disease in various parts of the world. In recent years, P. cinnamomi and 

some other exotic root-infecting Phytophthora species have been associated with mortality of forest trees and native shrubs at numerous northern California locations. Contaminated nursery plants appear to be the 

likely source of these pathogens at a number of the infested wildland sites. 

Phytophthora cinnamomi and other root-rotting Phytophthora species are 

common in nursery stock produced in native plant nurseries and other commercial nurseries. (Fichtner and others 2010, Swiecki and others 2011). 

From these examples, it is clear that nursery stock infested with any Phytophthora species should not be planted in or adjacent to natural areas. Unfortunately, 

most nurseries, including native plant nurseries, have not taken necessary steps to 

ensure that stock they produce is free of pathogenic Phytophthora species.

The U.S. Nursery Certification Program (USNCP, see http://www.aphis.usda.

gov/plant_health/acns/certification.shtml) is a pilot, voluntary phytosanitary certification program for U.S. nurseries that ship nursery stock to Canada. The phytosanitary standards include “practical freedom from nonregulated plant pests,” which 

would include many Phytophthora species. The Safe Procurement and Production 

Manual (Griesbach and others 2012) provides checklists, best management practices (BMPs), and information related to obtaining USNCP certification. Nurseries 

that adhere to the BMPs listed in this manual are highly likely to produce nursery 

stock that is free of Phytophthora species and other serious pests and diseases. 

If a nursery does not have USNCP certification, an inspection of the nursery 

production facilities can show whether the cultural practices are sufficient to 

produce clean stock. To produce plants free from P. cinnamomi and other Phytophthora species, nurseries need to follow phytosanitary procedures throughout the 

entire propagation cycle. These principles apply equally to large and small nurseries, including backyard propagators.

Clean nursery production is possible only if pathogen-free soil, disease-free 

propagation stock, and clean containers are used. Nursery-grown plants may 

become infested with Phytophthora species through the use of recirculated irrigation water and movement of infested soil via hoses, tools, pots, hands, shoes, 

and vehicles. To avoid contamination, do not place pots and flats directly on soil. 

Routinely disinfest benches, pots, tools, and other items used for propagation prior 

to each use. 
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Unless a nursery follows a systems approach to prevent contamination though 

the entire production cycle, planting stock can eventually become infested. Older, 

larger container planting stock is more likely to be infested with Phytophthora species than younger, smaller container stock. Liner stock grown in the ground (e.g., 

bareroot stock) may be infested at any age if the production field is contaminated. 

The presence of Phytophthora infection can be masked in fungicide-treated 

plants. Fungicide treatment can prevent disease symptoms but will not free plants 

and container soil of Phytophthora spores. Phytophthora activity increases and 

symptoms develop as fungicide residues decline. Nursery plants should be set aside, 

prior to planting, without fungicide treatment for at least 3 months to reveal symptoms of any existing Phytophthora infestations. 

Genetic considerations— 

If onsite natural regeneration is inadequate for accomplishing restoration goals, the 

next best source of planting material is locally collected seed or planting stock derived from it (table 3-11). Local germplasm represents the result of natural selection 

under local soil and climate conditions. Trees growing from locally collected seed 

should be adapted to site conditions and are likely to perform well over time (Millar 

and Libby 1991). 

It is important to realize that selection pressure can be high at the seedling 

stage under natural conditions. High rates of seedling mortality are common in 

nature. Seedling mortality can exert selection pressure for characteristics that are 

important for early survival. Such characteristics are likely to be important for 

long-term stand sustainability. If intensive inputs are used to ensure high rates of 

seedling survival, this type of adaptive selection is minimized or avoided. Consider 

overplanting with inexpensive planting material (e.g., seed) and allowing natural 

selection to function rather than using intensive inputs to ensure the survival of 

large expensive stock. 

Sources of information on tree planting and silviculture—

McCreary, D. 2009. Regenerating rangeland oaks in California. Publication 21601e. 

Oakland, CA: University of California, Agriculture and Natural Resources 

Publication Services. 62 p. http://anrcatalog.ucdavis.edu/Items/21601e.aspx. 

(7 December 2012).

Burns, R.M.; Honkala, B.H., tech, coords. 1990. Silvics of North America, 1. 

Conifers. 2. Hardwoods. Agric. Handb. 654. Washington, DC: U.S. Department 

of Agriculture, Forest Service. 877 p. http://www.na.fs.fed.us/spfo/pubs/silvics_

manual/table_of_contents.htm. (27 August 2012).
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Table 3-11—Recommendations for the management of genetic resources in forest restoration projects

Recommendation Notes

Use existing regeneration Conserve existing seedling regeneration that has been exposed to 

selective pressures.

Use local seed For trees such as oaks and tanoaks, trees within the same watershed 

or located within a few to about 10 km of the restoration site may 

be considered to be within the local seed zone. Wider local zones 

may apply for species that disperse seed and pollen more widely.

Match seed source and planting site characteristics When collecting seed within or outside of the local seed zone, match 

the microclimate (e.g., north slope) and soil characteristics (type 

and depth) of the seed source site to that of the target restoration 

site to the degree possible. It may be desirable to collect seed 

from sites that are hotter and drier than the target site to provide a 

potentially wider range of climate adaptation variation.

Ensure genetic diversity in seed collections Seed collected for planting should be from a large number of 

individuals spread widely over the local seed zone. Because of 

variation in seed production between individuals from year to year, 

seed collection and planting should be conducted over a series of 

years.

Avoid genetic pollution Avoid collecting seed from trees likely to be pollinated by 

horticultural specimens of unknown origin. 

Do not plant stock (nursery-grown or transplanted) of non-local or 

unknown origin within about 1 to 3 km of existing native stands in 

areas to prevent spread of pollen and seed from nonlocal genotypes 

into native stands.

Let natural selection function Allow for attrition resulting from natural selective pressures in 

restoration plantings. Plant seeds or the smallest planting stock size 

available and plan for relatively high rates of seedling mortality so 

that survival will be largely limited to the best-adapted genotypes.
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3.5.5—Monitoring Restoration Outcomes

Restoration of forests affected by SOD is largely experimental at this time. Through 

careful monitoring of your restoration projects, you can compile the data needed 

to determine the success of various techniques under the range of conditions that 

occur at your site. The locations of various treatments applied in each year (planting, weed control, etc.) can be documented using maps, geographic coordinates, 

or permanent field markers (e.g., metal tags or stakes). Photos taken from specific 

locations (photo points) are also useful for documenting locations and changes over 

time. 

Include information on factors that limit growth or survival in your monitoring 

data. These include soil conditions (texture, moisture, mulch), weather (precipitation, temperatures), shading, plant competition, browsing by livestock or wildlife, 

diseases, insect injury, and other damage agents (e.g., fire). Some of these constraints can be addressed by cultural inputs (e.g., fencing to exclude herbivores, 

thinning, weed control). If factors such as an extended drought or fire are limiting, 

it may be necessary to switch to species that will better tolerate these adverse 

conditions. 

Initial seedling and sapling survival rates are commonly the focus of restoration project monitoring. However, high seedling survival in the first few years of a 

project does not guarantee long-term survival. Continued monitoring is needed to 

determine whether the restoration project is succeeding, and if not, what additional 

steps are needed. 

Acknowledgments

Support for this publication was provided by the U.S. Department of Agriculture, 

Forest Service, State and Private Forestry, Forest Health Protection and Pacific 

Southwest Research Station; Midpeninsula Regional Open Space District; and the 

San Francisco Public Utilities Commission. The authors wish to thank the following 

for review and guidance; Everett Hansen, Oregon State University; Phil Cannon, 

USDA Forest Service, Pacific Southwest Region, Forest Health Protection; Chris 

Lee, University of California Cooperative Extension, Humboldt and Del Norte 

Counties; Ellen Natesan, San Francisco Public Utilities Commission; Cindy Roessler, Midpeninsula Regional Open Space District; and Susan Frankel, USDA Forest 

Service, Pacific Southwest Research Station.
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English Equivalents

When you know: Multiply by: To find

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Millimeters (mm) 0.0394 Inches

Kilometers (km) 0.6215 Miles

Square meters (m2

) 10.76 Square Feet (ft2)

Grams (g) 0.352 Ounces

Pounds per gallon 1.717 Kilograms per liter

Degrees Celsius (°C) 1.8 and add 32 Degrees Fahrenheit

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