A Field Guide

to Insects and 

Diseases of 

California Oaks

United States

Department of 

Agriculture

Forest Service

Pacific Southwest

Research Station

General 

Technical Report

PSW-GTR-197

July 2006

Tedmund J. Swiecki

Elizabeth A. Bernhardt
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Author information: Tedmund J. Swiecki and Elizabeth 

A. Bernhardt are plant pathologists and principals of 

Phytosphere Research, 1027 Davis Street, Vacaville, CA 

95687 (Email address: phytosphere@phytosphere.com)

Citation: Swiecki, Tedmund J. and Bernhardt, Elizabeth 

A. 2006. A field guide to insects and diseases of 

California oaks. Gen. Tech Rep. PSW-GTR-197. Albany, 

CA: Pacific Southwest Research Station, Forest Service, 

U.S. Department of Agriculture, 151 p.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

cm

0

mm
A Field Guide to 

Insects and Diseases

of California Oaks

Tedmund J. Swiecki

Elizabeth A. Bernhardt
The authors wish to thank the following persons for their review 

of material included in the manuscript: David Adams, California 

Department of Forestry and Fire Protection, retired; L.E. Ehler, 

Department of Entomology, University of California, Davis; 

Deborah Ellis, Consulting Arborist and Horticulturist; Susan J. 

Frankel, USDA-Forest Service, Pacific Southwest Research Station; 

James D. MacDonald, Department of Plant Pathology, University 

of California, Davis; Don Owen, California Department of Forestry 

and Fire Protection; Katie Palmieri, California Oak Mortality Task 

Force; David Rizzo, Department of Plant Pathology, University 

of California, Davis; Steve Seybold, USDA Forest Service, Pacific 

Southwest Research Station; and Sheri Smith, USDA-Forest Service, 

Forest Health Protection.

The authors thank John Muir Laws (California Academy of 

Sciences) for the use of his original illustrations. His work is 

supported by the Richard and Rhoda Goldman Fund. We also thank 

the following persons and organizations for providing photographs: 

Larry G. Bezark, California Department of Food and Agriculture; 

Robert Campbell, Department of Plant Pathology, University of 

California, Davis, retired; Jack Kelly Clark, University of California 

Statewide Integrated Pest Management Program; Larry Costello, 

University of California Cooperative Extension (UCCE); Bruce 

Hagen, California Department of Forestry and Fire Protection 

(CDF); Robert S. Kelley, Vermont Department of Forests, Parks 

and Recreation; T.W. Davis and California Academy of Sciences; 

Edward E. Gripp, Landscape Architect; Steve Koike, UCCE; Don 

Owen, CDF; Steve Tjosvold, UCCE; John A. Weidhass, Virginia 

Tech; Allison Wickland, Department of Plant Pathology, University 

of California, Davis; and www.forestryimages.org. Photographs 

without a specific photo credit and the photo-based illustrations are 

by the authors.

The photographs and illustrations reproduced in this publication are 

under copyright by the photographers or illustrators and may not 

be reproduced in any form without obtaining permission in writing 

from them.

Layout and design by Janice Alexander, University of California 

Cooperative Extension and the California Oak Mortality Task Force.

Support for this publication was provided by the US Department 

of Agriculture Forest Service, Pacific Southwest Region, State and 

Private Forestry, Forest Health Protection. 

Acknowledgements
1

5 Introduction

Insects and Mites

Acorn feeders

10 Filbert weevils (Curculio aurivestis, C. occidentis, C. pardus)

12 Filbertworm (Cydia latiferreana)

General foliar feeders

14 California oakworm, California oakmoth

 (Phryganidia californica)

16 Fruit tree leafroller (Archips argyrospila)

18 Oak ribbed casemaker (Bucculatrix albertiella)

20 Tent caterpillars: Western tent caterpillar (Malacosoma 

 californicum); Pacific tent caterpillar 

 (M. constictum); Forest tent caterpillar (M. disstria)

24 Western tussock moth (Orgyia vetusta)

Gall formers

26 Erineum mite (Eriophyes mackiei)

28 Gall wasps (family Cynipidae)

Sap feeders

32 Kuwana oak scale (Kuwania quercus)

34 Oak leaf phylloxera (Phylloxera spp.)

36 Oak lecanium scale (Parthenolecanium quercifex)

38 Obscure scale (Melanaspis obscura)

40 Pit scales: Oak pit scale (Asterolecanium minus, 

 A. quercicola); Golden oak scale (A. variolosum)

42 Treehoppers (Platycotis vittata, P. vittata quadrivittata)

Contents

Contents
2 A Field Guide to Insects and Diseases of California Oaks

44 Whiteflies: Crown whitefly (Aleuroplatus coronatus); 

 Gelatinous whitefly (A. gelatinosus); Stanford’s

 whitefly (Tetraleurodes stanfordi)

46 Woolly oak aphids (Stegophylla querci, 

 S. quercifoliae; S. essigi)

Twig borers

48 Oak twig borer (Styloxus fulleri californicus)

49 Other twig borers 

 (Aneflomorpha lineare, Scobicia suturalis)

50 Oak twig girdler (Agrilus angelicus)

Bark and wood boring insects

52 Ambrosia beetles (Monarthrum dentiger, M. scutellare)

56 Bark beetles: Western oak bark beetle 

 (Pseudopityophthorus pubipennis); Oak bark beetles 

 (P. agrifoliae, P. pruinosus)

60 Carpenterworm (Prionoxystus robiniae)

62 Flatheaded borers/Metallic wood boring beetles:

 Flatheaded appletree borer (Chrysobothris femorata);

 Pacific flatheaded borer (Chrysobothris mali) 

64 Lead cable borer (Scobicia declivis) 

65 Pigeon tremex (Tremex columba)

66 Roundheaded borers/Long-horned beetles: 

 California prionus (Prionus californicus); Banded alder

 borer, California laurel borer (Rosalia funebris); Nautical 

 borer (Xylotrechus nauticus); Neoclytus conjunctus

70 Western sycamore borer (Synanthedon resplendens)
Contents 3

Diseases

Acorn diseases

72 Drippy nut (Brenneria quercina)

Diseases affecting leaves and twigs

74 Early defoliation/leaf browning/drought stress

76 Oak anthracnose, twig blight, and leaf spots: 

 Apiognomonia errabunda (= Discula umbrinella),

 Cryptocline cinerescens, Septoria quercicola, and other fungi

80 Oak leaf blister (Taphrina caerulescens)

82 Powdery mildews: Brasiliomyces trina, Cystotheca lanestris, 

 Microsphaera extensa curta, Microsphaera penicillata (wide

 sense), Phyllactinia angulata

Branch diseases

88 Branch canker (Diplodia quercina)

90 Orange hobnail canker (Cryphonectria gyrosa=Endothia gyrosa)

92 Oak mistletoe (Phoradendron villosum)

Diseases affecting branches and trunks

96 Canker rots (Inonotus andersonii, I. dryophilus) 

100 Hedgehog fungus (Hericium erinaceus f. erinaceus)

102 Hypoxylon thouarsianum

106 Sulfur fungus (Laetiporus gilbertsonii=L. sulphureus)

110 Other wood decay fungi affecting branches and trunk

 (Phellinus gilvus, Phellinus robustus=Fomitiporia robusta) 

112 Sudden oak death, Phytophthora ramorum canker;

 Phytophthora stem canker (P. nemorosa, P. pseudosyringae)

118 Sunscald 

120 Wetwood and alcoholic flux
4 A Field Guide to Insects and Diseases of California Oaks

Root diseases

124 Armillaria root rot, oak root fungus

 (Armillaria mellea)

128 Ganoderma root rot (Ganoderma applanatum, 

 G. brownii, G. lucidum)

132 Phytophthora root rot (Phytophthora

 cinnamomi and other Phytophthora species)

136 Weeping conk (Inonotus dryadeus)

137 Western jack o’lantern fungus

 (Omphalotus olivascens)

138 Glossary

143 List of plants

146 References

149 Index
5

Introduction

California has more than twenty-five native species, natural 

hybrids, and varieties of oaks (Quercus species). The form of these 

oaks ranges from large trees, up to about 25 m tall, to shrubs 

no taller than about 1.5 m. California’s native oaks include 

representatives of three oak subgroups or subgenera (Table 1). 

Hybridization only occurs between oaks in the same subgroup. 

In addition, some insects, pathogens, and other agents may 

selectively colonize or damage oaks in certain subgroups.

Introduction

Subgroup 

(subgenus)

Species Common name Common 

form1

White oaks 

(Quercus or 

Lepidobalanus)

Q. douglasii 

Q. engelmannii

Q. garryana

Q. lobata

Q. berberidifolia

Q. cornelius-mulleri

Q. dumosa

Q. durata

Q. john-tuckeri

Q. sadleriana

Q. turbinella

blue oak

Engelmann oak

Oregon white oak

valley oak

scrub oak

Muller oak

Nuttall’s scrub oak

leather oak

Tucker’s oak

deer oak

desert scrub oak

tree

tree

tree

tree

shrub

shrub

shrub

shrub

shrub

shrub

shrub

Red/black oaks 

(Lobatae or 

Erythrobalanus)

Q. agrifolia

Q. kelloggii

Q. parvula var. shrevei

Q. wislizeni2

coast live oak

California black oak 

Shreve oak

interior live oak

tree

tree

tree

tree

Intermediate oaks 

(Protobalanus)

Q. chrysolepis

Q. tomentella

Q. vacciniifolia

canyon live oak

island oak

huckleberry oak

tree

tree

shrub

Table 1—Scientific and common names of California native oaks by 

subgroup.

1 Some of the species have both tree and shrub forms.

2 This species was formerly spelled Q. wislizenii.
6 A Field Guide to Insects and Diseases of California Oaks

As of the early 1990’s, oak woodlands and forest types 

dominated by oak trees covered about 3.6 million hectares 

in California, or roughly 8.9 percent of the state’s land area 

(Waddell and Barrett 2005). Oak savanna (grasslands with 

less than 10 percent tree cover) was estimated to occur on an 

additional 0.73 million hectares (Bolsinger 1988). California 

native oaks also occur as components in riparian and desert 

plant communities, chaparral, and conifer-dominated forest 

ecosystems (fig. 1). 

Remnant native oaks have also become incorporated into 

developed areas within many California communities. The 

importance of oaks in the California landscape is reflected in 

the large number of California cities, localities, and geographic 

features that include references to oaks in either English or 

Figure 1. Approximate composite range of all native oak 

species in California.
Introduction 7

Spanish (encina, roble). In many urbanized landscapes, as in 

wildlands throughout much of California, individual oaks 

and oak woodlands are critical elements of the ecosystem.

Oak woodlands provide extremely important wildlife habitat 

and have higher levels of biodiversity than virtually any other 

terrestrial ecosystem in California. At least 300 terrestrial 

vertebrate species (Guisti and others 1996), 1,100 native 

vascular plant species (CalFlora 1998), 370 fungal species, 

and an estimated 5,000 arthropod species (Swiecki and 

others 1997) are associated with California oak woodlands.

This publication focuses on the relatively small number of 

microorganisms (primarily fungi) and arthropods (primarily 

insects) that are capable of causing noticeable damage to 

oaks in California. We have included agents that cause 

serious damage to oaks, as well as some common agents that 

produce conspicuous impacts even if they are not especially 

detrimental to oak health. Conspicuous but relatively 

inconsequential agents, such as cynipid gall wasps, often 

attract more attention than more cryptic agents that can 

severely impact oak health and structural integrity, such as 

canker rot fungi. However, some agents that have little or 

no impact on oak health may still create a nuisance in urban 

settings as the result of materials that are shed (e.g., sap) or 

because they adversely affect the appearance of oaks in the 

landscape.

Sudden oak death, caused by the pathogen Phytophthora 

ramorum, has emerged as a major disease of several California 

oaks. This new disease, which was first recognized as a 

problem in 1995, has highlighted the need for information 

about agents that damage oaks in California. This publication 

is primarily intended to help arborists, land managers, pest 

management specialists, and other professionals identify 

and assess the likely impacts of common agents that attack 

oaks in California. While we have incorporated enough 
8 A Field Guide to Insects and Diseases of California Oaks

Figure 2. 

Multicounty regions 

used in agent range 

maps.

technical information to make this document useful for 

professionals, property owners and other members of 

the general public can also use this publication to better 

understand common oak diseases and pests. A much 

more comprehensive compilation of agents that feed 

on, colonize, and/or damage oaks can be found in the 

California Oak Disease and Arthropod (CODA) database 

(currently available at http://Phytosphere.com/coda).

Due to a lack of detailed range data for most agents on 

California oaks, the included range maps are approximate. 

For most agents, range maps are based on multicounty 

regions (fig. 2). On the range maps for individual agents 

in the following pages, the agent’s documented range is 

shown in maroon. The likely but unconfirmed range, based 

on both the likely distribution of the agents and the range 

of the known oak hosts, is shown in gold. More detailed 

range data was available for a few agents, and for these 

species, we have generally used specific county boundaries 

to show the reported range. Agents may also occur beyond 

the illustrated ranges and are not necessarily found in all 

portions of the illustrated ranges.
9

Information on agent biology, host range, and importance 

provided in this publication will be helpful for determining 

whether management actions are likely to be necessary 

or practical. However, this publication does not provide 

specific recommendations for the management of the 

described pests and diseases. Readers interested in current 

management recommendations should consult the 

University of California Integrated Pest Management 

website (http://www.ipm.ucdavis.edu/), their local 

University of California Cooperation Extension office, or 

their local County Agricultural Commissioner’s Office.

Introduction
10 A Field Guide to Insects and Diseases of California Oaks

Filbert weevils

Curculio aurivestis, C. occidentis, 

C. pardus (Curculionidae)

Distribution/Hosts

Curculio aurivestis occurs from Victoria, British 

Columbia, to southern California. It is reported on canyon 

live, Oregon white, and other oaks. C. occidentis is widely 

distributed throughout the western U.S. and California. It 

is reported on a wide variety of oaks as well as tanoak and 

California hazelnut. C. pardus occurs from Washington to Los 

Angeles County, California. It is reported on coast live, interior 

live, canyon live, blue, valley, and other oak species.

Symptoms

Pinhead-sized oviposition wounds on acorns are commonly 

surrounded by a discolored and/or raised area and may exude 

small amounts of sap. Round, open holes (about 1–2 mm 

diameter) are left in the acorn seed coat when larvae exit. When 

infested acorns are cut open, dark brown granular frass caused 

by larvae tunneling throughout the interior of the acorn is 

visible. One or more (up to at least eight) filbert weevil larvae 

may be found in a single acorn. Filbert weevil and filbertworm 

larvae may occur in the same acorn (fig. 3).

Agent 

Description

Larvae are white to 

cream-colored and 

about 6–8 mm long; 

the head is small 

and brown. They are 

legless, plump, and 

relatively sluggish, and 

assume a curved Cshape when removed 

from an acorn (fig. 4). 

Figure 3. This valley oak acorn contained five 

filbert weevil larvae and one filbertworm larva 

(second from left). The exit hole (right) indicates 

that at least one additional larva has already 

exited the acorn.
Acorn Insects 11

Adult weevils are about 5.5–6.5 mm long, yellowish-brown, and 

have long slender snouts (fig. 5).

Biology

Adult weevils emerge from pupae in debris or soil beneath trees 

in summer. Oviposition occurs in summer and fall. The female 

weevil chews a small hole in the shell of a developing acorn and 

lays eggs in these holes. Larvae tunnel throughout the acorn. 

Heavily damaged acorns drop early, beginning in August or 

earlier. Larval feeding and development continues after the acorns 

have fallen. When the larva matures in the fall and winter, it 

bores an exit hole through the acorn seed coat and enters the 

ground, where it spends the winter. Larvae pupate in spring 

or summer. Only one generation occurs per year.

Importance

Severely damaged acorns are unable to sprout. Acorns without 

damage near the embryo axis at the pointed end of the acorn 

may still germinate, but seedling survivability may be reduced. 

Infestation levels can 

vary substantially 

with locality, year, oak 

species, and between 

individual trees at a 

particular locality. 

Infestation levels 

among acorns from a 

single tree can range up 

to at least 75 percent. 

Figure 4. Filbert weevil larvae and 

associated damage to a blue oak acorn.

Figure 5. Adult filbert weevil.
12 A Field Guide to Insects and Diseases of California Oaks

Filbertworm

Cydia latiferreana (Tortricidae)

Distribution/Hosts

Filbertworm is widely distributed throughout the 

U.S. and California. It attacks the acorns of most 

oak species as well as hazelnuts or filberts.

Symptoms

Larvae tunnel throughout the interior of acorns (fig. 6), leaving 

brown granular frass and sometimes silken webbing. Round 

open holes (about 1–2 mm diameter) are left in acorn seed 

coats when larvae exit.

Agent Description

Larvae are beige to light gray, about 18–20 mm long at 

maturity, with three pairs of true legs; the head is dark brown. 

Usually only a single filbertworm larva colonizes an infested 

acorn, but filbert weevil larvae may occur in the same acorn 

(fig. 3). Compared to the slow-moving larvae of the filbert 

weevil, filbertworm larvae are typically active when removed 

from an acorn and may drop down on a strand of silk when 

disturbed. Adults are small, stout-bodied moths with wingspans of 

12–15 mm (fig. 7). They have rust-brown forewings with several 

irregular dark or metallic bands and dark hindwings.

Figure 6. Filbertworm larva & damage to valley oak acorn.
Acorn Insects 13

Biology

Moths emerge from pupal cases in litter beneath trees in late 

spring and early summer, up to about two months before 

oviposition occurs. Female moths lay eggs throughout the 

summer on acorns that are still attached to the tree. Eggs 

are laid singly on the acorn surface. Larvae bore into acorns 

and feed internally. Heavily damaged acorns drop early, 

beginning in August or earlier. Insect development continues 

after the acorns have fallen. When the larva matures, in the 

fall or winter, it bores an exit hole through the acorn seed 

coat and pupates in plant debris on the ground. There is 

typically one generation per year, but two generations may be 

possible in some areas.

Importance

Heavily damaged acorns are unable to sprout. Acorns without 

damage near the embryo axis at the pointed end of the 

acorn may still germinate, but survivability may be reduced. 

Infestation levels can vary substantially with locality, year, 

oak species, and between individual trees at a particular 

locality. Infestation levels among acorns from a single tree 

can range up to at least 80 percent.

Figure 7. Filbertworm adult and pupal case.

Photo: Don Owen, CDF.
14 A Field Guide to Insects and Diseases of California Oaks

California oakworm, 

California oakmoth

Phryganidia californica (Dioptidae)

Distribution/Hosts

The California oakworm is generally found in 

coastal areas from Del Norte to San Diego County, and inland 

to near Riverside in the south and Davis in the north. It occurs 

on most oak species present within its range, as well as on 

tanoak.

Symptoms

Young larvae feed between veins on the lower leaf surface. 

Although the upper leaf surface is left intact, it dries out and 

turns brown. Larvae in later instars chew completely through 

the leaf blade, often leaving only major leaf veins. Small frass 

pellets drop from the canopy as larvae feed. In outbreak 

years, individual trees or groups of trees may be almost 

entirely defoliated, typically by late summer or early fall.

Agent Description

Eggs are round and clustered in groups of 20 or more. Eggs are 

initially white, but develop red centers that become pinkish to 

brownish before hatching occurs. Larvae (fig. 8) are black with 

lengthwise yellow stripes, and are about 3 cm long at maturity; 

the head is large, globose, and brown. Pupae are white or yellow 

with black markings. Adults are tan to gray moths with 

Figure 8. California 

oakworm larva feeding 

on coast live oak leaf.
General Foliar Feeders 15

prominent wing veins. Moths are about 15 mm long with 

wingspans of about 25 mm (fig. 9). 

Biology

Young larvae overwinter on the lower leaf surfaces of evergreen 

oaks. In northern California, overwintering larvae mature in 

May to June. First generation adults are present in June and 

July and lay eggs for the second generation of larvae. The 

second larval generation defoliates trees from July through 

September. Second generation adults are present in October 

and November and lay eggs that give rise to overwintering 

larvae. Evergreen or live oaks are attacked by both spring (first) 

and summer (second) generations; deciduous oaks normally 

avoid attack by the spring (first) generation. Development is 

more variable in southern California where three generations 

can occur per year and moths may be present at various times 

between March and November. Oakworm population levels 

cycle over a period of years, so that numbers may range from 

very high to nearly absent in any given year. 

Importance

Within its range, this is probably the most serious defoliator of 

oaks in California. When defoliation is severe, tree appearance is 

degraded and frass production may be a nuisance. In years with 

high oakworm populations, all trees in affected areas may be 

infested. However, defoliation seldom, if ever, kills affected trees. 

Figure 9. California oakworm adult moth.

Photo: Bruce Hagen, CDF.
16 A Field Guide to Insects and Diseases of California Oaks

Fruit tree leafroller

Archips argyrospila (Torticidae)

Distribution/Hosts

Fruit tree leafroller is found throughout the U.S. 

In California, it is found from Siskiyou County 

southward to San Diego County and occurs on almost all 

oak species as well as pome fruits, shade, and ornamental 

plants. The obliquebanded leafroller (Choristoneura rosaceana), 

which has similar larvae, has been reported on coast live oak 

in California. It occurs on many hosts throughout the U.S. 

Other larvae that cause similar damage on oaks in California 

include the European leafroller (A. rosana), the oak leaftier 

(Croesia semipurpurana), the stenomid oak leaftier (Setiostoma 

fernaldella), and the phycitid oak leaftier (Trachycera caliginella).

Symptoms

Larvae feed only in the spring on new leaves. Feeding occurs 

in young leaves that are rolled and tied with silk. Larvae chew 

through the entire leaf blade. Damaged leaves become distorted 

and ragged in appearance as the uneaten portions of the leaf 

continue to grow (fig. 10). During a heavy infestation, most 

or all trees in a local area may be severely defoliated. In most 

instances, trees outgrow the damage by producing new leaves.

Figure 10. Damage to newly expanding valley oak 

leaves from feeding of fruit tree leafroller larvae.
General Foliar Feeders 17

Agent Description

Eggs are cemented together into flat, grayish to whitish masses 

that become riddled with small holes when larvae emerge. 

Larvae are green and 1.5–2.5 cm long at maturity; the head 

is shiny and black (fig. 11). Larvae have a dark band behind 

the head on the prothorax; this band 

may be absent in the last larval instar. 

If disturbed, larvae wriggle actively and 

may drop from the leaf on a strand of 

silk. Obliquebanded leafroller larvae 

resemble late-instar larvae of the fruit 

tree leafroller, but do not react vigorously 

when disturbed. Adult fruit tree leafroller 

moths are bullet-shaped when viewed 

from above with variegated dark brown 

or yellowish tan wings; the wingspan is 

20–25 mm. 

Biology

The insect overwinters as masses of eggs on twigs and branches. 

Eggs hatch in the spring, usually at the same time that new 

leaves emerge. Larvae construct nests by rolling or tying young 

leaves together and feed within the nests. At maturity they 

pupate inside the nests or on bark in thin brown cocoons. 

Adult moths emerge eight to 12 days after pupation. After adult 

moths mate, females lay the overwintering egg masses. There is 

one generation per year. 

Importance

This is a common, well-known, and important insect pest of 

oaks. A severe multiyear outbreak affecting California black 

oaks in the San Bernardino Mountains of southern California 

was reported in 1999–2003. Before this, the last serious 

outbreak in southern California was in 1951–1953. In some 

trees, repeated defoliation combined with drought and high 

levels of leafy mistletoe infection has reportedly increased 

overall mortality rates.

Figure 11. Fruit tree 

leafroller caterpillar 

hanging from silken 

thread.
18 A Field Guide to Insects and Diseases of California Oaks

Oak ribbed 

casemaker

Bucculatrix albertiella (Lyonetiidae)

Distribution/Hosts

The oak ribbed casemaker occurs throughout 

California. It is reported on coast live, interior live, valley, 

and California black oak, but probably occurs on other oak 

species as well.

Symptoms

Young larvae feed within the leaf, creating a linear leaf mine. 

Older larvae cause necrotic windowing: they feed on the 

undersides of the oak leaves, sparing the upper epidermis and 

veins, which become brown and somewhat translucent. Small, 

flat circles of webbing (molting shelters) and small, white, 

narrow-cylindrical cocoons with longitudinal ribs may be 

present on the undersides of affected leaves (fig. 12). 

Agent Description

Mature larvae are yellowish to olive green with rows of pale 

spots and are about 6 mm long. Adult moths are mottled white, 

brown, and black, with a wingspan of 8–9 mm.

Figure 12. Oak ribbed casemaker cocoons and feeding 

on coast live oak. Note the round first-instar larval molting 

shelter to the left near the midrib. 

Photo: Bruce Hagen, CDF.
General Foliar Feeders 19

Biology

The oak ribbed casemaker overwinters as pupae inside 

cocoons. Adult moths emerge in early spring, mate, and lay 

eggs on oak leaves. Eggs hatch and the young larvae mine 

inside the leaf. After the first molt, larvae feed on the lower 

epidermis and interior tissues of leaves. Larval instars molt 

within flat, rounded shelters. Mature third-instar larvae 

spin the characteristic white ribbed cocoon. Adults emerge 

from cocoons, mate and lay eggs that hatch into the second 

(summer) generation. Larvae of the summer generation form 

overwintering pupae. There are two generations per year.

Importance

The oak ribbed casemaker is one of the more common insects 

affecting oaks and may cause serious leaf damage over large 

geographic areas in some years.
20 A Field Guide to Insects and Diseases of California Oaks

Figure 13. Tent (left) and larva (right) of the western tent caterpillar on coast 

live oak.

Tent caterpillars

Western tent caterpillar - 

Malacosoma californicum

Pacific tent caterpillar - M. constictum

Forest tent caterpillar - M. disstria

(Lasiocampidae)

Distribution/Hosts

The western tent caterpillar occurs west of the Rocky Mountains 

and in New York. Four subspecies (Malacosoma californicum

ssp. ambisimilis, californicum, fragile, and recenseo) with distinct 

geographic ranges are reported in California. The species is reported 

on oak, willow, poplar, birch, alder, madrone, ceanothus, redbud, 

hazel, ash, toyon, apple, almond, apricot, cherry, prune, plum, 

coffeeberry, currant, antelope bitterbrush, and apple. 

The Pacific tent caterpillar occurs in California, Oregon, 

and Washington and is reported only on oaks. In California, 

subspecies M. constrictum austrinum is found from Santa Barbara 

County southward; M. constrictum constrictum occurs from Los 

Angeles County northward.

The forest tent caterpillar is widespread throughout the entire U.S. 

Its host plants include oak, poplar, birch, alder, willow, cherry, 

peach, plum, prune, pear, apple, quince, hawthorn, and rose.
General Foliar Feeders 21

Symptoms

Larvae feed on leaves, often defoliating affected branches. 

Western tent caterpillar larvae spin large tents of silk (fig. 13) 

and the larvae do most of their feeding in the tents. Pacific tent 

caterpillars spin more rudimentary tents that can be up to 10 

cm wide (fig. 14). The larvae feed in groups outside the tent, 

entering it only to molt. Sometimes tents are absent in local 

populations of both the Pacific and the western tent caterpillar. 

Forest tent 

caterpillars do not 

construct tents; 

they construct 

silken mats on 

branches or the 

trunk where the 

larvae congregate 

when at rest or 

during molting.

Agent Description

Tent caterpillar larvae are fairly large, 40–50 mm in length 

at maturity. Adult moths have stout bodies and are tan to 

brownish, with two oblique lines on each forewing (fig. 15).

Western tent caterpillar larvae are blackish, with light blue 

speckles on the sides, and are covered with orange hairs (fig. 

13). Egg masses are pale gray to dark brown.

Pacific tent caterpillar larvae are blackish, with light blue 

speckles on the sides, orange hairs on top and grayish or cream 

colored hairs on the sides (fig. 16). The head is dark blue. Egg 

masses are golden yellow (fig. 17).

Forest tent caterpillar larvae are mostly dark blue, with wavy 

reddish brown lines and white, keyhole-shaped spots on their 

backs (fig. 18). Egg masses are dark brown.

Figure 14. Pacific tent caterpillar tent on blue oak.

Photo: Don Owen, CDF.
22 A Field Guide to Insects and Diseases of California Oaks

Figure 16. Pacific tent caterpillar larva on blue oak.

Photo: Don Owen, CDF; www.forestryimages.org.

Figure 15. Pinned specimen of the Pacific tent caterpillar female moth.

Photo: Don, Owen, CDF.

Figure 17. Pacific tent caterpillar egg mass cut away to show 

individual eggs.

Photo: Don Owen, CDF.
General Foliar Feeders 23

Biology

Overwintering eggs are 

laid during the summer in 

masses that encircle twigs. 

Eggs are coated with a 

varnish-like substance 

called spumaline. The 

young larvae are active 

from early to late spring. 

Young larvae congregate 

and spin communal tents. 

Mature larvae may spin 

cocoons (fig. 19) in folded 

leaves, or under bark or 

litter. Adult moths emerge 

in mid-summer, mate, 

and lay eggs. There is one 

generation a year.

Importance

Other than obvious aesthetic impacts, tent caterpillars typically 

do not cause major damage on California oaks. When present 

in high numbers, caterpillars may be a nuisance.

Figure 18. Forest tent caterpillar larvae.

Photo: Robert S. Kelley, Vermont Department of Forests, Parks and Recreation; www.forestryimages.org.

Figure 19. Pacific tent caterpillar cocoons.

Photo: Don Owen, CDF.
24 A Field Guide to Insects and Diseases of California Oaks

Western tussock 

moth

Orgyia vetusta (Lymantriidae)

Distribution/Hosts

Western tussock moth occurs from southern 

California to British Columbia. It is reported on virtually all 

California oak species as well as various fruit and nut trees, 

ceanothus, hawthorn, manzanita, pyracantha, toyon, walnut, 

and willow. 

Symptoms

Larvae feed on leaves, especially young growth, sometimes 

causing substantial defoliation. Masses of cocoons may be 

present on branches in late spring.

Agent Description

Eggs are white, nearly spherical, 1.2 mm in diameter, and 

depressed on top. Egg masses may contain over 100 eggs. 

Egg masses are gray and felt-like, and include hairs and 

scales from the adult female’s body. Mature larvae are 13–22 

mm long, hairy, and have red spots on their sides and four 

prominent white tufts of hair (tussocks) on their backs (fig. 

20). The adult male moth is about 2.5 cm across when at rest. 

Its wings are brown and have two evenly spaced irregular dark 

Figure 20. Western tussock moth larva on blue oak.
General Foliar Feeders 25

Figure 21. Western tussock moth cocoons on coast live oak.

brown lines which run perpendicular to the body. The female 

is covered with wavy light gray hair, has greatly reduced wings, 

and cannot fly.

Biology

Western tussock moth overwinters as eggs laid in masses on 

twigs, on bark of larger stems, or on old cocoons. Eggs hatch 

from March through early June. Larvae are present for six to 

eight weeks. Mature larvae spin loosely woven grayish-white 

cocoons that may contain larval hairs (fig. 21). Pupae mature in 

two to three weeks. Adult moths are active from May to July. 

There is usually one generation a year, but two generations can 

occur in southern California. Where the second generation 

occurs, larvae are present late August through October, and 

adults lay overwintering eggs in September and October.

Importance

Oaks may occasionally be seriously defoliated. Damage on 

other hosts can sometimes cause branch dieback. Hairs from 

western tussock moth larvae can be very irritating to humans. 

Frass may be a nuisance when larval populations are high.
26 A Field Guide to Insects and Diseases of California Oaks

Figure 22. Symptoms of erineum mite on coast live oak.

Erineum mite

Eriophyes mackiei (Eriophyidae)

Distribution/Hosts

Eriophyes mackiei is distributed through much of 

California. It is reported on evergreen oaks in the 

red/black oak subgroup (coast live and interior live oak) and 

intermediate oak subgroup (canyon live and huckleberry oak).

Symptoms

The mites cause blisterlike swellings, 1–12 mm long, that 

protrude from the upper leaf surface (fig. 22). On the lower 

leaf surface, the blisters appear as depressions filled with mats 

of rusty brown hairs (erinea). The blisters are initially green 

but later become brown. Leaves may become distorted if large 

populations of mites are present.

Agent Description

Mites are 0.2 mm long and difficult to see without 

magnification. They are whitish or yellowish, slender and 

wormlike, and have two pairs of legs. 
Gall Formers 27

Biology

Erineum mites overwinter in leaf blisters or in leaf buds. 

They establish new colonies on the underside of spring 

foliage. The mites induce the host leaf to produce the 

characteristic erinea in the blisterlike depressions that form 

on the underside of leaves. Mites feed and reproduce within 

the blisters. 

Importance

The damage caused by erineum mites is generally limited and 

of little or no consequence to affected trees.
28 A Field Guide to Insects and Diseases of California Oaks

Gall wasps

Over 100 species in about 20 genera 

including Andricus spp., Antron spp., 

Callirhytis spp., Disholcaspis spp., Dros

spp., Neuroterus spp. (Cynipidae)

Distribution/Hosts

Many gall wasp species are found throughout the range of their 

hosts, whereas others are more limited in distribution. Virtually 

all native California oak species are hosts to one or more gall 

wasp species. Most gall wasp species can colonize several to 

many oak species within a given subgroup such as the white 

oaks, red/black oaks, or intermediate oaks. Andricus californicus, 

the California gallfly, produces galls known as oak apples on at 

least eight species in the white oak subgroup (fig. 23). Others, 

such as Andricus chrysobalani, which causes galls on canyon live 

oak acorns, are reported only from a single host species.

Symptoms

Galls may be produced on catkins, acorns, leaves, petioles, or 

twigs (fig. 24–27). They appear as simple swellings, amorphous 

growths, or unique, highly organized structures, often with 

strange shapes and 

bright coloration. Galls 

may be detachable 

(distinct structures that 

are readily removable 

from underlying tissues) 

or integral (swellings or 

distortions of the plant 

part itself that cannot 

be removed without 

cutting or breaking the 

affected part). Galls 

range from 1 mm to 

more than 50 mm 

Figure 23. Oak apple galls on valley oak across. Some galls are produced by Andricus californicus.
Gall Formers 29

associated with localized chlorosis or necrosis of underlying 

host tissue. Galls affecting branches or major leaf veins may 

interfere with water and food transport within the branch or 

leaf, causing tissues beyond the gall to decline or die.

Agent Description

Adults are small wasps, typically no more than about 4 mm 

long (fig. 28). Larvae are small, whitish, and legless and 

develop completely inside larval chambers that form within 

the galls. Depending on the wasp species, each gall may have 

either one (monothalamous galls) or many (polythalamous 

galls) larval chambers.

Biology

Gall wasp larvae can apparently only induce galls in 

undifferentiated meristematic or cambial cells, so female wasps 

must lay eggs in specific host tissues (buds, leaves, etc.) when 

the tissues are at the correct growth stage. Developing larvae 

produce substances that induce the plant to form a larval 

chamber of a structure that is unique to the wasp species and 

generation. The gall wasp larvae typically remain small while 

they induce the gall to grow rapidly. As the gall nears its mature 

size, larvae feed on nutritive tissue produced in the center 

of the gall and begin to grow rapidly. As the larvae mature, 

they induce 

lignification 

of the gall. In 

detachable galls, 

lignification may 

induce the gall to 

fall off the plant. 

Figure 24. Andricus 

crystallinus galls on 

blue oak leaves.
30 A Field Guide to Insects and Diseases of California Oaks

Larvae pupate within the gall and adult wasps emerge from the 

galls to initiate the next generation. 

Many oak gall wasp species have two alternating generations; 

a sexual generation with females and males followed by an 

asexual generation with only females, which can lay fertile 

eggs without mating. Each of these generations produces a 

unique gall, often on different parts of an oak. Galls produced 

by the sexual generation are usually found during winter 

Figure 26. Stem galls produced by Andricus 

chrysolepidicola on blue oak.

Figure 25. Andricus fullawayi galls on leaves of valley oak.
Gall Formers 31

Figure 27. Disholcaspis plumbella

gall on scrub oak.

Figure 28. Adult gall wasps. 

Photo: Don Owen, CDF.

on permanent parts of the tree, such as twigs. The asexual 

generation typically is produced during spring and summer, 

and galls are often on leaves and catkins. Some oak gall wasps 

are known to produce only a single generation, which is 

typically asexual.

Importance

Most wasp galls, especially foliar galls, cause little or no lasting 

damage to oaks. However, a few galls cause limited leaf or twig 

dieback by blocking the vascular tissue.
32 A Field Guide to Insects and Diseases of California Oaks

Kuwana oak scale

Kuwania quercus (Margarodidae)

Distribution/Hosts

In California, Kuwania quercus has been found in 

Amador, Napa, San Joaquin, San Mateo, Santa 

Clara, Solano, Sonoma, and Yolo Counties, but may be more 

widespread. It has been observed on blue oak and unidentified 

live oaks (probably coast live and/or interior live oak) and in an 

American chestnut orchard. It was originally described on oaks 

in Japan, but also occurs on oaks in China and Formosa. Gill 

(1993) indicates that California specimens may actually be a 

different species from the Asian species. 

Symptoms

In blue oak, infestations are associated with obvious roughening 

and exfoliation of bark (fig. 29). The ground close to the trunk of 

infected trees may be covered with pieces of bark that have flaked 

off the tree. Hard grayish or whitish waxen capsules that surround 

the pre-adult females can be seen under loose bark in spring. 

Figure 29. Exfoliating bark of blue oak infested with Kuwana oak scale.
Sap Feeders 33

Agent Description

Bright red ovate-shaped adults are about 1.75–2.25 mm long. 

Adult females produce a dorsal mass of white, waxy threads. 

They are secretive and often hide under rough, loosened bark. 

The pre-adult female is bright red, legless and covered with 

a hard grayish or whitish 

waxy capsule. Only waxy 

residue may be visible 

on the bark of affected 

trees through much of 

the summer and fall (fig. 

30, 31).

Biology

The life cycle of this insect 

in California is unknown. 

In Solano County, preadult females have been 

observed in mid-March; 

mature females laid eggs 

by mid- to late April.

Importance

Infestations tend to 

be highly localized in 

distribution. Other than 

obvious visual impacts due 

to bark flaking, long-term 

impacts on tree health are 

unknown.

Figure 30. Reddish Kuwana oak scale 

females and associated waxy filaments 

under blue oak bark.

Figure 31. Reddish Kuwana oak scale 

females and associated waxy filaments 

(arrows).
34 A Field Guide to Insects and Diseases of California Oaks

Oak leaf phylloxera

Phylloxera spp. (Phylloxeridae)

Distribution/Hosts

One or more Phylloxera spp. are found through 

much of California west of the Sierra Nevada, as 

well as in the southeastern U.S. and the Pacific Northwest to British 

Columbia. Oaks in the white oak subgroup are attacked, including 

valley, Oregon white, Engelmann, and blue oak. 

Symptoms

Small (about 1 mm diameter) yellow and brown spots are visible 

on the upper and lower surfaces of affected leaves (fig. 32); spots 

are most noticeable in mid summer to early fall. Spots form where 

the small, yellow to brownish, wingless insects feed. Spots may be 

located primarily along veins or spread over the entire leaf surface. 

Different species may have different distribution patterns on leaves. 

If population levels are high enough, part or all of an affected leaf 

may turn brown and die (fig. 33). Symptoms are commonly most 

severe on oak seedlings and saplings. 

Agent Description

Eggs are oval, about 0.3 mm long. Nymphs and adults are oval, 

with fine ridges and short spine-like projections (tubercles), and 

very short legs (fig. 34). The taxonomic status of some species 

reported in California 

is not completely 

resolved. At least five 

species are described 

on oaks in California. 

P. davidsoni wingless 

adults (apterae) 

are light orange to 

Figure 32. Oak leaf 

phylloxera symptoms 

on valley oak; this 

unidentified species tends 

to congregate along veins. 
Sap Feeders 35

yellowish-brown and 

about 0.7 mm long. 

Winged adults (alatae) 

are orange with a black 

thorax. Immatures are 

pale yellow. P. querceti

apterae are yellow 

to orange and 0.6–1 

mm long. P. reticulata

apterae are bright 

orange-yellow, about 

0.8 mm long, and have 

reddish eyes. P. rileyi

apterae are dark brown 

with almost black 

dorsal tubercles and are about 0.6 mm long. Paler forms also occur. 

P. stanfordiana apterae are pale yellow and about 0.75 mm long. 

Biology

Life cycles of these species have not been studied in California. Data 

from other locations indicate that overwintering occurs as eggs or 

first-instar nymphs in sheltered locations on the branches. Multiple 

generations occur over the spring and summer, some of which 

involve asexual reproduction. Winged adults develop in at least some 

species. Populations build up over the summer and are highest in 

September and October. 

Importance

The species of oak leaf phylloxera known 

from California are apparently of minor 

importance, although damage to seedlings 

may be significant in some situations. The 

oak leaf phylloxera (P. glabra) is reported 

to be a severe pest of Oregon white oak 

in British Columbia, causing significant 

scorching and defoliation of mature 

trees. P. glabra has not been reported in 

California to date.

Figure 33. Damage associated with high oak leaf 

phylloxera populations, here on valley oak, may 

include leaf scorch (right). Lady beetles (note 

adult at left and pupal case at right) prey upon 

phylloxera.

Figure 34. Close-up of 

several phylloxera nymphs 

on the underside of a 

valley oak leaf.
36 A Field Guide to Insects and Diseases of California Oaks

Oak lecanium scale

Parthenolecanium quercifex (Coccidae)

Distribution/Hosts

Oak lecanium scale is found on oaks in many parts 

of the eastern U.S. In California, it is reported on 

coast live and valley oak from the Sacramento Valley and the 

San Francisco Bay Area to southern California. Other reported 

hosts include other oak species, sycamore, pecan, chestnut, 

birch, persimmon, and pricklyash. 

The very similar and related European fruit lecanium, P. corni, 

is reported on California black oak in the northern Sierra 

Nevada. P. corni is common throughout California and the 

U.S. on many hosts.

Symptoms

Globose brown scale insects occur on twigs and produce 

honeydew. Black sooty mold grows on the honeydew that drips 

on leaves and twigs. Very heavy infestations can cause twig 

dieback and stunted leaves.

Figure 35. Close-up of oak lecanium scale.

Photo: Bruce Hagen, CDF.
Sap Feeders 37

Agent Description

Mobile nymphs (crawlers) are flat and orangish and occur on 

leaves or twigs. Adult scales are immobile and occur on twigs. 

Adults are hemispherical, 4–6 mm long, brown, and have a 

smooth, shiny surface that may include some white powdery 

material (fig. 35, 36). Females have a pair of lateral bumps. 

Biology

Nymphs overwinter and mature into adults in the spring. 

Winged adult males mate with wingless females. Eggs are laid 

beneath the female in late spring. First-instar nymphs emerge 

from beneath the female and crawl to the leaves where they 

feed. After settling they retain their tiny legs and antennae and 

are able to move very slowly. The second-instar nymphs migrate 

back to twigs in late summer, where they overwinter. This insect 

has one generation per year.

Importance

Honeydew dripping and subsequent sooty mold growth may 

become a nuisance. Greatest impacts of this insect reportedly 

occur on coast live oak in southern California.

Figure 36. Oak lecanium scale.

Photo: Don Owen, CDF.
38 A Field Guide to Insects and Diseases of California Oaks

Obscure scale

Melanaspis obscura (Diaspididae)

Distribution/Hosts

Obscure scale is widely distributed in the eastern 

and southeastern U.S., particularly in warmer areas. It has been 

established in Sacramento, CA, for an extended period, but 

is now controlled by an introduced parasitoid. Infestations in 

San Diego and Los Angeles Counties occurring before 1940 

were eradicated. In California, obscure scale has been reported 

on native coast live, interior live, and oracle oak (natural 

hybrid between interior live and California black oak), and 

the introduced pin, willow, and northern red oaks. It prefers 

oak and pecan, but is found on many other woody species 

including hickory and walnut.

Symptoms

These scale insects are inconspicuous due to gray covers 

and may be found on twigs, branches, trunks, and exposed 

roots, sometimes under loose bark. They often settle close 

together, resulting in masses of overlapping scale insects. Heavy 

infestations may cause twig and branch dieback, which can 

weaken trees, increasing their susceptibility to other opportunistic 

agents. Obscure scale does not produce honeydew.

Agent Description

Adult scale covers are gray, circular to oval, 2–3 mm in 

diameter, flat to slightly convex, and may have an off-center 

dark spot, which is the first-instar cast skin. Scales are often in 

overlapping clusters (fig. 37). A noticeable white spot is left on 

the host stem if the scale is removed. 

Biology

Eggs hatch beneath females in mid summer. Crawlers (mobile 

nymphs) are very tiny, about the size of a period, and flat. 

They settle close together on the bark of the host, often 

under existing adult shells. Once the crawler begins to feed 
Sap Feeders 39

it produces a clear wax shell. Nymphs continue to grow to 

adulthood beneath the protective wax shell. Winged adult 

males crawl out from under their armored shells in the spring 

to mate with immobile females, which lay eggs beneath their 

bodies. Egg laying can last for an extended period. There is one 

generation per year.

Importance

This scale was introduced to California from the eastern 

U.S., where it is a significant pest of oaks in landscapes, but 

is not a forest pest. It was originally discovered in 1933 in 

southern California infesting pecans, although the Sacramento 

infestation may date to 1897, when trees from Civil War 

battlefields were transplanted into Capitol Park. In Sacramento, 

a tiny wasp parasitoid (Encarsia aurantii) introduced from Texas 

has provided complete biological control (Ehler 2005). 

Figure 37. Overlapping clusters of obscure scale 

on an infested twig.

Photo: John A. Weidhass, Virginia Tech; www.forestryimages.org
40 A Field Guide to Insects and Diseases of California Oaks

Pit scales

Oak pit scale - 

Asterolecanium minus, A. quercicola

Golden oak scale - A. variolosum

(Asterolecaniidae)

Distribution/Hosts

Asterolecanium minus and A. quercicola are widely distributed in 

California but A. variolosum is reportedly rare (Gill 1993). All 

three species are common in other parts of the U.S. on oaks. In 

California, A. minus and A. quercicola prefer white oaks including 

valley, blue, and Engelmann oak, and some non-native white 

oaks, but are also found on coast live and California black oak. A. 

variolosum is reportedly restricted to English oak and California 

black oak in California. Reports of this species on other oaks may 

be due to confusion between the three species of Asterolecanium. 

Additional oaks in the white and red/black oak subgroups are 

hosts of these pit scales in the eastern U.S.

Symptoms

Pinhead-sized scales are found on twigs and 

small branches (fig. 38-40). Each scale is in 

a depression that develops in the center of 

a ring of swollen bark. Heavy infestations 

cause the normally smooth bark of young 

twigs to become rough. Poor twig growth 

Figure 38. Oak pit scale. Swellings that give rise 

to pitted appearance have not yet formed around 

some of the younger scales.

Photo: Bruce Hagen, CDF.

Figure 39. Small 

coast live oak 

branch with 

tangential slice 

of bark and 

wood removed 

showing necrosis 

associated with 

oak pit scales.
Sap Feeders 41

and dieback are common results of pit scale infestation. Deciduous 

oaks retain dead leaves on killed twigs throughout the winter. Damage 

to oaks is caused by the removal of plant fluids and perhaps the 

injection of toxins into twigs. Pit scales do not produce honeydew. 

Agent Description

Diameters of adults vary somewhat by species: 1–1.5 mm for A. 

minus, 1.25–1.75 mm for A. quercicola, 1.75–2.25 mm for A. 

variolosum. All three species are otherwise similar in appearance. 

Scales are rounded and green, gold, or brown, depending on age. 

Biology

Only females are known. There is only one generation a year, but 

an individual female may produce young for up to five months. 

Females overwinter in pits on twigs. Eggs are laid beneath the 

female’s protective cover. Most mobile nymphs (crawlers) emerge 

from beneath the females from April to June, but some may emerge 

as late as October. The crawlers settle near the parent on twigs not 

more than a year old. Once settled, the scale remains immobile for 

the rest of its life.

Importance

Pit scales are serious pests of oaks in California, especially in 

southern California. Pit scales tend to be more common on 

deciduous oaks 

than on live 

oaks, and more 

common on oaks 

in landscapes than 

in wildlands.

Figure 40. Golden 

oak scale. Pits 

have not yet 

formed around 

these scales.

Photo: Jack Kelly Clark, 

courtesy UC Statewide 

IPM Program
42 A Field Guide to Insects and Diseases of California Oaks

Treehoppers

Platycotis vittata, P. vittata quadrivittata

(Membracidae)

Distribution/Hosts

Platycotis vittata is widely distributed in California 

and is found throughout the U.S. It is reported on various oaks 

in California, including coast live and valley oak. In other parts 

of the U.S., P. vittata is reported on oaks, birch, chestnut, and 

other hardwoods. P. minax occurs on coast live oak and possibly 

other oaks in southern California. 

Symptoms

Slit-like egg-laying (oviposition) wounds on fine branches can 

cause leaves to wilt and die and may cause twig dieback. Callus 

developing around these longitudinal oviposition slits may leave 

scars on affected twigs. Honeydew produced by treehoppers 

drips on foliage and branches and can support growth of black 

sooty mold on these surfaces.

Agent Description

P. vittata nymphs are black with yellow and red markings 

(fig. 41). Adults are 9–12 mm long, and have an enlarged 

hardened plate 

(pronotum) 

on the back 

of the thorax. 

The pronotum 

covers the 

insect’s head, 

much of its 

abdomen, and 

part of the 

Figure 41. 

Platycotis vittata

adults and 

nymphs.

Photo: Bruce Hagen. 

CDF.
Sap Feeders 43

prominently-veined wings (fig. 42). In P. vittata, two lateral 

horns project from the pronotum; between these two horns 

there may be a central forward facing horn. P. vittata adults vary 

in color (dull olive, bronzy, or sea green) and have small red pits 

on the pronotum. P. vittata quadrivittata adults are a pale blue 

color with four red longitudinal stripes and usually have welldeveloped central and lateral pronotal horns. Both immatures 

and adults jump.

Biology

P. vittata adult females overwinter and oviposit in slits or 

crescent-like punctures they cut in the bark of twigs in spring. 

Females remain with eggs and nymphs as they mature. In the 

spring generation, nymphs are gregarious and aggregate along 

twigs. Spring-generation adults give rise to a second generation 

later in the summer.

Importance

Although damage may sometimes be conspicuous, damage to 

mature trees is generally minor. Large amounts of twig dieback 

may stunt affected seedlings or saplings.

Figure 42. P. vittata adults.

Photo: Bruce Hagen, CDF.
44 A Field Guide to Insects and Diseases of California Oaks

Figure 43. Close-up of crown 

whitefly pupae on underside of a 

coast live oak leaf.

Whiteflies

Crown whitefly - Aleuroplatus coronatus; Gelatinous 

whitefly - A. gelatinosus; Stanford’s whitefly - 

Tetraleurodes stanfordi (Aleyrodidae)

Distribution/Hosts

Crown whitefly is found throughout California. It 

prefers coast live oak, but is also reported on canyon live, 

interior live, valley, and scrub oak, as well as chinquapin, 

tanoak, coffeeberry, toyon, madrone, and Rhus.

Gelatinous whitefly is reported from the San 

Francisco Bay Area southward through the San 

Joaquin Valley and southern California on coast live 

and interior live oak.

Stanford’s whitefly is found mainly throughout 

coastal California. It is reported on coast live, 

California black, and interior live oak as well as 

chinquapin, tanoak, and coffeeberry.

Symptoms

Non-mobile nymphs and pupae on leaves are the most 

conspicuous and identifiable sign of an infestation. 

They appear as small (less than 1 mm) black or black and white 

spots on leaves which can be wiped off. Crown and gelatinous 

whiteflies occur only on lower leaf surfaces, whereas Stanford’s 

whitefly mostly feeds on the upper leaf surface. At certain times 

of the year, the adult whiteflies may fly around infested plants, 

sometimes in great numbers. The honeydew secreted by nymphs 

accumulates on leaves and 

supports the growth of black 

sooty mold. Very heavy 

infestations may cause some 

leaf distortion, yellowing, and 

premature leaf drop. 
Sap Feeders 45

Agent Description

Adult whiteflies look like tiny (about 1 mm) white moths. Eggs are 

white to pink and are attached to the leaf surface by a short stalk. 

Nymphs are initially translucent but become black as they mature. 

Pupae are tiny, black, oval, and flat. Species differ in the amount 

and color (clear or white) of wax associated with the pupae. Crown 

whitefly pupae are covered with somewhat rectangular plates of white 

wax that project outward in all directions (fig. 43, 44). Gelatinous 

whitefly pupae are surrounded by a matrix of clear, gelatinous wax. 

Stanford’s whitefly pupae are surrounded by a flat fringe of white 

waxy plates (fig. 45). Nymphs of crown whitefly resemble Stanford’s 

whitefly pupae.

Biology

Adults emerge from overwintering 

pupae in spring and lay eggs on 

the lower leaf surface. Eggs hatch 

to give rise to tiny mobile nymphs 

called crawlers. Within hours, 

crawlers settle down and insert 

their mouthparts into the leaf to 

suck plant sap from phloem cells. 

Whitefly nymphs remain immobile 

as they go through successive molts. 

The last molt gives rise to pupae which overwinter. Crown whitefly 

has a single generation per year.

Importance

Whiteflies do not cause significant damage to oaks and are typically of little 

or no importance. Crown whitefly populations can sometimes reach high 

enough populations to pose 

a nuisance, mainly due 

to honeydew production 

and related sooty mold 

growth. The gelatinous 

and Standford’s whitefly 

generally occur in lower 

numbers than the crown 

whitefly. 

Photo: Bruce Hagen

Figure 44. Close-up of a crown 

whitefly pupa on a coast live oak leaf.

Figure 45. Stanford’s whitefly pupae on coast 

live oak.

Photo: Bruce Hagen, CDF.
46 A Field Guide to Insects and Diseases of California Oaks

Woolly oak aphids

Stegophylla querci, S. quercifoliae, S. essigi

(Aphididae)

Distribution/Hosts

Stegophylla querci (=S. quercicola) is widely 

distributed in North America and much of 

California. In California, it occurs on blue, valley, 

coast live, interior live, California black, and scrub 

oak and probably other oaks. 

S. quercifoliae is found in the San Francisco Bay Area 

on at least blue oak and coast live oak. It occurs on 

other oak species in other portions of the U.S. 

S. essigi is known only from the West Coast, and 

is reported from many California oaks, including 

blue, valley, Oregon white, coast live, interior live, 

California black, and scrub oak. However, there is 

only limited information on its distribution.

Symptoms

Masses of white wax filaments produced by S. querci and S. 

quercifoliae nymphs and adults are visible on infested leaves (fig. 

46). S. querci forms small, scattered colonies on upper or lower 

leaf surfaces. S. quercifoliae infests the undersides of leaves. Large 

colonies may cause leaves to curl. Feeding by first generation S. 

essigi in spring causes the 

edges of young leaves to 

fold over to the upper 

side of the leaf, forming 

a protective shelter in 

which the aphids feed and 

where wax accumulates 

(fig. 47). The folded leaf 

area may show a reddish 

discoloration initially and 

eventually may die. Figure 46. Woolly oak aphid (Stegophylla 

querci) colonies on valley oak.
Sap Feeders 47

Agent Description

Wingless adults (apterae) of all species are covered in white wax 

wool. S. querci apterae are oval, pale greenish- or brownish-yellow, 

1–1.5 mm long, and have dense wax wool (fig. 48). S. quercifoliae

apterae are yellowish to yellowish green with a brown head, 1.1–1.5 

mm long, and have loose wax wool. S. essigi apterae are broadly 

oval, pale gray-green to olive and 1.2–1.8 mm long.

Biology

Woolly oak aphids overwinter as eggs. Several generations develop 

each year. In the spring, females reproduce parthenogenically 

(without mating). In the fall, male and female aphids are produced. 

Adult males may be either winged or wingless. S. querci has 

sexual females (oviparae) and both wingless and winged males in 

September and October. For S. essigi, oviparae and both wingless 

and winged males are produced in November. Populations of S. 

essigi may continue to reproduce parthenogenically on evergreen 

oaks such as coast live oak, especially in leaves tied together by 

caterpillar silk.

Importance

Woolly wax deposits may become conspicuous and possibly 

unsightly, especially late in the season. Damage to trees is normally 

minimal.

Figure 47. Symptoms of S. essigi on Figure 48. Close-up of S. querci nymph.

coast live oak; affected leaf margins 

are folded upward and become 

reddish.
48 A Field Guide to Insects and Diseases of California Oaks

Oak twig borer

Styloxus fulleri californicus 

(Cerambycidae)

The California subspecies of oak twig borer 

(Styloxus fulleri californicus) is widespread in 

California and has been reported on coast live, interior live, 

and California black oak. S. fulleri californicus makes a large, 

oval, frass-filled tunnel in the heartwood of twigs, particularly 

those that are 0.5–1.25 cm in diameter. Larvae are yellowish 

roundheaded borers, 12.5–19 mm long. Adult beetles (fig. 49) 

are slender and greenish brown.

Figure 49. Pinned specimen of adult oak twig borer.

Photo: Larry G. Bezark, CDFA.
Twig Borers 49

Other twig borers

Aneflomorpha lineare (Cerambycidae)

Aneflomorpha 

lineare is a twig 

borer reported on coast 

live and scrub oak in 

southern California; it 

is also found in Oregon. 

Other hosts of A. lineare

include bitterbrush and chamise. Larvae of A. 

lineare bore through small branches, making 

holes at frequent intervals through which 

frass is expelled. Upon reaching the point 

where the branch diameter is about 2–4 cm 

in diameter, larvae construct tunnels that 

spiral outward from the center of the branch. 

Adults are slender and brown (fig. 50). Adults 

are present from July to September and are 

attracted to light. 

Scobicia suturalis (Bostrichidae)

Scobicia suturalis, a false powder post beetle, 

is reported from the San Francisco Bay 

Area. It mines the small branches and twigs 

of coast live oak, California black oak, and 

acacia, as well as grape canes. Adults of S. 

suturalis are cylindrical and 5 mm long. The 

head and abdomen are black, the prothorax 

and posterior half of the elytra are dark reddish brown, and the 

antennae, legs, and front half of the elytra are pale amber in color.

Figure 50. Pinned 

specimen of adult 

Aneflomorpha 

lineare.

Photo: Larry G. Bezark, 

CDFA.
50 A Field Guide to Insects and Diseases of California Oaks

Oak twig girdler

Agrilus angelicus (Buprestidae)

Distribution/Hosts

The oak twig girdler is widespread in California 

and is reported from other states including Oregon 

and Nebraska. In California, coast live oak may be the most 

commonly affected species, but oak twig girdler is also reported 

on canyon live, interior live, California black, Engelmann, 

leather, and valley oak. 

Symptoms

Mining by the oak twig girdler beneath the bark girdles small 

diameter twigs, causing the portion of the twig beyond the 

mine to die. Heavy infestations of oak twig girdler, which are 

relatively common on coast live oak in southern California, 

give rise to numerous patches of dry, dead leaves scattered 

in the canopy (fig. 51). Because other twig borers and agents 

such as twig blight fungi can cause similar symptoms, it 

is necessary to remove the bark from affected twigs at the 

junction between live and dead tissues to determine the cause 

of the damage. Oak twig girdler larvae form flattened tunnels 

under the bark that spiral around the twig from the younger 

(typically dead) portion of the 

twig toward the point of branch 

origin (fig. 52). Dark brown 

granular frass and whitish larvae 

may be found in the tunnels.

Figure 51. Twig dieback due to oak 

twig girdler damage on coast live oak.
Twig Borers 51

Agent Description

Oak twig girdler adults are 7 mm long and dark metallic 

brownish bronze. The larvae are up to 19 mm long, white, 

legless, somewhat flattened, and constricted at each body 

segment (fig. 53). The flattened head is large relative to the 

body. 

Biology

Adults emerge from pupae between May 

and September. Adults mate and the female 

lays eggs singly on the youngest twigs. 

Eggs hatch after two weeks and larvae bore 

directly into twigs. The young larva makes 

a slender linear tunnel for up to three to 

six months as it moves toward the older 

portion of the twig. The tunnel then begins 

to spiral around the twig. The larva feeds for 

two years in spiral tunnels, which can extend 

30 cm or more down the twig. The larva then bores to the center 

of the twig and tunnels up to 15 cm back toward the killed 

portion, where it pupates near the twig surface. After several 

weeks it emerges as an adult. A single generation thus requires 

about two years. 

Importance

Oak twig girdler is believed to be attracted to trees weakened 

by drought. High levels of damage can be unsightly, but overall 

impacts to tree health are minor to insignificant.

Figure 53. Oak twig 

girdler larva.

Photo: Bruce Hagen, CDF.

Figure 52. Oak twig girdler boring on coast live oak; bark has been 

removed to show the spiraling larval tunnel. 
52 A Field Guide to Insects and Diseases of California Oaks

Ambrosia beetles

Monarthrum dentiger, M. scutellare (Scolytidae)

Distribution/Hosts

Monarthrum scutellare is widespread throughout 

California and Oregon on various oak species, 

including coast live, California black, and valley 

oak. It also occurs on tanoak. 

M. dentiger is less common than M. scutellare and 

is reported in Arizona and California. It occurs 

on coast live and valley oak, and probably on 

other oak species. It has also been reported on 

avocado and walnut.

Symptoms

The most obvious symptom of infestation is fine, light-colored 

boring dust that accumulates on the bark surface and/or around 

the tree’s base (fig. 54). Small, cone-shaped piles of boring dust, 

which are pushed out of small (about 1–1.5 mm diameter) 

holes in the bark, may be visible on the surface of infested 

trunks and branches. In cross-sections through affected areas, 

thin, black-stained tunnels can be seen extending 2–15 cm into 

the wood (fig. 55, 56). Tunnels often branch from a single point 

near the cambium or at the end of a short length of tunnel that 

extends into the outer sapwood. 

Figure 54. White 

boring dust 

produced by 

ambrosia beetles 

on the surface of 

a tanoak infected 

with Phytophthora 

ramorum.
Bark and Wood Boring Insects 53

Figure 55. Broken 

trunk of a failed 

coast live oak 

that was killed 

by Phytophora 

ramorum (sudden 

oak death) shows 

a very high density 

of ambrosia beetle 

galleries. Although 

ambrosia beetle 

galleries are 

common in trees 

with this disease, 

only some trees 

show such high 

gallery densities. 

Agent Description

Adults are small dark brown to blackish cylindrical beetles (fig. 

57). From above, the head is completely hidden beneath a 

bullet-shaped plate (pronotum) over the thorax. M. scutellare

adults are 3.5–4.1 mm long. The smaller M. dentiger adults are 

1.9–2.4 mm long. 

Biology

Both species tunnel into the wood of trunks or branches that 

are severely stressed, diseased, dying, or recently dead. Ambrosia 

beetles commonly invade trees with Phytophthora ramorum

cankers (sudden oak death), initially in the vicinity of cankers. 

As diseased trees decline, parts of the tree well away from 

cankers may be colonized. Ambrosia beetles bore galleries in 

wood, but feed on ambrosia fungus (Ambrosiella brunnea) that 

grows on the inner surfaces of the galleries after it is introduced 

by the adult female. Galleries are maintained free of frass, 

resulting in accumulations seen outside the bark.

In M. scutellare, the adult male initiates colonization by 

tunneling up to about 6 cm into the wood and excavating 
54 A Field Guide to Insects and Diseases of California Oaks

Figure 57. Monarthrum scutellare adult beetle.

Photo: Jack Kelly Clark, courtesy UC Statewide IPM Program

Figure 56. Detail showing ambrosia beetle larval tunnels 

branching from the end of the main entrance tunnel 

excavated by the adult male.
Bark and Wood Boring Insects 55

a chamber. The male is joined by a female, who carries the 

inoculum of the ambrosia fungus in specialized leg cavities. 

After mating, eggs are laid in niches along the sides of the 

chamber where the fungus also becomes established. Gallery 

extension and oviposition may last two to four months. Larvae 

extend the niches into “larval cradles” that branch off of the 

chamber. Larval development lasts six to eight weeks and 

pupation takes two to three weeks. Two generations occur 

per year in California. Most egg laying takes place in March 

and October, but adults may be found much of the year. The 

biology of M. dentiger is presumed to be similar to that of M. 

scutellare.

Importance

Ambrosia beetles only attack trees or branches that are already 

dead, diseased, or declining. Heavy attacks commonly occur 

on oaks and tanoak with trunk cankers caused by the sudden 

oak death pathogen, Phytophthora ramorum. In some situations, 

heavy attacks appear to hasten the decline of dying trees 

or branches, but the effects of beetle attacks are difficult to 

separate from those of wood decay fungi that are also common 

in declining trees. Extensive ambrosia beetle tunneling may 

increase the risk of branch or trunk failure, especially in 

branches and trunks less than about 30 cm in diameter. 
56 A Field Guide to Insects and Diseases of California Oaks

Bark beetles

Western oak bark beetle - 

Pseudopityophthorus pubipennis

Oak bark beetles - P. agrifoliae, P. pruinosus

(Scolytidae)

Distribution/Hosts

Pseudopityophthorus pubipennis is reported 

throughout California from the coast to the 

western slopes of the Sierra Nevada and Cascade 

Range. It occurs north to southern British 

Columbia, at least in the coastal zone. It is 

common on various oaks, including coast live, 

California black, and Oregon white oak, but 

has also been reported on tanoak, chestnut, and 

California buckeye. 

P. agrifoliae is reported from at least Marin to Los 

Angeles County on coast live, California black, and 

canyon live oak. Both P. agrifoliae and P. pubipennis

are often very abundant in oak firewood. 

P. pruinosus (=P. pulvereus) is reported as 

uncommon in southern California, and occurs on 

various oaks. P. pruinosus also occurs in Arizona, 

Texas, Mexico, and some states in the eastern U.S.

Figure 58. Pseudopityophthorus engraving in outer sapwood of 

an oak branch; bark has been removed to expose the bark beetle 

galleries.

Photo: Don Owen, CDF.
Bark and Wood Boring Insects 57

Symptoms

Oak bark beetles colonize the bark of branches and trunks. 

The outermost sapwood just under the bark may be 

extensively engraved due to tunneling by the adult beetles. 

The engravings score the wood surface across the grain but 

do not enter deeply into wood (fig. 58). Tunnels produced 

by P. pubipennis adults are limited to the inner bark and 

do not score the sapwood (Bright and Stark 1973). In dry, 

dead stems, deposits of sawdust-like frass and/or pinhead 

sized emergence holes are typical symptoms. Oak bark beetle 

boring dust is typically dark or reddish due to the feeding of 

the beetles in the phloem (fig. 59). 

Ambrosia beetles, Monarthrum spp., which bore deeply into 

the wood, produce light-colored boring dust. On live stems, 

bleeding sap or foam may be associated with oak bark beetle 

attack (fig. 60). 

Figure 59 (left). Boring dust on bark of coast live oak resulting from 

infestation by bark beetles (darker brown dust) and ambrosia beetles 

(lighter boring dust).

Figure 60 (right). White ooze associated with oak bark beetle attack.

Photo: Don Owen, CDF.
58 A Field Guide to Insects and Diseases of California Oaks

Figure 61. Pseudopityophthorus pubipennis adult beetle.

Photo: Jack Kelly Clark, courtesy UC Statewide IPM Program

Agent Description

Larvae are whitish, legless grubs, 1–2.5 mm long. Adult beetles 

are very small (1.7–2.3 mm long), cylindrical, dark brown to 

black, and shiny (fig. 61). The pronotum and elytra are finely 

and densely punctate and covered with fine hairs. 

Biology

Adults bore through the bark to the interface with the sapwood. 

From the entrance hole, two tunnels, about 5 cm long 

combined, are excavated in the inner bark perpendicular to 

the wood grain. Eggs are laid in niches along these tunnels. As 

larvae hatch they begin tunneling in the inner bark (phloem) 

at right angles to the adult gallery, i.e., with the wood grain. 

Larval tunnels are up to 2.5 cm long for P. pubipennis and 1–2 

cm long for P. agrifoliae. At maturity, the larvae tunnel to just 

below the bark surface and pupate at the end of the tunnel. 

Adults emerge and chew through the bark, leaving behind small 

exit holes (“shotholes”). Depending on the location, there may 

be two or more generations per year. Generations overlap, so 

adult beetles are always present during the growing season. 

Larvae and adults overwinter under bark. 
Bark and Wood Boring Insects 59

Importance

Oak bark beetles colonize trees or parts of trees that are severely 

weakened, dying, or dead. Beetles may also colonize freshly cut 

oak firewood. When adult beetles emerge from cut logs, they 

may attack nearby healthy oaks. P. pubipennis is commonly 

found in association with trunk cankers caused by the sudden 

oak death pathogen, Phytophthora ramorum, in coast live oak, 

California black oak, and tanoak. Attacks by oak bark beetles 

may hasten the decline of dying branches and trees. Healthy, 

non-stressed trees are not likely to be substantially harmed by 

oak bark beetle attack.
60 A Field Guide to Insects and Diseases of California Oaks

Carpenterworm

Prionoxystus robiniae (Cossidae)

Distribution/Hosts

The carpenterworm feeds on decidous trees 

throughout the U. S., including ash, aspen, 

cottonwood, poplar, elm, locust, oak, and willow. In California, 

coast live and California black oak are the most common oak 

hosts.

Symptoms

The larvae feed by tunneling through the inner bark and into 

the wood of trees. Larvae keep the tunnels open to the outside 

of the tree. Tunnel ends appear as 13–18 mm diameter holes 

in the bark. Bleeding often occurs from the infested area and 

sawdust-like frass accumulates below the open-ended tunnels. 

The bark over tunneled areas is sometimes roughened or 

missing in patches (fig. 62). After adults emerge, empty pupal 

cases may be found protruding from tunnel openings.

Agent Description

Larvae are off-white, pinkish, or greenish-white, and up to 64 

mm long at maturity; the head is dark (fig. 63). Unlike larvae 

of wood-boring beetles, 

carpenterworm larvae have 

abdominal prolegs, each of 

which has a band of hooks 

(crochets) at the bottom. 

Pupae are shiny, dark brown, 

and have a double row 

of spines (fig. 64). Adults 

are large, heavy-bodied 

moths with mottled grayish 

Figure 62. Areas of roughened 

and missing bark on this valley oak 

are symptoms of carpenterworm 

damage.

Photo: Bruce Hagen, CDF.
Bark and Wood Boring Insects 61

forewings (fig. 65). Female moths are large, with a wingspan of 

about 75 mm and grayish hindwings. Male moths are about twothirds the size of females, generally darker, and have an orange to 

red area on their hindwings. The male’s antennae are more comblike (pectinate) than the female’s.

Biology

Adult moths emerge from 

pupal cases from May through 

July, or as early as April in 

southern California. Females 

are poor fliers and frequently 

lay eggs on the same tree from 

which they have emerged. Eggs 

are laid in sticky clumps of 

two to six on the bark, often 

in cracks or crevices or in areas 

of rough bark from previous 

infestations. After hatching, 

the larva immediately bores 

through the bark. Larval 

feeding initially occurs mostly 

in the sapwood, but later instars 

tunnel into the heartwood, 

returning to the tunnel 

entrance to expel frass. The 

gallery is mostly vertical and 

about 15–25 cm long. After the 

mature larva pupates, the pupa 

wiggles toward the bark surface, 

where the adult emerges. The larval stage takes up most of the 

two to four year life cycle.

Importance

Bleeding and scarring of the affected area may be unsightly. 

Girdling associated with tunneling may induce branch dieback. 

Wounds may also serve as an entry point for wood decay fungi or 

canker pathogens. Extensive boring and associated wood decay 

can make trees more susceptible to breakage. 

Figure 65. Carpenterworm adult 

female moth.

Photo: Bruce Hagen, CDF.

Figure 63. Carpenterworm larva.

Photo: Bruce Hagen, CDF.

Figure 64. Carpenterworm pupal 

case.

Photo: Bruce Hagen, CDF.
62 A Field Guide to Insects and Diseases of California Oaks

Flatheaded borers/

Metallic wood boring 

beetles

Flatheaded appletree borer - 

Chrysobothris femorata

Pacific flatheaded borer - Chrysobothris mali

(Buprestidae)

Chrysobothris femorata occurs throughout the U.S. and 

southern Canada. C. mali occurs in western North America 

from the Rocky Mountains and southern Canada to the Pacific 

Coast. Both species attack a 

wide variety of woody plants, 

including both young and 

mature shrubs and trees. They 

primarily attack plants that are 

stressed, diseased, or injured. 

Eggs are laid singly in bark 

crevices or near injuries. The 

larva emerges from the egg by 

boring through the bottom 

of the shell directly into the 

bark. Larvae are whitish to 

yellowish, legless, and have 

a wide, flattened segment 

near the head (fig. 66). They 

mine the inner bark, sapwood 

Figure 66 (top). Flat-headed borer 

larva and frass-packed tunnels in 

the outer sapwood of a recently-cut 

coast live oak log. The enlarged 

thoracic segments give the larva a 

“flat-headed” appearance.

Figure 67 (bottom). Flat-headed 

borer tunnels in coast live oak wood. 
Bark and Wood Boring Insects 63

and heartwood of trees, forming tunnels that are wide and 

flattened in cross section (fig. 67). The tunnels are partially 

filled with powderlike frass. Sap may ooze from the affected 

area. In vigorous trees, heavy sap flow can kill invading borers. 

Tunneling may girdle and kill trees, especially young trees. 

Larvae construct a chamber for pupation in the outer wood 

or heartwood and overwinter as larvae in the chamber. They 

pupate in the spring or early summer. Adults chew oval holes 

through the bark as they emerge. One generation is produced 

per year, although in cooler areas more than a year may be 

required to complete a generation (Solomon 1995).

C. femorata adults emerge from March through September and 

feed at the bases of twigs. Adult beetles, 7–16 mm long, are 

dark metallic brown to dull gray, with greenish blue mottled 

areas (fig. 68). Mature larvae reach 25 mm in length. Larvae 

feed in the inner bark and surface of the sapwood, migrating 

into the sapwood when they reach maturity.

C. mali adults, found from April to August, are about 6–11 

mm long and dark bronze, gray, or a mottled copper color (fig. 

69). Mature larvae reach 15–18 mm long. Larvae feed in both 

the phloem and the sapwood.

Figure 68. Pinned 

specimen of 

adult flatheaded 

appletree borer.

Photo: Larry G. Bezark, 

CDFA.

Figure 69. 

Pinned specimen 

of adult Pacific 

flatheaded borer.

Photo: Larry G. Bezark, 

CDFA.
64 A Field Guide to Insects and Diseases of California Oaks

Lead cable borer

Scobicia declivis (Bostrichidae)

The lead cable borer is common throughout 

California and southern Oregon. Larvae mine 

and completely destroy dead and seasoned wood, 

but also attack living hardwoods including oak. The larvae are 

heavy bodied, 10 mm long at maturity, white to pale yellow, 

C-shaped, and enlarged at the front end. They have three 

pairs of true legs. Frass-filled tunnels produced by larvae are 

mostly aligned along the wood grain. Adult beetles are 5–6 mm 

long, cylindrical, and dark brown to black (fig. 70). Antennae 

and portions of the legs and mouthparts are reddish in color. 

The head is bent downward and completely hidden by the 

prothorax when viewed from above. Adults emerge from April 

through September, and are most common in July and August. 

Adults sometimes bore through non-wood materials, including 

the sheathing of aerial telephone cables. 

Figure 70. Adult lead cable borer.

Illustration: John Muir Laws, California Academy of Sciences.
Bark and Wood Boring Insects 65

Pigeon tremex

Tremex columba (Siricidae)

This wood-boring wasp is found in the 

northern part of the U.S. from coast to coast 

and throughout California. Larvae bore almost 

perfectly round tunnels, 6–7 mm in diameter, through the 

heartwood of declining and dead hardwoods including oak, 

pear, apple, beech, sycamore, hickory, elm, and maple. Tunnels 

are not open to the outside of the tree. Larvae are up to about 

50 mm long. Adults are 25–38 mm long and have goldenyellow wings and a yellow abdomen with black bands (fig. 71). 

The adult female pigeon tremex inserts her ovipositor into solid 

wood to a depth of 13 mm and deposits a single egg at each 

site. Often, the ovipositor cannot be extracted and the female 

dies on the tree. 

Figure 71. Adult pigeon tremex.

Illustration: John Muir Laws, California Academy of Sciences.
66 A Field Guide to Insects and Diseases of California Oaks

Roundheaded borers/

Long-horned beetles

(Cerambycidae)

California prionus - Prionus californicus

The California prionus is found along the Pacific 

Coast from Alaska to Sonora, Baja California, 

and the Rocky Mountains south to New Mexico. 

Larvae feed in the lower trunk and roots of oaks 

and many other hardwood trees, conifers, and 

woody shrubs. Eggs are laid in the soil near host 

roots. Larvae are large white grubs up to 60–75 

mm long that tunnel in the wood and sometimes in the bark 

of roots. Larvae attack living roots, but also feed in dead and 

decaying roots. The mature larva leaves the root and constructs a 

pupation tunnel up to 1 m long in the soil. Adult beetles are 40–

60 mm long, shining, dark reddish brown, with three sharp teeth 

on each lateral margin of the prothorax (fig. 72). Adults are active 

in twilight hours and at night, and are attracted to light. The adult 

flight period is from June to September. The life cycle takes three 

to five years. Extensive feeding by larvae can completely girdle 

the trunk of the host tree just 

below the ground. Injury is 

greater among trees growing 

in light, well-drained soil.

Prionus lecontei larvae also 

tunnel within living oak roots. 

This species is found from 

British Columbia to Baja 

California. P. lecontei adults fly 

from June to August and are 

sometimes captured with P. 

californicus adults.

Figure 72. Adult California prionus.

Illustration: John Muir Laws, California Academy of 

Sciences.
Bark and Wood Boring Insects 67

Banded alder borer, California 

laurel borer - Rosalia funebris

The banded alder borer is found along 

the Pacific Coast from southern Alaska to 

southern California and the Rocky Mountains 

south to New Mexico. The larvae bore in dead 

or declining hardwoods including oak, alder, willow, ash, and 

California bay. Adults are 25–38 mm long, with alternating 

bands of black and light blue, gray, or white on their wing 

covers (elytra) and long antennae (fig. 73). Adults are attracted 

to fresh paint and are also occasionally found in large numbers 

on alder or California bay.

Figure 73. Adult banded alder borer.
68 A Field Guide to Insects and Diseases of California Oaks

Nautical borer - Xylotrechus nauticus

The nautical borer is common throughout 

California on oaks and other hardwoods. This 

beetle colonizes dead or declining trunks and limbs 

but normally does not attack healthy trees. Larvae, 

which grow to a length of about 18 mm, bore into 

the inner bark, producing an engraved pattern on the surface 

of the sapwood. Tunnels differ from those of bark beetles in 

that no central egg gallery is formed, and the larval tunnels 

are longer, larger in diameter, and oval in cross-section. Larvae 

also tunnel into heartwood and pack the tunnels with dry, 

powder-like frass. Adult beetles may emerge in large numbers 

from infested wood. Adults are about 8–15 mm long and dark 

gray (fig. 74). The elytra (hard outer forewings) are crossed 

with three light-colored zigzag-shaped markings. Antennae are 

relatively short. Adult beetles are found in homes when they 

emerge from firewood stored indoors. 

Figure 74. Adult nautical borer.

Photo: T.W. Davis © California Academy of Sciences.
Bark and Wood Boring Insects 69

Neoclytus conjunctus

This beetle is found along the Pacific 

Coast from British Columbia to southern 

California. Hosts of this borer include 

oak, eucalyptus, ash, pear, manzanita, and 

madrone. The larvae are common in freshlycut deciduous trees and adults may emerge from firewood. 

The larvae initially tunnel under the bark before progressing 

to the sapwood and outer heartwood, which are extensively 

tunneled. The adult flight period is March to June. Adults 

have light-colored markings on the black elytra (fig. 75). The 

pronotum is broadly rounded with many elevated ridges. 

Antennae are long and clubbed. 

Figure 75. Pinned 

specimen of adult 

Neoclytus conjunctus.

Photo: Larry G. Bezark, CDFA.
70 A Field Guide to Insects and Diseases of California Oaks

Western sycamore 

borer

Synanthedon resplendens (Sesiidae)

Distribution/Hosts

The western sycamore borer occurs from 

Washington to California and east to New Mexico. It is reported on 

sycamore, ceanothus, coast live oak, and other oaks. It is reported to 

be more common on oaks in southern than in northern California. 

A related species, Synanthedon mellinipennis, has been reported 

on coast live oak and possibly other oaks in California along the 

Central Coast and San Francisco Bay Area.

Symptoms

Larvae mine the bark below the surface, producing numerous 

meandering feeding tunnels (fig. 76). Reddish sawdust-like frass, 

which larvae expel from tunnels, accumulates in branch crotches or 

on the ground below the infested area. The outer bark surface over 

the tunnels becomes roughened after repeated infestations and may 

produce wet ooze (fig. 77). Pupal skins may be visible in emergence 

holes, which are 3 mm in diameter. 

Figure 76. Western sycamore borer 

damage on coast live oak. The outer 

surface of bark has been cut away to 

show tunneling in inner bark.

Figure 77. Symptoms of western 

sycamore borer damage on 

coast live oak.

Photo: Bruce Hagen, CDF.
Bark and Wood Boring Insects 71

Agent 

Description

Adult moths 

resemble 

yellowjacket wasps 

and have a wingspan 

of about 18 mm 

(fig. 78). Adults are 

mostly yellow with 

black markings 

on the thorax and 

abdomen; the head 

is brownish black. 

Wings are mostly 

clear with yellow to orange edges. Larvae are pinkish to whitish 

and reach 18 mm in length; the head is brown (fig. 79).

Biology

One generation occurs per year. Adults emerge from May through 

early August, with greatest emergence in June and July. The 

ovoid, gold-colored eggs are laid singly on the bark in small cracks 

or depressions. Old or slow-growing trees and injured tissues are 

favored sites for egg laying and feeding. The larva tunnels into the 

inner bark after hatching, creating winding tunnels that extend 

over about 100 cm2. Larvae rarely damage the cambium or feed 

in the wood. Pupae are formed just below the bark surface, and 

protrude through the bark when the adult is ready to emerge. 

Overwintering occurs as larvae or pupae within the bark tunnels. In 

southern California, larvae feed through most of the winter months 

and pupate in early spring.

Importance

Bark of infested trees may become 

unsightly, but the amount of 

damage caused by this insect is 

generally considered to be of minor 

importance. Repeated infestations 

may retard tree growth.

Figure 79. Western sycamore borer 

larva.

Photo: Bruce Hagen, CDF.

Figure 78. Western sycamore borer adult.

Illustration: John Muir Laws, California Academy of Sciences.
72 A Field Guide to Insects and Diseases of California Oaks

Drippy nut

Brenneria quercina (=Erwinia quercina)

Distribution/Hosts

Drippy nut is reported on both coast live 

and interior live oak. The disease may occur 

throughout the ranges of these species but is especially common 

in the San Francisco Bay Area counties. It has also been 

reported on holly oak and Quercus pyrenaica in Spain.

Symptoms

The most obvious symptom is copious oozing of frothy, 

sticky sap from affected acorns (fig. 80). Sap may continue 

to drip from acorn caps attached to the tree after acorns have 

dropped. The ooze is more watery under humid conditions. 

In affected trees, the total amount of dripping sap tends to 

increase with the size of the acorn crop. Affected acorns may 

show discoloration at insect oviposition wounds and evidence 

of internal decay. In Spain, the bacterium is reportedly 

associated with bark cankers and oozing buds in addition to 

oozing acorns.

Agent Description

The agent is visible only with a high-magnification microscope. 

The causal bacterium, originally described under the name 

Erwinia quercina, is a non-spore forming, gram negative 

rod with flagella distributed over the entire cell surface (i.e., 

peritrichous flagella).

Biology

Bacteria apparently enter the acorn through insect oviposition 

wounds, including those caused by acorn weevils, filbertworms, 

and some cynipid wasps. The disease is typically more severe 

during warm weather. Drippy nut disease severity varies from 

year to year, but the disease is more likely to recur in trees that 

have shown symptoms in previous years.
Acorn Diseases 73

Figure 80. Frothy sap oozing from coast live oak 

acorns affected by drippy nut.

Importance

In California, this disease has not been associated with any 

obvious damage to trees, although the seed crop is reduced. The 

sticky sap can be abundant and can pose a substantial nuisance, 

especially around dwellings.
74 A Field Guide to Insects and Diseases of California Oaks

Early defoliation/

leaf browning/

drought stress

Distribution/Hosts

Early defoliation related to prolonged drought 

stress is most commonly seen in blue oak and can occur 

throughout its range. It is most common on drought-prone sites, 

such as areas with shallow or rocky soils. 

Symptoms

In low rainfall years, leaves of mature trees turn brown 

prematurely and begin to drop well in advance of the normal 

time for leaf fall (fig. 81). This drought–deciduous characteristic 

is seen in a more extreme form in California buckeye and is 

also common in chaparral and desert plants. Drought-induced 

defoliation can be differentiated from branch dieback by the fact 

that buds and twigs of drought-defoliated branches remain alive 

and normally leaf out the following spring. In 1987, following 

extreme drought conditions in the preceding rainy season, foliar 

browning and leaf drop in blue oak occurred by mid-August in 

affected areas. During droughts lasting multiple years, symptoms 

may be widespread. In less extreme drought years, only blue oaks 

growing in drought-prone areas may be affected, and defoliation 

may occur later in the season, but still earlier than the usual 

November-December leaf drop.

Seedling advance 

regeneration in 

the understory 

(seedlings less than 

about 10–15 cm tall 

that are typically 

several to many 

years old) commonly 

Figure 81. Early leaf browning and defoliation 

associated with drought in a stand of blue oak.
Diseases Affecting Leaves and Twigs 75

defoliates much earlier 

than overstory trees, 

sometimes losing leaves 

in response to drought 

stress by June or July 

(fig. 82). In seedling 

advance regeneration, 

the shoots may also 

partly or completely die 

back to the ground by 

midsummer in response 

to severe drought stress.

Biology

Blue oak is the most drought-tolerant of California’s tree-sized 

oaks. Early defoliation in response to extreme drought reduces 

water loss to a minimum and presumably favors tree survival 

under these conditions. One year of early defoliation had no 

discernable effect on tree health in one study (McCreary 1991). 

Trees in this study that defoliated early were among the earliest 

to leaf out the following spring. However, some trees that 

defoliated early in response to the extreme 1976–1977 drought 

(the most severe drought in California during the 20th century), 

were subsequently leafless for an entire growing season before 

leafing out in the following year. Well-established seedling 

advance regeneration can resprout from the base to produce new 

shoots if the previous season’s shoot has been drought-killed. 

However, successive years of early defoliation and shoot death 

are associated with reduced seedling survival.

Importance

Mature blue oaks periodically endure drought-induced early 

defoliation without obvious ill effect. If repeated in successive 

years, or if coupled with root damage due to constructionrelated activity, such defoliation could contribute to tree decline. 

Drought-induced defoliation and shoot dieback reduce the 

survival of blue oak seedlings and are commonly a contributing 

factor to poor regeneration in blue oak stands.

Figure 82. Early browning in blue oak seedling at 

right compared to green seedling at left.
76 A Field Guide to Insects and Diseases of California Oaks

Oak anthracnose, twig 

blight, and leaf spots

Apiognomonia errabunda (=Discula 

umbrinella), Cryptocline cinerescens, 

Septoria quercicola, and other fungi

Distribution/Hosts

A number of fungi cause necrotic spotting of leaves 

and/or twig dieback symptoms on oaks. The fruiting bodies of 

these fungi are minute, making them difficult to distinguish from 

each other.

Apiognomonia errabunda (asexual stage = Discula umbrinella, 

synonyms include Discula quercina, Gnomonia errabunda, G. 

quercina, G. veneta, Gleosporium nervisequum, G. quercinum, 

G. quercuum, and many others) is widely distributed across 

temperate North America and Europe and occurs on oaks, beech, 

and sycamore. In California, it attacks both white oaks, including 

valley, Oregon white, and blue oak, and red/black oaks, including 

California black, coast live, and interior live oak.

Cryptocline cinerescens occurs on oaks in North America and 

Europe, and in California is reported on valley, coast live, and 

interior live oak. Susceptibility varies across species and among 

individual plants. In general, coast live oak appears to be the most 

susceptible host in California.

Septoria quercicola is distributed throughout temperate regions 

on oaks and is reported on California black, coast live, interior 

live, and canyon live oak.

Other leaf spotting fungi reported on native oaks in California 

include Cercospora polytricha on canyon live oak; Colletotrichum 

gloeosporioides on California black oak; Cylindrosporium kelloggii

on coast live, California black, and Oregon white oak; Dicarpella 

bina on coast live oak; Marssonina martinii on blue and coast live 

oak; Pseudomassaria agrifolia on coast live oak; Septoria dryina

on California black and possibly other oaks; and several other 

described fungi that may or may not be distinct from other 

species reported above.
Diseases Affecting Leaves and Twigs 77

Figure 84. Anthracnose symptoms on 

coast live oak.

Symptoms

Oak anthracnose is a term used 

to describe diseases caused 

by fungi that produce dark, 

pinpoint-sized asexual fruiting 

bodies and cause localized 

necrotic spots or lesions on 

leaves and young shoots (fig. 83, 

84). Some of these fungi may 

completely kill young leaves and 

twigs under certain conditions. 

Dead leaves may remain attached 

to killed twigs. C. cinerescens has been associated primarily with twig 

dieback (fig. 85). Symptoms are usually scattered throughout the 

canopy, and can resemble those caused by twig boring insects. C. 

cinerescens symptoms are reported to be more severe in twigs colonized 

by oak pit scale. S. quercicola produces small, circular to angular 

necrotic leaf spots visible on both leaf surfaces. It may cause defoliation 

if disease levels are severe.

Agent Description

The sexual stages of these anthracnose and leafspot fungi are in the 

Ascomycotina, which contains fungi that produce spores in microscopic 

saclike structures (asci). For some anthracnose fungi (e.g., C. cinerescens) 

the sexual stage has not been observed. Most anthracnose fungi produce 

asexual spores in acervuli (spore-bearing structures without distinct 

walls that arise beneath the epidermis) or in pycnidia (minute globose 

or flask-shaped fruiting bodies). 

Active lesions usually show only 

asexual fruiting bodies.

A. errabunda is the sexual 

stage of D. umbrinella. Asci are 

formed in long-necked black 

perithecia (nearly globose or 

flask-shaped fruiting bodies) that 

are 130–440 μm in diameter. 

Ascospores are colorless and have 

two unequal sized cells.

Figure 83. Anthracnose symptoms on 

California black oak.

Photo: Bruce Hagen, CDF.
78 A Field Guide to Insects and Diseases of California Oaks

Figure 85. Branch dieback of coast live oak caused by Cryptocline 

cinerescens. 

Photo: Bruce Hagen, CDF.

D. umbrinella forms acervuli in lesions on leaves, most 

commonly near major veins, or on dead twigs. Acervuli on 

leaves appear as small (about 250 μm diameter) raised brown 

flecks. On twigs they are orange-brown, turning dark with age. 

Conidia (asexual spores) produced in acervuli are single celled, 

colorless, and elliptical (9–14 by 3.5–5 μm). Microconidia half 

the size of these conidia are also produced. 

C. cinerescens acervuli occur within discolored areas of leaves 

and twigs. They are round in outline, approximately 100–180 

μm in diameter and rupture through the epidermis and cuticle. 

Acervuli appear black on twigs (fig. 86) and yellowish to 

light brown on leaves. Conidia are elliptical to clavate with a 

truncate base, 10–25 by 3.5–5 μm, single-celled, clear, smoothwalled, and have internal oil droplets.

S. quercicola produces small, black pycnidia that are 

embedded in host tissue within lesions. Conidia have 

several cells, and are colorless and narrowly elongate to 

filiform (thread-like). 

Biology

Symptom severity and appearance can vary with host, time of 

season, and weather. Spores of anthracnose and leaf spot fungi 

are primarily dispersed by splashing and wind-blown rain. 

Wet conditions are required for infection to occur. Immature 
Diseases Affecting Leaves and Twigs 79

Figure 86. Dark spots are C. cinerescens acervuli erupting 

through bark of an oak twig.

Photo: Bruce Hagen, CDF.

host tissue is most susceptible, so disease severity is greatest if 

rains occur during spring growth flushes. Prolonged periods of 

rain throughout the spring provide the optimum conditions 

for disease development. Because such weather patterns are 

uncommon in California, severe disease levels rarely develop.

Perithecia of the sexual stage of A. errabunda form in fallen 

leaves. In cold winter areas, perithecia mature in spring and 

release ascospores during spring rains. The fungus can also 

overwinter within lesions in infected twigs. New infections 

occur when leaves and twigs remain wet for several hours. 

Temperatures suitable for infection range from 15–27  ̊C. 

Wet conditions at 15–20  ̊C are most favorable for infection 

and disease development. Spores produced in acervuli initiate 

new infections and multiple disease cycles continue as long as 

moisture conditions are favorable. Some acervuli produced on 

newly-infected tissue in spring do not produce mature spores 

until the following fall.

C. cinerescens and S. quercicola have similar disease cycles but 

do not produce the sexual perithecial stage. In S. quercicola, 

asexual spores are produced in pycnidia rather than acervuli. 

Importance

Anthracnose and leafspot diseases are generally of minor 

importance to California native oaks, but can cause substantial 

amounts of defoliation and dieback in years with frequent and 

abundant rainfall that continues through late spring. Unless 

several wet years occur in succession, long-term impacts of a 

severe disease year are minimal.
80 A Field Guide to Insects and Diseases of California Oaks

Oak leaf blister

Taphrina caerulescens

Distribution/Hosts

Although Taphrina caerulescens is reported to 

affect more than 50 species of oaks in the U.S., 

specialists believe that this taxon may actually include multiple 

similar species. In California, this pathogen is reported on 

native blue, valley, Engelmann, California black, and coast live 

oak and on some non-native oaks. Oaks in the red/black oak 

subgroup are generally more susceptible than those in the white 

oak subgroup.

Symptoms

Rounded, wrinkled blisters develop on the upper surface of 

young leaves. These blisters appear as depressions when the 

lower leaf surface is viewed (fig. 87). Blisters range in size from 

3 to 30 mm and sometimes coalesce, affecting large areas of 

the leaf. Severe leaf distortion may result, and young leaves 

may drop if heavily infected. Blisters are initially light green, 

Figure 87. Oak leaf blister symptoms on the upper and lower surfaces of 

California black oak leaves.

Photo: Jack Kelly Clark, courtesy UC Statewide IPM Program.
Diseases Affecting Leaves and Twigs 81

yellowish, or whitish on the upper leaf surface and yellowbrown to gray on the underside of the leaf. As blisters age, their 

upper surfaces may turn reddish or purplish and then brown. 

On coast live oak, symptoms caused by erineum mites may 

appear similar to oak leaf blister. Inspection of symptomatic 

tissues under a microscope may be necessary to confirm 

whether symptoms are associated with erineum mites or T. 

caerulescens.

Agent Description

All life stages of this fungus are microscopic. Fungal mycelium 

develops just below the leaf cuticle and between epidermal cells. 

No distinct fruiting body is formed. Sexual spores (ascospores) 

are produced in club-shaped, sac-like structures (asci) that form 

in a layer beneath the upper or lower leaf cuticle and eventually 

break through the leaf surface. Eight ascospores are formed in 

each ascus, but ascospores can produce additional asexual spores 

(conidia) by budding, which may occur within the ascus. All 

fungal tissue is initially colorless, but the layer of asci darkens 

after spores are released. 

Biology

The fungus overwinters as spores (ascospores and conidia) 

in bark cracks and under bud scales. In the spring, young, 

germinating spores infect expanding leaves primarily through 

stomata. After ascospores are released, they may continue 

to grow saprophytically on plant surfaces in a yeast-like 

form, reproducing asexually by budding. Initial infection 

and subsequent saprophytic growth are favored by mild, wet 

conditions. Usually only one disease cycle occurs per year.

Importance

In California, oak leaf blister is generally uncommon and 

typically does not cause substantial damage when present. 

In the southeastern U.S. and Gulf Coast states, disease is 

commonly more severe, leading to a high amount of early 

defoliation in some years.
82 A Field Guide to Insects and Diseases of California Oaks

Powdery mildews

Brasiliomyces trina, Cystotheca 

lanestris, Microsphaera extensa curta, 

Microsphaera penicillata (wide sense), 

Phyllactinia angulata

Distribution/Hosts

Powdery mildews are widespread and occur on most oaks 

throughout California. The taxonomy of the powdery mildews 

has undergone a number of revisions, with some genera and 

species being renamed.

Brasiliomyces trina (= Erisyphe trina) is relatively uncommon and 

has been reported from coast live oak, canyon live oak, and tanoak.

Cystotheca lanestris (= Sphaerotheca lanestris) is widespread 

and has been reported from most oaks in California.

Microsphaera penicillata (= M. alni) has been split into four 

species (M. alphitoides, M. abbreviata, M. calocladophora, 

and M. extensa). As noted below, at least some of the records 

previously reported as M. penicillata have been reassigned to 

M. extensa var. curta. However, most of the host/distribution 

reports based on the older name cannot be reassigned to the 

new species without microscopic examination of the original 

specimens. M. penicillata was reported on a wide variety of 

native oaks in California, including coast live, interior live, 

blue, valley, leather, Engelmann, Oregon white, California 

black, and various scrub oaks. It was also reported on several 

cultivated non-native oaks in California, including English 

oak. Microsphaera extensa var. curta, previously included within 

M. penicillata, occurs on a wide variety of oak species in 

North America. In California, M. extensa var. curta has been 

confirmed on blue, valley, Oregon white, and California black 

oak, but may occur on other species. 

Phyllactinia angulata (= Phyllactinia corylea, P. guttata) is 

distributed throughout North America on trees in the families 

Fagaceae (includes oak, chestnut, beech) and Ulmaceae 

(includes elm, hackberry) and is reported in California on 

coast live, California black, and valley oak. According to Braun 
Diseases Affecting Leaves and Twigs 83

(1987), the species of Phyllactinia previously identified as P. 

corylea on oaks in California should be assigned to this species. 

Symptoms

All of the powdery mildews on California oaks produce dry, 

whitish fungal growth (mycelium) on leaf surfaces. Young 

leaves are often affected more than mature leaves. High 

infection levels may cause leaf distortion and discoloration. 

Symptom location and appearance varies somewhat among 

the powdery mildew species:

 B. trina produces well 

defined, grayish to brownishwhite, rounded to irregular 

colonies on the upper leaf 

surface (fig. 88, 89).

 C. lanestris mycelium 

primarily grows on the lower 

leaf surface (fig. 90, 91). The 

white mycelium turns tan to 

brown with age. The fungus 

can cause systemic infections 

that induce the formation of 

witches’ brooms (abnormal 

clusters of shoots that are 

thickened, elongated, and 

highly branched). Leaves on 

witches’ brooms are small 

and senesce early (fig. 92). 

The entire broom becomes 

white when the fungus 

sporulates, and may either 

die out over the winter or 

produce either more brooms 

or normal shoots the 

following season. 

Figure 89. Brasiliomyces trina on 

coast live oak. Masses of brown 

cleistothecia give a mottled brown or 

grayish appearance to the lesions.

Figure 88. Brasiliomyces trina on 

coast live oak.
84 A Field Guide to Insects and Diseases of California Oaks

M. extensa var. curta

mycelium is normally present 

on the upper leaf surface. It 

commonly forms a persistent 

thin whitish film over the 

entire leaf (fig. 93, 94).

P. angulata mycelium 

primarily occurs on the lower 

leaf surface but is also found 

on the upper surface. 

Agent Description

Powdery mildew fungi 

on California oaks are 

differentiated primarily 

on the basis of their 

sexual fruiting bodies 

(cleistothecia), which are just 

large enough to see without 

magnification, and to a lesser 

degree on their microscopic 

asexual spores (conidia). 

B. trina cleistothecia are 

abundant, yellow-brown, 

somewhat transparent, small (60 μm diameter) and lack 

appendages. Each cleistothecium contains two to three 

saclike structures (asci), each of which contains two spores 

(ascospores). B. trina produces almost no conidia. 

C. lanestris cleistothecia are of intermediate size (95 μm 

diameter) with unbranched, flexuous appendages. Each 

cleistothecium has a single ascus with eight ascospores. 

Conidia are abundant, barrel-shaped, and stick together end to 

end in chains. 

M. extensa var. curta cleistothecia are intermediate to fairly 

large (80–170 μm diameter). Each has five to 20 somewhat 

stiff appendages which are one to 2.5 times as long as the 

cleistothecium. Ends of mature appendages have many 

Figure 90. Cystotheca lanestris on 

lower surfaces of valley oak leaves.

Figure 91. Cystotheca lanestris

symptoms on upper surfaces of valley 

oak leaves; note leaf chlorosis and 

distortion.
Diseases Affecting Leaves and Twigs 85

Figure 92. Cystotheca lanestris witches’ broom symptom on 

coast live oak.

Figure 94. Microsphaera 

extensa var. curta on 

blue oak.

Figure 93. Microsphaera 

extensa var. curta on 

valley oak.
86 A Field Guide to Insects and Diseases of California Oaks

short dichotomous branches (fig. 95). Each ascus has four 

to six (rarely three to eight) ascospores. Conidia are roughly 

cylindrical to somewhat barrel shaped.

P. angulata cleistothecia are large (about 270 μm 

diameter) and have needle-like appendages that are inflated 

at the base. Cleistotheica contain about 10 asci, each 

with two ascospores. Conidia are roughly cylindrical or 

somewhat constricted in the middle.

Figure 95. Micrograph of M. extensa var. curta cleistothecia showing 

unbranched developing appendages and characteristic dichotomously 

branched appendages. 
Diseases Affecting Leaves and Twigs 87

Biology

Powdery mildew fungi derive nutrition from living host cells, 

so infected host tissues are generally not directly killed by these 

parasitic fungi. Most powdery mildew leaf infections affect only 

the outermost layer of leaf cells (epidermis), but the systemic 

infections caused by C. lanestris affect multiple cell layers.

Powdery mildew fungi overwinter as cleistothecia or, in the case 

of C. lanestris, as mycelium in infected buds. Cleistothecia may 

be less likely to form on evergreen oaks than on deciduous oaks. 

In spring, ascospores are released from the cleistothecia and are 

blown or splashed onto host leaves. After ascospores germinate, 

they form swollen structures (appressoria) on the leaf surface. 

Hyphal pegs formed under the appressoria penetrate host 

epidermal cells. Fungal structures (haustoria) are formed within 

living host cells and absorb nutrients that the fungus uses 

for growth and reproduction. Most of the fungal mycelium 

grows on the outer surface of infected leaves. Dry conidia are 

formed on this superficial mycelium and are blown to young 

succulent shoots where they initiate new infections throughout 

the growing season. Infections by powdery mildew conidia are 

favored by warm days and cool nights and inhibited by wet 

conditions. Vigorous new growth produced late in the growing 

season, such as stump sprouts and epicormic shoots produced 

after heavy pruning, may develop severe powdery mildew 

infections. Unlike the other powdery mildew infections, which 

are most noticeable in late summer and fall, infections by B. 

trina can be found in the spring.

Importance

Levels of infection are not normally severe enough to 

substantially affect plant health, except possibly in small 

seedlings. Symptoms, including mycelium, witches’ brooms, 

and leaf distortion, may be unsightly.
88 A Field Guide to Insects and Diseases of California Oaks

Branch canker

Diplodia quercina

Distribution/Hosts

This pathogen has been reported only from oaks in 

California and Europe. In California, it has been 

reported to occur on coast live, valley, and California black oak.

Symptoms

Infections are normally found on branches up to 10 cm in 

diameter. The fungus can girdle infected branches causing 

leaves beyond the point of infection to wilt and die. Affected 

trees show discrete dead branches in the crown (fig. 96) 

as opposed to the widely scattered twig dieback caused by 

anthracnose/twig blight fungi. Infections cause extensive 

dark brown discoloration of the wood. The border between 

discolored and healthy tissue is generally well-defined 

(fig. 97). Fruiting bodies emerge from the bark, causing a 

roughened appearance. 

Agent Description

Diplodia quercina produces 

tiny flask-shaped asexual 

fruiting bodies (pycnidia). 

On branches with bark, 

pycnidia are in black masses 

of fungal tissue (stromata). 

Pycnidia also occur singly 

on the surface of wood that 

has lost its bark. Asexual 

spores (conidia) are about 

30 by 15 μm or larger. 

Figure 96. Dead branches 

on California black oak (top) 

and coast live oak (left) due to 

Diplodia canker.

Photo: Larry Costello, UC Cooperative 

Extension.
Branch Diseases 89

Conidia are initially one-celled 

and colorless but become dark 

with one or more cross walls 

(septae). The sexual stage of 

D. quercina is reported to be 

Botryosphaeria stevensii, but this 

stage has not been reported on 

oaks in California.

Biology

The fungus may enter branches through wounds and possibly 

natural openings such as leaf scars. The fungus can only be 

recovered from within a few centimeters of the infection point 

even though discoloration may extend 0.6–1 m further (HechtPoinar and others 1987, Costello and others 1989). In artificial 

inoculations, asexual fruiting bodies are produced within 24 

weeks and continue to produce spores for about two years. Spore 

production and germination are favored by moisture. Drought 

stress may predispose trees to infection. The disease became 

widespread following the extreme drought of 1976–1977. 

Importance

Historically, this disease has been commonly reported only after 

severe drought conditions and is not a widespread problem in 

most years. Drought-stressed trees with wounded branches are 

most likely to be affected.

Figure 97. Diplodia canker on an 

artificially inoculated, greenhousegrown coast live oak seedling.

Photo: Steve Koike, UC Cooperative Extension.
90 A Field Guide to Insects and Diseases of California Oaks

Orange hobnail 

canker

Cryphonectria gyrosa

(=Endothia gyrosa)

Distribution/Hosts

Cryphonectria (=Endothia) gyrosa has been reported on coast live 

oak in the San Francisco Bay Area but may also occur on other 

oaks and in other areas. It has also been observed on tanoaks 

in coastal Sonoma County. This pathogen is widespread in the 

eastern U.S. and the southeast from Virginia to Texas, where 

it occurs on many hardwood trees, including oaks, beech, 

sweetgum, American elm, and American chestnut. Oak hosts 

are primarily in the red/black oak subgroup and include eastern 

black, cork, English, southern live, pin, post, northern red, 

southern red, scarlet, water, and willow oak. The fungus is also 

reported from Europe, Asia, and New Zealand.

Symptoms

The fungus causes somewhat sunken, slowly expanding cankers 

which may girdle and kill branches and trunks of trees under 

stress. Cankers sometimes extend from smaller branches to the 

larger parent branch. Small (1–2 mm diameter), cushion-shaped 

masses of fungal tissue containing fruiting structures (stromata) 

erupt from the bark of killed stems (fig. 98). Stromata are initially 

bright orange but become cinnamon brown with age. Stromata 

Figure 98. Cryphonectria 

gyrosa stromata on coast 

live oak. 
Branch Diseases 91

remain bright orange internally and release a beet-red pigment 

when wetted with a solution of potassium hydroxide (fig. 99). The 

common name, hobnail canker, refers to the masses of rounded, 

bumpy stromata that appear on affected stems.

Agent Description

Stromata that emerge from killed bark initially produce asexual 

spores (conidia) in irregular chambers. Conidia of the asexual stage 

(Endothiella gyrosa) are colorless, straight to somewhat curved 

rods that are 3–4 by 0.75–1.5 μm. Flask-shaped sexual fruiting 

structures (perithecia) are produced in the stromata late in the 

season. Perithecia are 150–300 μm in diameter, with elongate necks 

that protrude slightly above the surface of the stroma. Sexual spores 

(ascospores) are produced in saclike structures (asci) within the 

perithecia. The one-celled, curved ascospores are 7–11 by 1–3 μm.

Biology

The biology of this fungus in California has not been studied. The 

fungus is reported to infect through wounds, such as those which 

form naturally through branch breakage, or through pruning cuts 

or other mechanical wounds. Moisture favors dispersal of spores, so 

presumably most infections occur during the rainy season. Asexual 

spores are most likely spread by rain splash, and possibly by insects 

or pruning tools. Ascospores are probably wind-dispersed. On many 

hosts, canker expansion occurs most rapidly on drought stressed trees 

during summer months.

Importance

On coast live oak, C. gyrosa is typically a branch pathogen, often 

affecting suppressed or wounded branches. 

Figure 99. Close-up 

of C. gyrosa stromata. 

At right is a slide 

mount of stromata in a 

solution of potassium 

hydroxide, showing 

the characteristic 

release of red 

pigment.
92 A Field Guide to Insects and Diseases of California Oaks

Oak mistletoe

Phoradendron villosum

Distribution/Hosts

Oak mistletoe occurs from northwestern Oregon, 

throughout California, and into Mexico. Reported 

oak hosts in California include valley, blue, Oregon white, 

coast live, interior live, canyon live, and California black oak. 

Phoradendron villosum prefers oaks as a host but is also found 

on some other hardwoods, including California bay, manzanita, 

Adenostoma, and Rhus. The related P. macrophyllum (big-leaf 

mistletoe) attacks other hardwoods such as alder, ash, walnut, 

sycamore, cottonwood, mesquite, locust, and willow, but not oaks. 

Symptoms

Oak mistletoe forms spherical to ellipsoidal clumps of light 

green leafy shoots that emerge from oak branches. The oak 

branch is slightly to significantly swollen at the point where 

clusters of mistletoe shoots emerge. Over time, the portion 

Figure 100. Oak mistletoe clumps in the upper canopy of a dormant blue 

oak. Mistletoe infestations are most obvious in deciduous oaks during the 

winter.
Branch Diseases 93

of the oak branch beyond the mistletoe clump may decline 

and die. Clumps of oak mistletoe are often located in the 

upper portion of oak canopies (fig. 100). Mistletoe infections 

tend to be clustered and multiple clumps commonly occur in 

individual trees or in groups of adjacent trees. 

Agent Description

P. villosum plants are evergreen with thick obovate-elliptic leaves 

that are 1.5–4.7 cm long by 1–2.5 cm wide (fig. 101). The 

leaves and young stems are covered with whitish or yellowish 

hairs, but older stems are hairless (glabrous). Branches are 

generally erect, although longer stems, which can be up to 1 m 

long, may become pendulous under the weight of the foliage. 

Flowers are present between July and September with male and 

female flowers occurring on separate plants. Fruit are globose, 3–4 

mm in diameter, pinkish white, and have short hairs near the tip.

Figure 101. Shoots and leaves of oak mistletoe, showing inflorescences 

in leaf axils. A few of the white berries are visible in the lower half of the 

photo.
94 A Field Guide to Insects and Diseases of California Oaks

Figure 102. Clump of oak mistletoe seeds deposited via bird droppings on 

an oak branch.

Biology

Oak mistletoe fruit mature in early winter. The fruit are eaten 

by various birds, including robins, bluebirds, thrushes, and 

cedar waxwings. Seeds in consumed fruit pass quickly through 

birds’ digestive systems and are defecated onto the source tree 

or nearby trees (fig. 102). Seeds are coated with a sticky fluid 

which helps them adhere to bark surfaces. Wet conditions 

favor seed germination. Germinating seeds produce a root-like 

structure that grows along the bark surface to a point where it 

produces a specialized structure (haustorium) that penetrates 

into the bark. Within the host branch, the mistletoe plant 

forms connections between its own phloem and xylem cells 

and those of the host. The connections allow the mistletoe 

plant to absorb water and nutrients from its host. One or more 

years may elapse between infection of the host stem and the 

emergence of green mistletoe shoots. Although dependent on 

the host oak for water and mineral nutrients, leafy mistletoe 
Branch Diseases 95

plants meet all of their carbon-based energy and nutritional 

needs through their own photosynthesis. Mistletoe plants are 

perennial and generally live until the branch of the host on 

which they are situated dies.

Importance

Leafy mistletoe usually has little impact on healthy oaks, 

although it may be unsightly in urban settings. Swellings on 

stems may serve as points of entry for wood decay fungi. P. 

villosum uses water at a higher rate than its host oaks. Oaks 

with heavy mistletoe infestations may therefore deplete 

available soil moisture reserves earlier in the season. Earlier 

and/or more severe water stress levels related to heavy mistletoe 

infestations can lead to early defoliation in deciduous oaks and 

may contribute to decline in some trees.
96 A Field Guide to Insects and Diseases of California Oaks

Canker rots

Inonotus andersonii, I. dryophilus

Distribution/Hosts

These canker rot fungi are distributed throughout 

oak woodlands in California, the Pacific Coast and 

the southwestern U.S. as well as in the oak-hardwood forests of 

the eastern U.S.

Inonotus andersonii is reported from coast live, California 

black, blue, and valley oak in California, but probably infects 

other oak species as well. In addition to oaks, it reportedly 

occurs on Carya, Populus, and willow.

I. dryophilus is reported on coast live, interior live, canyon 

live, California black, Oregon white, blue, and valley oak in 

California. It is primarily found on living oaks, but is also 

reported on maple, Populus, Prunus, Schinus, and willow.

Symptoms

These fungi cause a white rot of the heartwood of living oaks 

but also decay strips or sections of sapwood. For I. andersonii

especially, wood with advanced decay appears bleached, is 

very light weight, and 

crumbles easily. Where 

sapwood decay reaches the 

cambium, the cambium 

may be killed, giving rise 

to an externally visible 

canker. Cankers are 

commonly elongate and 

may become callused 

at the edges (fig. 103). 

Cankers are sometimes 

Figure 103. Blue oak with 

typical elongate canker rot 

cankers and fruiting body of 

Inonotus dryophilus. 
Diseases Affecting Branches and Trunks 97

associated with an elongate, decayed cavity. The bark near 

expanding or developing cankers may ooze varying amounts 

of dark sap. 

Affected branches or trees may show general symptoms of 

decline including poor growth, thinning, dieback, epicormic 

shoots (short twiggy branches arising from dormant buds on 

large-diameter stems) and branch or trunk failures. Generally, 

trees with canker rots decline slowly, often appearing to die 

from the top down as they slowly fall apart over a period of 

many years. 

The fruiting bodies (basidiocarps) may appear on living trees or 

on recently-failed stems. 

Agent Description

I. andersonii produces a sheet-like (resupinate) annual 

basidiocarp on dead wood under the bark, or occasionally 

between outer sapwood layers. Basidiocarps are commonly 

0.3–1 m long. Fresh basidiocarps are cinnamon brown (fig. 

104), but become black as they age (fig. 105). The basidiocarp 

consists of two layers: a thin (up to 1 mm thick) layer of sterile 

tissue (context) that is attached directly to the woody substrate 

and a layer of downward-pointing tubes (up to 1 to 2 cm long) 

attached to the context that terminate in an uneven layer of 

circular to angular pores (1–6 per mm). Bright chrome yellow 

Figure 104. Close-up of 

I. andersonii fruiting on 

coast live oak, showing 

resupinate pore layer 

formed under bark on 

recently killed wood. 

Bright yellow spores 

have been deposited 

on bark, which has 

been peeled away 

from the wood for the 

picture. 
98 A Field Guide to Insects and Diseases of California Oaks

Figure 106. 

Fallen coast 

live oak branch 

with distinctive 

deposit of 

chrome yellow 

I. andersonii

spores at the 

bark/wood 

interface.

Figure 105. 

Old resupinate 

pore layer of 

I. andersonii

has dark 

brown to black 

appearance.

Figure 107. 

Close-up of 

I. dryophilus

fruiting on 

interior live oak. 
Diseases Affecting Branches and Trunks 99

deposits of basidiospores develop on the inner surface of bark 

that initially covers new basiodiocarps (fig. 104, 106). Spore 

deposits become brown over time. 

I. dryophilus basidiocarps are annual, hoof-shaped, up to 11 

by 19 by 9 cm, usually solitary, and firm and corky when fresh 

(fig. 107). The upper surface of the basiodiocarp (pileus) is buff 

to reddish-brown. The lower surface is covered with a layer 

of tubes up to 3 cm thick that terminate in a smooth layer of 

small angular pores (1–3 per mm). The pore surface is white 

to buff at first, but becomes dark reddish-brown with age. 

Basidiospores are brownish. The context (sterile tissue between 

the tube layer and the pileus) mostly consists of a hard, granular 

core of intermixed brown and whitish mycelium. 

Biology

Basidiocarps mature mainly between fall and spring. I. 

dryophilus typically fruits on living stems. I. andersonii only 

fruits on dead wood, including dead portions of living 

stems, dead branches, or on stems that have failed due to 

decay. Basidiospores are produced along the inner walls of 

the tubes in the basidiocarp. They are discharged though 

the pores on the lower surface of the basidiocarp and are 

disseminated by air currents. Basidiospores initiate new 

infections, most likely by invading wounds such as broken 

branch stubs or pruning cuts.

Importance

I. andersonii and I. dryophilus are among the most frequently 

observed wood decay fungi on living oaks in many types 

of hardwood forests. These fungi are serious pathogens of 

California oaks and are associated with oak decline, failure, and 

mortality in many California oak woodlands.
100 A Field Guide to Insects and Diseases of California Oaks

Hedgehog fungus

Hericium erinaceus f. erinaceus

Distribution/Hosts

Hericium erinaceus is known by several common 

names including lion’s mane and hedgehog 

fungus. It is widely distributed throughout temperate North 

America and is also found in Europe and Asia. It occurs on a 

variety of oaks and other hardwoods, including maple, ash, 

and eucalyptus. In California, H. erinaceus f. erinaceus has been 

reported on coast live, canyon live, interior live, California 

black, blue, and valley oak. 

Figure 108. Fresh fruiting body of Hericium erinaceus f. erinaceus at the 

edge of a decayed wound on coast live oak.
Diseases Affecting Branches and Trunks 101

Symptoms

The fungus is usually associated with a wound and causes a 

white pocket rot of living trees. Decayed tissue becomes spongy 

and eventually disintegrates to form a cavity. The distinctive 

fruiting bodies (basidiocarps) generally appear near the edges of 

old wounds in the fall (fig. 108).

Agent Description

The basidiocarps are annual, usually solitary, and 10–20 

cm across at maturity. They commonly form on wounds or 

recently-cut log ends. The spore-bearing surface consists of 

many closely-packed, slender, icicle-like teeth that are 2–5 

(rarely 1–10) cm long. The teeth are white when fresh, and 

become pale yellow, brownish, or rarely reddish with age. 

Spores are nearly globose, 5–6 by 4–5.5 μm, and smooth to 

slightly roughened. The spore print is white. Young basidiocarps 

are edible but may cause allergic reactions in some individuals.

Biology

Basidiocarps are formed in the fall months, often at the site 

of a wound. Basidiospores are disseminated by air currents. 

Infection presumably occurs at wounds such as cracks, fire 

scars, or branch stubs. 

Importance

This fungus has the potential to affect tree health and increase 

failure in infected trees. It is not observed as commonly as other 

decay fungi in most stands. 
102 A Field Guide to Insects and Diseases of California Oaks

Hypoxylon thouarsianum

Distribution/Hosts

Hypoxylon thouarsianum occurs throughout much 

of California. Its reported range also includes the 

southeastern U.S., Central and South America, 

and parts of Africa, including South Africa and Tanzania. It 

occurs on California black, blue, coast live, interior live, canyon 

live, and valley oak, as well as tanoak. It is occasionally found 

on other species including hackberry and avocado.

Symptoms

Black hemispherical fruiting bodies (stromata) of H. 

thouarsianum develop, sometimes in great numbers, on the 

surface of dead bark or wood (fig. 109). The fungus causes 

a white rot of the sapwood of living trees and dead wood. 

Sapwood decay caused by H. thouarsianum in trees affected by 

Phytophthora ramorum canker may be quite extensive and may 

lead to branch or trunk failure.

Agent Description

Figure 109. Mature H. thouarsianum fruiting bodies on coast live oak.
Diseases Affecting Branches and Trunks 103

Immature stromata that emerge through the bark over 

wood colonized by H. thouarsianum are initially rounded, 

black, smooth, and glossy and covered with a thin, fragile 

membrane. This short-lived membrane ruptures, revealing a 

layer of powdery, dark olive green asexual spores (conidia) on 

the surface of the new stromata (fig. 110, 111). Conidia are 

subglobose, about 6 μm in diameter, and are borne on densely 

packed specialized spore bearing hyphae (conidiophores). 

Mature stromata are hard, hemispheric to nearly spherical, 

1–4 cm in diameter, and blackish brown to black. Stromata 

may occur singly or in merged overlapping clusters. Internally, 

mature stromata are carbonaceous with some faint concentric 

zonation (fig. 112). Sexual spores (ascospores) are formed 

in flask-shaped structures (perithecia) arranged in a single 

layer just below the outer surface of the stroma. The rounded 

openings of the perithecia are visible as fine bumps that cover 

the outer surface of the stroma. Ascospores are dark brown, 

smooth, ellipsoidal, and 14–24 by 4–5.5 μm.

Figure 110. Young Hypoxylon thouarsianum fruiting body on tanoak 

covered with powdery asexual spores (conidia).
104 A Field Guide to Insects and Diseases of California Oaks

Biology

Little is known about the biology of this species. Many 

Hypoxylon species initiate latent (dormant or inactive) 

infections in healthy hosts or grow to a limited degree within 

host tissues without causing obvious disease symptoms. When 

the host is severely stressed, some Hypoxylon species become 

opportunistic pathogens, and may cause rapid and extensive 

decay of the sapwood. H. thouarsianum commonly fruits 

abundantly on or near bark cankers caused by Phytophthora 

ramorum. It is also commonly found on dead or declining 

branches and on downed wood. It is unclear whether conidia 

and/or ascospores establish infections, but infections are 

probably initiated during the wet season.

Figure 111. Maturing H. thouarsianum fruiting bodies (center) show 

remnants of conidia between bumps formed by the openings of the 

underlying perithecia. Small newly emerging fruiting bodies to the left and 

below are still covered by a shiny thin membrane.
Diseases Affecting Branches and Trunks 105

Figure 112. Sliced H. thouarsianum fruiting body shows slightly zonate 

interior and outer layer of perithecia which contain sexual spores 

(ascospores).

Importance

This fungus appears to be of little consequence in healthy 

trees, but can quickly overrun severely stressed trees and trees 

with extensive Phytophthora ramorum cankers. Sapwood decay 

caused by this pathogen probably plays a role in rapid tree 

death that occurs in some Phytophthora ramorum-infected trees. 

H. thouarsianum contributes to the failure potential of both 

living and dead P. ramorum-infected stems.
106 A Field Guide to Insects and Diseases of California Oaks

Sulfur fungus

Laetiporus gilbertsonii

(=L. sulphureus in part)

Distribution/Hosts

The taxonomy of this fungus in North America 

has recently been revised by Burdsall and Banick (2001). 

Fungi previously described as Laetiporus sulphureus in the 

Pacific Coast and southwestern states have been renamed L. 

gilbertsonii if they occur on hardwoods and L. conifericola if 

they occur on conifers. In California, L. gilbertsonii has been 

observed on blue, valley, California black, coast live, interior 

live, and canyon live oak, as well as eucalyptus, Prunus, and 

probably other hardwoods.

Symptoms

This fungus causes a distinctive brown cubical wood decay of 

living trees, as well as dead trunks and logs (fig. 113). Wood 

decayed by L. gilbertsonii is a cinnamon to dark brown color 

and tends to fracture into flat-sided, somewhat cubical pieces. 

Daedalea quercina, which is uncommon in California, is the 

only other reported cause of brown rot in living oaks in the 

state. Brown rot caused by L. gilbertsonii commonly leads 

to failure of large branches or the trunk (fig. 114). White to 

pale yellow felt-like masses of fungal tissue (mycelium) are 

prominent in shrinkage cracks in the decayed wood. Fruiting 

bodies (basidiocarps) are initially bright yellow and become 

orange on top before fading to a pale color. 

The distinctive bright yellow basidiocarps may be the first 

obvious symptoms of infection by L. gilbertsonii. The presence 

of basidiocarps indicates that substantial internal decay is 

present and often presages failures at the root crown, trunk, 

or major branches. However, basidiocarps may not form on 

affected trees until after failure has already occurred. 
Diseases Affecting Branches and Trunks 107

Figure 113. Brown cubical wood decay of California black 

oak caused by Laetiporus gilbertsonii.

Figure 114. This L. gilbertsonii basidiocarp 

on valley oak developed on the scar left by a 

large scaffold failure.
108 A Field Guide to Insects and Diseases of California Oaks

Figure 116. Close-up of fresh L. gilbertsonii basidiocarps on California 

black oak.

Figure 115. Cluster of L. gilbertsonii basidiocarps on interior live oak. 
Diseases Affecting Branches and Trunks 109

Agent Description

Basidiocarps are annual and occur singly or in stacked clusters 

(fig. 115). Newly emerging basidiocarps may appear as bright 

yellow lumpy masses, but mature to form moist, fleshy, shelflike fruiting bodies. Basidiocarps are edible when young but 

may cause allergic reactions in some individuals. Mature 

basidiocarps are yellow with orange to brownish-orange 

banding on the upper surface; the lower pore surface is a light 

to intense lemon yellow and covered with fine circular pores 

(3–4 per mm) (fig. 116). Basidiocarps fade to pale brown 

or white with age and become dry, chalky, and crumbly. 

Individual basidiocarps are variable in size, ranging up to at 

least 20 cm in diameter.

Biology

Basidiocarps are formed in the fall months, often before the 

first fall rains, and may form in the same location for multiple 

years. These fruiting bodies release basidiospores, which are 

disseminated by air currents. Infection presumably occurs 

most commonly through wounds such as broken branch stubs, 

pruning cuts, or fire scars. Basidiocarps are typically formed 

only after the amount of internal decay is considerable.

Importance

L. gilbertsonii is the most important brown rot fungus on oaks 

in California and is a major cause of tree failure and failureassociated tree mortality.
110 A Field Guide to Insects and Diseases of California Oaks

Other wood decay fungi 

affecting branches and trunk

In addition to the decay fungi described below and elsewhere in this 

publication, species of Pleurotus, Trametes, and Stereum are commonly 

found fruiting on dead wood in living trees, such as on dead branches 

or fire-scarred areas of the trunk. These fungi cause a white rot of 

dead wood. They generally do not appear to attack live wood, but will 

decay wood that has been killed by other agents.

Phellinus gilvus

In California, Phellinus gilvus occurs on coast live, 

California black, and blue oak and probably other 

oaks. It is found on oaks and many other hardwoods 

throughout much of the U.S. It decays both living 

and dead trees, causing a uniform 

white rot. In California oaks, P. 

gilvus usually colonizes dead wood 

and severely declining stems. It is 

also often found on coast live and 

California black oak with advanced 

symptoms of Phytophthora 

ramorum canker, where it can 

cause extensive sapwood decay 

and is associated with an increased 

likelihood of branch and trunk 

failures. P. gilvus fruiting bodies 

(basidiocarps) are shelf-like, 5–15 

cm wide, 1.5–3 cm thick, corky, 

annual or perennial, and solitary 

or in overlapping clusters (fig. 

117, 118). The upper surface is 

generally dark yellowish brown but 

is variable in appearance and may 

be wrinkled, woolly, and/or zonate. 

The lower surface is covered with 

minute round pores (6–8 per mm) 

Figure 117. Phellinus gilvus and 

Hypoxylon thouarsianum fruiting on 

failed coast live oak.

Figure 118. Close-up of P. gilvus

basidiocarp on coast live oak 

showing light pore surface.
Diseases Affecting Branches and Trunks 111

and is initially buff-colored becoming dark purplish brown at 

maturity. The pores are white within.

Phellinus robustus (=Fomitiporia robusta)

Phellinus robustus is found across the U.S. in forests 

which contain oaks. In California, it has been 

observed on coast live and interior live oak, and 

probably occurs on other species as well. P. robustus

causes a white rot of the trunk and large woody 

roots of living 

oaks. Infections on the 

trunk or large branches may 

be associated with canker 

rot symptoms. P. robustus

decay is also associated 

with stem failure and tree 

decline. Fruiting bodies 

(basidiocarps) are produced 

at the base of the tree or 

higher up the stem (fig. 

119, 120). They are hooflike, perennial, very hard, 

often massive (up to 20 

cm wide and 11 cm thick), 

and usually in clusters. The 

upper surface is brown to 

blackish, concentrically 

zonate, irregularly cracked, 

and lacks hairs. The lower 

surface is covered with fine 

circular pores (7–9 per mm) 

and is yellowish to grayish 

brown.

Figure 119. Phellinus robustus

basidiocarps high on the trunk of 

this coast live oak are associated 

with a trunk cavity and canker rot 

symptoms. 

Figure 120. P. robustus basidiocarps 

at the base of a coast live oak.
112 A Field Guide to Insects and Diseases of California Oaks

Sudden oak death, 

Phytophthora ramorum canker

Phytophthora ramorum

Phytophthora stem canker

P. nemorosa, P. pseudosyringae

Distribution/Hosts

Disease and tree mortality due to Phytophthora ramorum was 

first noted in Marin and Santa Cruz counties 

in the mid 1990s, but the causal agent was not 

positively identified until 2000. This introduced 

pathogen is currently known in California forests 

from the counties bordering the San Francisco 

Bay and the Coast Ranges from Humboldt 

to Monterey Counties, and Curry County in 

southern Oregon. Future expansion of the known range in 

California is likely due to both spread of the pathogen and 

intensive efforts to detect infestations. P. ramorum causes 

cankers on the main stems of coast live, California black, and 

Shreve oak, and tanoak. It also causes lesions on leaves, twigs, 

and/or small stems in a variety of native species, including 

tanoak, canyon live oak, California bay, madrone, California 

huckleberry, manzanita, toyon, poison oak, Douglas-fir, and 

coast redwood. In California, the disease is found in mixed 

hardwood forests containing oaks, usually in association with 

California bay, and in forests containing tanoak. P. ramorum

also causes foliar and stem diseases in a wide variety of 

ornamentals and has been found in nurseries in North America 

and Europe. In Europe, disease caused by P. ramorum has also 

been found in some landscape plantings and in trees near these 

plantings. 

P. nemorosa is currently known only from 

California coastal forests from Monterey County 

north to Central Oregon and from one location 
Diseases Affecting Branches and Trunks 113

in Mariposa County in the Sierra Nevada. It is common in 

redwood-tanoak forests, causing cankers on tanoak and foliar 

symptoms on redwood. P. nemorosa is uncommon in coast live 

oak woodlands, where it primarily causes foliar symptoms on 

California bay. It also causes foliar symptoms on manzanita and 

rarely causes stem cankers on coast live oak.

P. pseudosyringae is currently known to occur 

in California from San Luis Obispo County to 

Humboldt County and has been found at the 

same location in Mariposa County as noted for 

P. nemorosa above. This species appears to occur 

in somewhat drier, more inland sites, almost 

exclusively in coast live oak woodlands, where it 

causes cankers on coast live oak and leaf lesions on California 

bay. P. pseudosyringae also occurs in Europe where it is known to 

cause root and collar rot of oak, alder, and beech.

Symptoms

P. ramorum causes small to extensive bark cankers. In tanoak, P. 

ramorum also infects leaves 

and twigs, causing twig 

dieback on both mature 

trees and understory 

seedlings and saplings. In 

oaks, cankers are normally 

confined to the lower 

trunk and sometimes low 

scaffold branches, but oak 

twigs, canopy branches, 

and main stems less than 

about 10 cm diameter are 

not affected. Cankers do 

not extend more than a 

few cm below the soil line, 

but can occur on exposed 

buttress roots. 

Figure 121. Extensive older P. ramorum

canker on coast live oak showing both 

recent bleeding and older area of killed 

bark that has begun to crack.
114 A Field Guide to Insects and Diseases of California Oaks

Cankers generally ooze 

dark reddish brown 

to black sap (fig. 121, 

122), but some cankers, 

especially on small 

diameter tanoak stems, 

do not ooze. Shaving 

away outer bark in 

affected areas reveals 

a brownish discolored 

lesion in the bark, 

with a sharp margin 

generally delimited by 

a dark line (fig. 123, 

124). Bark (phloem) 

tissues are killed and 

dark discoloration may extend a few mm to several cm into the 

wood. Cankered areas are commonly attacked by secondary 

agents, including Hypoxylon thouarsianum, Phellinus gilvus, 

ambrosia beetles, and bark beetles. 

Trees with extensive girdling cankers may die one to two years 

after cankers develop without showing substantial decline 

of the canopy beforehand. Especially in warm weather, the 

drying foliage may turn brown over a period of a few weeks; 

the name “sudden oak death” was based on this pattern of 

Figure 124. P. ramorum canker on 

coast live oak; outer bark has been 

sliced away showing the irregular 

margin of the canker. 

Figure 122. Bleeding Phytophthora 

ramorum canker on coast live oak; this 

canker is less than one year old.

Figure 123. Same P. ramorum

canker shown in Figure 122 with 

outer bark sliced away to show the 

dark canker margin.
Diseases Affecting Branches and Trunks 115

symptom development. In 

trees with less extensive or 

slower-spreading cankers, 

a gradual thinning and 

progressive dieback of the 

canopy may develop over 

several years. Trees that have 

not been completely girdled 

by cankers may survive 

for a number of years 

after infection, but many 

eventually die once the 

stem is completely girdled 

or when the trunk fails due 

to wood decay associated 

with cankered areas. Small 

cankers in some trees 

apparently become inactive 

and may not be colonized by secondary agents. Inactive cankers 

cease bleeding, may become sunken, and are often bounded by 

host callus tissue (fig. 125).

P. nemorosa causes bark cankers similar to those described 

above on tanoak primarily, but appears to be less virulent 

than P. ramorum.

P. pseudosyringae causes bark canker symptoms similar 

to those caused by P. ramorum on coast live oak (fig. 126). 

Cankers can be quite extensive, occasionally girdling and 

killing trees. Overall, this species appears to be somewhat less 

aggressive than P. ramorum.

Agent Description

Phytophthora species fill an ecological niche similar to various 

fungi, but are members of the kingdom Chromista, which 

also includes diatoms and brown algae. All structures of 

Phytophthora species are microscopic. Phytophthora species 

reproduce asexually through production of swimming spores 

(zoospores) which are produced in sporangia. Species are 

Figure 125. Coast live oak tree with 

extensive P. ramorum cankers at 

least 4 years old; sunken canker at 

the base of the tree shows callus 

development along right side 

(arrows).
116 A Field Guide to Insects and Diseases of California Oaks

distinguished based on the 

sizes and characteristics of 

sexually produced spores 

(oospores), sporangia, 

thick-walled asexual spores 

(chlamydospores), and 

swellings in the vegetative 

filaments (hyphae).

 P. ramorum produces 

abundant deciduous, 

nonpapillate sporangia 

singly or in clusters of two 

to 12 or more arranged 

sympodially on long stalks. 

It produces numerous 

chlamydospores when grown 

on most synthetic media and 

on some foliar hosts. Colony 

morphology on synthetic growth media is distinctive, with a 

highly branched growth pattern. 

 P. nemorosa produces abundant deciduous, scarcely 

semipapillate sporangia in terminal clusters on stalks 

(sporangiophores) that branch sympodially. No chlamydospores are produced.

P. pseudosyringae produces abundant deciduous, semipapillate, 

sporangia in loose to dense sympodially branched clusters. It 

produces hyphal swellings but no chlamydospores.

Biology

Disease caused by P. ramorum is favored by moderate 

temperatures (near 20  ̊C) and rain or free surface moisture. 

Sporangia are formed on the surface of leaves and on twigs of 

some host plants and can be detached and spread by rain splash 

and possibly air currents. Infected California bay leaves are 

probably the most important source of sporangia in woodlands 

where coast live and California black oak are the main stem 

canker hosts. Sporangia produced on tanoak twigs may be the 

Figure 126. P. pseudosyringae

canker on coast live oak.

Photo: Allison Wickland, UC Davis.
Diseases Affecting Branches and Trunks 117

most important source of inoculum for that host. Sporangia 

may germinate as a unit and infect directly or, in the presence 

of free water, each sporangium can release multiple zoospores. 

Zoospores swim in water and seek out suitable host tissues. 

Flow of water down stems during rain storms may concentrate 

sporangia and zoospores in the lower trunks of oaks and tanoaks 

where most stem cankers are initiated. Sporangia do not form 

on bleeding oak and tanoak stem cankers, so reproduction and 

spread of the pathogen depend primarily on spores produced on 

infected California bay leaves, tanoak twigs, and possibly some 

other foliar hosts. P. ramorum has been detected in streams and 

rivers and may also reproduce on fallen leaves of foliar hosts or 

other substrates in standing or flowing water. P. ramorum does 

not appear to cause substantial root disease in oak and tanoak 

stands, but has been recovered from the roots of some host 

species. In contrast, many other Phytophthora species primarily 

attack the roots of their hosts. P. ramorum can be detected in soil 

during and after the wet season but becomes difficult to recover 

from soil after extended dry periods. 

Less is currently known about the biology of P. nemorosa

and P. pseudosyringae in California forests. Both are favored 

by wet conditions, have deciduous sporangia, and produce 

zoospores that are likely to be dispersed by rainwater. P. 

nemorosa is adapted to cooler temperatures, and grows 

optimally at 15  ̊C on synthetic media. For P. pseudosyringae

optimum growth on synthetic media occurs at 20  ̊C. At least 

in Europe, P. pseudosyringae is capable of living in the soil and 

causing root disease. 

Importance

At present, P. ramorum is probably the most important agent 

causing mortality of coast live oak, California black oak, and 

tanoak in affected forests. Compared with P. ramorum, P. 

nemorosa and P. pseudosyringae affect far fewer trees and are 

much less likely to kill trees.
118 A Field Guide to Insects and Diseases of California Oaks

Sunscald

Distribution/Hosts

Sunscald can occur in most parts of California, but 

is more common in hot, interior valleys than along 

the coast. Transplanted young oaks of most species 

can be affected. Among mature trees, species with thin, smooth 

outer bark, including coast live and interior live oak, are more 

likely to develop sunscald. 

Symptoms

Symptoms appear on areas of bark that are exposed to the 

highest sunlight intensity, generally on the west or southwest 

side. Affected bark may become sunken and darkened, and 

typically shrinks and cracks as it dries out (fig. 127, 128). 

Dark sap commonly bleeds from recently affected bark or 

the margin of the affected area. In mature trees with thick 

bark, only the outer layers of bark may be affected (fig. 129). 

Localized beetle boring or opportunistic decay fungi such as 

Trametes versicolor or Stereum spp. may colonize the wood in 

areas where the cambium 

is completely killed. Over 

time, callus may develop 

around the affected area 

and eventually the dead 

bark falls off. 

Biology

Sunscald develops in trees 

whose bark is suddenly 

subjected to a greatly 

increased level of solar 

radiation. This commonly 

occurs in newly transplanted 

young trees. Nursery 

stock that is grown under 

relatively shaded conditions 

and/or lacks low branches 

Figure 127. Lower trunk of a 

transplanted coast live oak shows 

evidence of abundant past oozing 

associated with sunburn injury.
Diseases Affecting Branches and Trunks 119

Figure 128. Mature coast live oak 

stem sunburned due to loss of 

surrounding tree canopy shows 

darkening, cracking, and some 

bleeding. 

needed to shade the trunk is most likely to develop sunscald 

when it is planted in hot, exposed landscape settings. In mature 

trees, sunburn is most likely to develop where previously shaded 

bark is exposed to high sun intensity as the result of pruning, 

tree failure, or other rapid changes in canopy cover. Sunscald 

is more likely to develop in water-stressed trees and following 

episodes of unusually high temperatures.

Importance

Sunscald occurs most commonly on transplanted nursery 

stock and can contribute to poor health, decline, or death, 

especially in small trees. It may develop in mature stems 

that have been overpruned or have lost overtopping canopy, 

especially in species such as coast and interior live oak, which 

have relatively thin outer bark. Species with relatively thick 

outer layers of corky or flaky bark seldom develop sunscald. 

Areas affected by sunscald may be susceptible to invasion by 

some wood decay fungi.

Figure 129. Bark cut away from 

bleeding area shows that bark 

tissue death (necrosis) is limited to 

the outermost layers of the inner 

bark.
120 A Field Guide to Insects and Diseases of California Oaks

Wetwood and 

alcoholic flux

Various microorganisms

Distribution/Hosts

Wetwood is common in California and occurs 

worldwide on oak and many other tree species, especially ash, 

birch, elm, maple, poplar, and willow. Wetwood that occurs in 

conifers appears to differ from that found in hardwoods.

Alcoholic flux, also known as white, frothy, or foamy flux is 

reported to occur on various hardwoods, including oak, willow, 

sweetgum, and elm. It occurs throughout California and is 

common in the midwestern and southwestern U.S.

Symptoms

Wetwood – Cracks or wounds on branches or trunks of affected 

trees ooze a sour, rancid-smelling liquid, known as slime flux 

or wetwood slime, that is initially clear but darkens over time 

(fig. 130, 131). Microorganisms growing in the slime flux 

contribute to the foul odor and slimy texture of the ooze. Slime 

flux oozing commonly lasts for years. The slime flux stains the 

bark and other materials it contacts. Affected wood, primarily 

heartwood, appears water-soaked and discolored (fig. 131). 

Slime flux may kill cambium 

and bark (phloem) tissues 

it contacts, but necrosis 

(tissue death) associated 

with wetwood is typically 

Figure 130. Bleeding (slime flux) 

due to wetwood on coast live oak. 

Ooze is initially clear and darkens 

with exposure to air.

Photo: Steve Tjosvold, UC Cooperative 

Extension.
Diseases Affecting Branches and Trunks 121

discrete and restricted to 

areas contacted by the ooze. 

Occasionally branches 

beyond the infected wood 

will wilt or die back. 

Alcoholic flux – Symptoms 

most commonly occur 

in the summer and are 

generally limited to stressed trees. A clear to white, frothy 

exudate typically oozes from multiple spots for a relatively short 

time (fig. 132). The exudate has a pleasant, fermented, alcoholic 

odor. Alcoholic flux is sometimes found in trees in which the 

cambium and outer sapwood is decayed, white, and mushy.

In contrast with wetwood and alcoholic flux, cankers caused 

by Phytophthora species, including P. ramorum, may exude a 

relatively thick liquid that is initially dark colored, and has 

only a faint odor. Secondary organisms may invade P. ramorum

cankers and give rise to alcoholic or fermented odors. Ooze 

from Phytophthora cankers is a by-product of tissue destruction 

caused by the pathogen, so the ooze is normally associated with 

relatively extensive discoloration and necrosis of the underlying 

bark and cambial tissues.

Figure 131. Bark of this coast live 

oak was chipped away at the point 

where wetwood ooze originated to 

show a small area of watersoaked, 

necrotic bark. Foul-smelling, clear 

liquid streamed from the area for 

several seconds when the bark was 

cut open.
122 A Field Guide to Insects and Diseases of California Oaks

Figure 132. Foamy flux bleeding from the bark of a coast live oak in 

southern California that was severely stressed due to extensive root 

damage; detail (lower left) shows moist dead bark beneath oozing area.

Photo: Edward E. Gripp, Landscape Architect.

Agent Description

Wetwood is commonly caused by various species of anaerobic 

soil bacteria, including Clostridium spp. in oaks. The 

microorganisms responsible for alcoholic flux are not well 

characterized, but probably include aerobic bacteria, yeasts, and 

possibly other fungi.
Diseases Affecting Branches and Trunks 123

Biology

Wetwood in oaks is a bacterial infection of the heartwood 

or inner sapwood. Bacteria that normally live in the soil 

enter the wood through cracks in the bark or wounds. Wood 

colonized by these bacteria develops a higher water content 

than normal wood. Metabolic activity of the bacteria releases 

gases, especially methane, leading to a buildup in pressure 

within affected wood. The elevated pressure helps force fluid 

out of affected wood through external openings in the tree. The 

liquid contains bacteria and their metabolic products including 

rancid-smelling fatty acids. Once the liquid is exposed to the air 

it can be colonized by a wide assortment of bacteria and fungi. 

The fermentation of the ooze by these organisms may cause 

it to darken and bubble or froth. Wetwood-infected wood is 

resistant to decay by fungi.

Alcoholic flux is poorly understood and is probably associated 

with several different causes. In some cases, it appears to result 

from the fermentation of sap in shallow wounds in the bark 

and cambium. In other cases, alcoholic flux may develop 

when areas of the cambium and/or bark killed by heat stress 

or other factors are colonized opportunistically by various 

microorganisms. Foamy exudation associated with some bark 

beetle attacks may be alcoholic flux.

Importance

Wetwood is not generally considered to have much impact on 

tree health, although the associated ooze may be considered 

a nuisance. Alcoholic flux appears to generally result from 

a secondary invasion of tissues that are damaged, severely 

stressed, or recently dead, and so may be an indicator of a more 

severe underlying condition.
124 A Field Guide to Insects and Diseases of California Oaks

Armillaria root rot, 

oak root fungus

Armillaria mellea

Distribution/Hosts

The name Armillaria mellea was previously 

applied to what is now known to be a number of distinct 

Armillaria species that vary in host and geographic range 

and pathogenicity. As currently understood, the range of A. 

mellea (narrow sense) extends throughout California to central 

Mexico, in the U.S. east of the Rocky Mountains, and to 

portions of Europe and Asia. In California forests, A. mellea

is primarily associated with oaks and other hardwoods but 

also attacks conifers, especially in mixed conifer-hardwood 

stands. A. mellea also attacks a wide variety of woody cultivated 

hosts and is common in California’s urban landscapes and 

agricultural areas, many of which were formerly oak woodlands.

The related A. gallica is also common in California oak 

woodlands and some conifer forests, but is rare in developed 

areas. It is commonly associated with oak roots, but does not 

appear to cause disease in oaks.

Symptoms

Trees with Armillaria root rot show general symptoms of poor 

root health, such as overall unthriftiness, canopy thinning, 

and branch dieback. Under conditions that favor disease, A. 

mellea can kill mature oaks. Trees with severe root rot also have 

an elevated risk 

of toppling due 

to root failure. 

Both woody 

Figure 133. Coast live 

oak with Armillaria 

mellea mycelial 

plaques (fans) under 

bark and associated 

bleeding. 
Root Diseases 125

roots and the lower trunk may be colonized by the fungus, 

which kills the bark and cambium and causes a white rot of 

the underlying wood. The bark of diseased root crown areas 

or exposed buttress roots may ooze a thick, dark fluid. White 

sheet-like mycelial mats or plaques, often fan-shaped, form 

beneath the bark of diseased roots and the root crown (fig. 

133). These mats have a mushroom odor when fresh. Affected 

wood initially has a water-soaked appearance, then becomes 

light colored and spongy as decay progresses. Dark, root-like 

structures (rhizomorphs) are commonly formed by the fungus 

on the surface of infected roots (fig. 134) and may extend 

into the soil. Rhizomorphs can also form under the bark of 

infected trees, generally late in the infection process. The 

combination of root decay coupled with the presence of both 

mycelial plaques (fans) and rhizomorphs is typically sufficient 

to diagnose A. mellea root rot in California oaks. A. mellea

mushrooms sometimes form in the autumn or winter at the 

base of infected trees. A. gallica also produces rhizomorphs that 

may be associated with living oak roots, but is not likely to 

cause serious root disease in oaks. 

Figure 134. A. mellea rhizomorphs and mycelial plaque (right 

side) growing on decayed root.
126 A Field Guide to Insects and Diseases of California Oaks

Agent Description

Rhizomorphs resemble small dark roots. Rhizomorphs are cylindrical 

to flattened in cross-section, up to about 2 mm in diameter, brown or 

blackish on the outside and white on the inside when fresh. Rhizomorphs 

lack any internal fibers and are not firm, whereas roots have a relatively 

tough, fibrous center of vascular tissue.

The fruiting bodies (basidiocarps) of A. mellea are gilled mushrooms 

(fig. 135-137), produced in clusters of eight to 10 but sometimes 30 or 

more. Clusters of mushrooms arise from a common base, and each stalk 

in the cluster tapers to a point at the bottom (fig. 136). Stalks are 5–20 

cm tall, usually with a persistent ring on the upper part (fig. 137). The cap 

(pileus) is honey colored, 3–13 cm in diameter, with a smooth surface; it 

lacks the small scales seen 

in some other Armillaria

spp. The spore print is 

white and spores are 7–9.5 

by 5–7 μm. A. mellea is 

the only Armillaria species 

in North America that 

lacks a clamp connection 

(a rounded swelling of the 

hypha) at the base of the 

basidium. This is the most 

definitive characteristic 

of A. mellea, but can only 

be determined with a 

microscope.

Biology

A. mellea infections are 

normally restricted to 

small root lesions in 

healthy oaks growing 

in natural stands, but 

the fungus thoroughly 

colonizes dead roots 

and can cause serious 

disease in oaks that are 

severely stressed. Although Figure 136. Young cluster of A. mellea

mushrooms.

Figure 135. Close-up A. mellea mushroom 

showing attached, slightly decurrent gills.
Root Diseases 127

mushrooms release airborne 

basidiospores that can initiate 

new infections, this mode of 

spread appears to be very rare in 

California. Almost all spread of 

the disease is via spread between 

infected and noninfected roots 

that are in direct contact with 

each other or by growth of 

rhizomorphs through the soil to 

noninfected roots. Rhizomorphs 

can sometimes extend 1 m or 

more through the soil away from 

an infected root. 

The fungus can survive for 

decades or longer in the soil on dead, decaying roots. Infections occur 

when healthy roots contact rhizomorphs or decayed root fragments. 

If high levels of inoculum are present in the soil, severe disease may 

develop even in resistant hosts such as oaks. High inoculum levels 

commonly develop on the dead roots of trees that have been cut or 

killed by other agents. Consequently, clusters of diseased trees may 

develop around old stumps or dead trees. Disease is also favored by 

warm, wet soils. Severe Armillaria root rot commonly develops in 

native oaks that are exposed to prolonged periods of irrigation in the 

summer. This typically occurs when irrigated landscaping is installed 

around existing oaks. Other factors that stress or damage roots, 

including soil compaction, addition of fill soil, and cutting or crushing 

of roots by construction equipment, favor the development of severe 

Armillaria root rot in oaks. 

Importance

A. mellea is one of the most important pathogens of mature 

native oaks that are retained in urban developments and become 

incorporated into irrigated landscapes. It can also be a major 

problem in native stands subjected to altered soil moisture regimes, 

such as in oaks near reservoirs or other water impoundments. In 

contrast, A. mellea is normally a minor pathogen of undisturbed 

oaks in natural stands, typically only attacking oaks that are in severe 

decline due to other factors.

Figure 137. Old cluster of A. mellea

mushrooms fruiting from buried root, 

showing clustered habit, with ring on 

central stalk.
128 A Field Guide to Insects and Diseases of California Oaks

Ganoderma root rot

Ganoderma applanatum, G. brownii, 

G. lucidum

Distribution/Hosts

Ganoderma applanatum is widely distributed in 

forests throughout North America and much of the world. It 

has a wide host range, occurring on many living hardwoods and 

conifers. It also fruits on dead trees and stumps. In California, G. 

applanatum occurs on coast live, interior live, and California black 

oak and possibly other oaks, and is common on California bay. 

G. brownii is known only from California on living and dead 

hardwoods including oak, California bay, citrus, Hakea, 

Prunus, and Schinus. It appears to be less common in California 

hardwood forests than G. applanatum or G. lucidum. 

G. lucidum is widely distributed throughout the U.S. in 

hardwood forests and urban areas, except in the Rocky 

Mountain and Pacific Northwest regions. It occurs on a wide 

variety of living hardwoods, but also fruits on dead trees and 

stumps. In California, it has been observed on coast live, 

interior live, California black, blue, and valley oaks, and 

probably occurs on oaks throughout the state.

G. annularis and G. lobatum also occur in California, but these 

species reportedly only decay dead hardwoods, including oaks.

Symptoms

The presence of the fruiting body (basidiocarp), usually near 

the base of the tree, is generally the most obvious sign of 

Ganoderma root rot. Many trees can tolerate extensive root 

loss without showing obvious canopy symptoms. Ganoderma 

root rot may also cause affected trees to fail at the roots or root 

crown. Other symptoms of root and crown rot may be present, 

including canopy thinning and dieback, and affected trees 

may undergo either a slow or rapid decline. G. applanatum

causes a mottled white rot of the roots and lower trunk. G. 
Root Diseases 129

brownii causes a white rot of the roots and lower trunk. G. 

lucidum causes a white rot of the heartwood, but can also decay 

sapwood and kill bark. 

Agent Description

Like other polypore fungi (including Phellinus, Inonotus, and 

Laetiporus), sexual spores (basidiospores) are produced on the 

inside walls of thin vertical channels that terminate as open 

pores on the lower surface of the basidiocarp.

G. applanatum produces shelf-like, woody to corky, perennial 

basidiocarps at or near ground level or on the lower trunk 

(fig. 138). Basidiocarps may be associated with old wounds. 

Basidiocarps are variable in size, ranging from 6 to 60 cm 

wide and from 5 to 10 cm thick. The upper surface is hard, 

gray-brown to brown. Basidiospores (6–9 by 4.5–6 μm) are 

brown and are commonly deposited on the upper surface of 

the basidiocarp in great numbers, forming a brown dusty layer. 

The lower surface is covered with fine circular pores (4–6 per 

mm) and is white when fresh, quickly bruising brown when 

handled or inscribed. The basidiocarp is commonly referred to 

as the “artist’s conk”. 

Figure 138. Ganoderma applanatum basidiocarp on coast live oak; 

brown basidiospores released from the fruiting body are visible both 

beneath it and on its upper surface
130 A Field Guide to Insects and Diseases of California Oaks

G. brownii produces perennial basidiocarps up to 20 cm 

wide and 5 cm thick. Basidiocarps are similar to those of G. 

applanatum except that the pore surface has a bright yellowish 

cast, which persists after drying. The growing margin of the 

basidiocarp is also yellowish. The fresh pore surface bruises dark 

brown. Basidiospores are 11–12 by 7–8 μm.

G. lucidum basidiocarps are annual, shelf-like or with a short 

stalk, and leathery to corky when fresh. They develop singly 

or in overlapping clusters at or near ground level. The upper 

surface is dark reddish brown and has a thin, shiny, varnishlike crust that becomes coated with a layer of dull brown 

basidiospores (9–12 by 5.5–8 μm) (fig. 139). The pore surface 

is creamy white, becoming light buff and bruising dark purple 

brown (fig. 140). Pores are circular to angular (4–5 per mm).

Figure 139. G. lucidum basidiocarp on California black oak; the 

coating of brown basidiospores has been rubbed away in one area to 

show the glossy upper surface.
Root Diseases 131

Figure 140. Lower 

surface of G. lucidum

basidiocarp; the white 

pore surface has 

turned brown where 

bruised by handling.

Biology

Fruiting bodies produce great numbers of basidiospores which 

are disseminated by air currents. New infections appear to 

be initiated primarily by basidiospores, with wounds being 

common infection sites. Extensive wood decay develops over 

the course of many years. Decline and mortality associated with 

Ganoderma decay may be more pronounced during periods 

of environmental stress, such as drought. In hardwoods, decay 

commonly extends 1–2 m above and below the location of 

the basidiocarp on the host. Although the white rot produced 

by Ganoderma spp. involves the degradation of lignin 

and cellulose, at least some Ganoderma spp., including G. 

applanatum and G. lucidum, cause selective delignification of 

the wood. Selective delignification, which can cause a mottled 

appearance in the decayed wood, occurs because lignin is 

degraded at a faster rate than cellulose and other polysaccharide 

components of the wood cell wall. 

Importance

G. applanatum and G. lucidum are considered to be important 

causes of root decay in natural hardwood forests. Though 

widely distributed in California, basidiocarps of these fungi are 

found at relatively low frequencies on oaks in most stands. Trees 

with basidiocarps typically have extensive decay and should be 

considered to have an elevated risk of failure. 
132 A Field Guide to Insects and Diseases of California Oaks

Phytophthora root rot

Phytophthora cinnamomi and other 

Phytophthora species

Distribution/Hosts

Phytophthora cinnamomi is a serious pathogen 

of many plant species in forest ecosystems and other native 

plant communities as well as in agricultural and horticultural 

situations. It affects plants in temperate, subtropical, and 

tropical regions on all continents except Antarctica. The 

known host range is in excess of 900 species, most of which 

are woody plants. It was probably introduced into California 

in the late 19th or early 20th century and is best known here 

as a pathogen of agricultural crops, including avocados and 

various orchard trees, and ornamental plants, including nursery 

stock and Christmas trees. It commonly affects oaks growing 

in irrigated landscapes, particularly coast live oak and cork oak, 

a non-native oak grown horticulturally. P. cinnamomi has been 

found in a native stand of coast live oak in San Diego County 

and has also become established in native manzanita stands in 

parts of Amador County where it also affects some native oaks.

Other species of Phytophthora, including P. citricola, P. cactorum, 

and P. citrophthora have also been reported to attack coast live, 

valley, and cork oaks in irrigated landscapes. 

Symptoms

Root-rotting Phytophthora species decay fine roots and can also 

invade larger woody roots and cause bleeding cankers on the 

root crown and lower trunk. In the early stages of infection, 

trees show little new growth, and develop small, yellowing 

leaves and a sparse canopy (fig. 141). Trees decline and die 

as cankers girdle the trunk. Phytophthora species attack the 

inner bark and cambium, causing dark brown discoloration 

of affected tissues (fig. 142). Dark brown to black lines are 

commonly found at the outer margin of the discolored infected 

tissue in the inner bark. Reddish brown to dark brown to 

black fluid may bleed from fissures in the bark in the cankered 
Root Diseases 133

Figure 141. Early (left) and advanced (right) decline of scrub oak (Q. 

berberidifolia) due to Phytophthora cinnamomi root rot. Both trees are 

along a small creek in an area infested with P. cinnamomi in Amador 

County.

area (fig. 143). Trunk cankers on oaks caused by P. cinnamomi

and other soil-borne Phytophthora species typically originate 

below the soil or occasionally at the soil line and are normally 

associated with root rot. Although cankers caused by P. 

ramorum are similar in appearance, P. ramorum cankers are 

initiated above the soil level, seldom extend more than a few 

cm below the soil level, and are not associated with root rot.

Agent Description

Phytophthora species fill an ecological niche similar to various 

plant pathogenic fungi, but are members of the kingdom 

Chromista, which includes both diatoms and brown algae. 

All structures of Phytophthora species are microscopic. 

Phytophthora species reproduce asexually through production 

of swimming spores (zoospores) which are produced in 

sporangia (fig. 144). Species are distinguished based on the 

sizes and characteristics of sexually produced spores (oospores), 

sporangia, thick-walled asexual spores (chlamydospores), and 

swellings in the vegetative filaments (hyphae).
134 A Field Guide to Insects and Diseases of California Oaks

Figure 142. P. cinnamomi cankers at base of cork oak. Bark 

around one canker has been partially removed on left side 

of trunk.

P. cinnamomi produces non-deciduous, nonpapillate sporangia 

with a slight apical thickening. Sporangia are formed on stalks 

that are occasionally branched but more often proliferate 

through empty sporangia. It also produces abundant numbers 

of chlamydospores and has prominent hyphal swellings. 

Chlamydospores are rarely produced by P. cactorum and P. 

citricola. 

Biology

Soil-inhabiting Phytophthora spp. can survive in the soil for 

many years. These pathogens are readily moved via infested soil 

on equipment and shoes and in infected or infested nursery 

stock. Disease development is favored by high levels of soil 

moisture, especially prolonged periods of soil saturation. Under 

moist conditions, dormant spores (chlamydospores or oospores) 

can germinate to produce hyphae that can directly infect roots 

or may form sporangia. In the presence of free water, swimming 

zoospores can be released from sporangia. Zoospores are spread 

with splashing or flowing water and are attracted to substances 

that exude from roots. Zoospores become immobile cysts on 

or near roots. Zoospore cysts produce hyphae that penetrate 

and infect roots. Hyphae proliferate through roots, killing 

root cells, which may be secondarily colonized by other soil 
Root Diseases 135

microorganisms. Sporangia 

can be produced on infected 

roots in as little as one to 

two days after infection and, 

under favorable conditions, 

can initiate additional disease 

cycles. Chlamydospores may 

be formed in dead roots. 

Chlamydospores tolerate 

drying and allow the pathogen 

to survive for extended periods 

in the soil. P. cinnamomi and 

P. citricola require warm soil 

temperatures (optimum about 

24–28  ̊C) for significant 

pathogenic activity. P. cactorum

can grow at temperatures as 

low as 2  ̊C and can cause disease at 

cool temperatures (10–20  ̊C).

Importance

Phytophthora root rots are common causes of decline in 

transplanted nursery stock and mature trees when soil moisture 

conditions are favorable for disease development, especially in 

irrigated landscapes. Disease caused by P. cinnamomi has also 

been seen in some native oaks stands into which the pathogen 

has been introduced.

Figure 143. Bleeding due to 

Phytophthora canker on cork oak.

Photo: Robert Campbell, UC Davis.

Figure 144. Micrographs of P. cinnamomi showing sporangia (left), 

sporangium releasing zoospores (center) and chlamydospores (right).
136 A Field Guide to Insects and Diseases of California Oaks

Weeping conk

Inonotus dryadeus

Inonotus dryadeus is found throughout much 

of North America and also occurs in Eurasia. It 

occurs primarily on oaks and other hardwoods 

but also occurs on some conifers. In California, it has been 

observed on coast live oak, Oregon white oak, and tanoak. I. 

dryadeus causes a mottled white rot of the roots and root crown 

of both living and dead trees. Infections reportedly begin in 

the roots and spread into the root crown, but decay does not 

extend much above ground level. Fruiting bodies (basidiocarps) 

are annual, but old basidiocarps may persist for several years. 

They develop near the base of the trunk at ground level or 

from roots below the soil 

surface. Basidiocarps vary 

in size, but can be massive, 

up to 75 cm wide and 15 

cm thick (fig. 145). Fresh 

basidiocarps are yellowish 

to brownish above and may 

be covered with drops of 

amber liquid, hence the 

name “weeping conk” (fig. 

146). The lower surface 

is buff with fine circular 

to angular pores (4–6 

per mm), but tubes can 

become obscured in older 

basidiocarps. Basidiocarps 

eventually turn blackish 

and become cracked. Trees 

with I. dryadeus fruiting 

bodies have substantial 

amounts of root decay 

and an elevated risk of 

windthrow or root crown 

failure. Figure 146. Newly emerging basidiocarp of 

I. dryadeus above small basidiocarp formed 

the previous year.

Figure 145. Large basidiocarp of Inonotus 

dryadeus.
Root Diseases 137

Western jack 

o’lantern fungus

Omphalotus olivascens

Omphalotus olivascens is known only from 

California and is found on the wood of dead 

or living hardwoods including coast live and California black 

oak, tanoak, manzanita, and redbud. Fruiting of this fungus at 

the base of living oaks suggests that it can attack live trees, but 

pathogenicity of this species has not been studied. Basidiocarps 

are stalked, gilled mushrooms (6–25 cm tall) that usually occur 

in clusters on or near trunks, stumps, or buried wood in the 

fall and winter (fig. 147). The gills extend down the stalk and 

are olive to yellow-orange and dimly luminescent when fresh. 

The caps are 4–16 (rarely to 25) cm across, bright goldenyellow to orange, brownish, or reddish, often with olive tones. 

Basidiocarp caps are initially 

broadly convex but later 

become flat to depressed 

with upturned margins. The 

spore print is cream to pale 

yellow.

Figure 147. Basidiocarps of 

Omphalotus olivascens.
138 A Field Guide to Insects and Diseases of California Oaks

Glossary

μm (micrometer) – A metric unit of length equal to one millionth of 

a meter, or one thousandth of a millimeter.

Acervulus (pl. acervuli) – A small, saucer- or cushion-shaped mass 

of conidiophores and conidia (asexual spores) that forms beneath 

the epidermis of leaves or twigs and is eventually exposed as the host 

tissue ruptures; no fungal tissue encloses the conidiophores. 

Aptera (pl. apterae) – Wingless aphids.

Ascocarp – Sexual fruiting body produced by fungi in the 

Ascomycotina. Common types of ascocarps are perithecia and 

cleistothecia. Sexual spores are produced in asci within or on the 

surface of the ascocarp.

Ascomycotina – A taxonomic group (considered variously as a 

phylum, subphylum, division, or subdivision) of the true fungi 

which produce sexual spores in asci.

Ascus (pl. asci) – A sac-like structure in which sexual spores develop, 

produced by fungi in the Ascomycotina. 

Basidiocarp – Sexual fruiting body produced by fungi in the 

Basidiomycotina. These vary widely in shape and size, and include 

mushrooms and conks. Sexual spores are produced on basidia within 

or on the surface of the basidiocarp.

Basidiomycotina – A taxonomic group (considered variously as 

a phylum, subphylum, division, or subdivision) of the true fungi 

which produce sexual spores on basidia.

Basidium (pl. basidia) – Microscopic club-shaped structures 

produced by fungi in the Basidiomycotina that produce sexual 

spores (basidiospores). Basidiospores are produced on the external 

surface of the basidium at the end of peg-like structures.

Brown rot – A type of fungal wood decay in which cellulose and 

hemicellulose are preferentially decayed, leaving behind browncolored lignin. 

Chlamydospore – A thick-walled asexual spore. Chlamydospores 

are commonly resistant spores that are adapted for surviving adverse 

conditions.
139

Chlorosis (adj. chlorotic) – Yellowing of leaves or other normallygreen plant parts.

Cleistothecium (pl. cleistothecia) – A globose fruiting body 

(ascocarp) that lacks a natural opening or pore; asci are entirely 

enclosed by the wall of the ascocarp. 

Conidiophore – A fungal hypha, often specialized in structure, that 

bears one or more conidia.

Conidium (pl. conidia) – An asexually (i.e., vegetatively) produced 

spore that is typically deciduous at maturity.

Context – The sterile tissue in a basidiocarp between the 

basidiospore-bearing tubes and the top surface (pileus). In 

resupinate basidiocarps, which lack a pileus, the context is also 

referred to as the subiculum; the subiculum is attached directly 

to the substrate.

Crawler – The young, mobile life stage of a scale insect.

Elytra – The thick, hardened forewings of beetles.

Embryo axis – within a seed, the portion of the embryo that 

includes the two growing points that develop into the root and 

shoot. In oaks, the embryo axis is at the pointed end of the acorn.

Flagellum (pl. flagella) – A threadlike or whiplike projection on a 

cell that functions to propel the cell through fluids.

Frass – Solid insect excrement, especially that produced by larvae. 

For larvae which feed in wood and/or bark, frass also includes xylem 

and/or phloem fragments (boring dust).

Honeydew – A sweet liquid excreted by some insects that suck plant 

sap, including aphids and soft scales.

Hypha (pl. hyphae) – Microscopic thread-like structures which are 

the simplest vegetative structures of most fungi (see also mycelium).

Inoculum – Units of a pathogen (such as viable spores or 

mycelium growing through soil) that can initiate infection of a 

susceptible host.

Glossary
140 A Field Guide to Insects and Diseases of California Oaks

Instar – The life stage of an immature insect or mite between 

successive molts.

Intermediate oak subgroup – Also known as the section 

Protobalanus of the genus Quercus, a taxonomic group between 

genus and species containing oaks that share characteristics 

including short flower styles; acorns that mature in two years, have 

a densely hairy inner shell, and are very bitter; leaf lobes, if present, 

are pointed. 

Larva (pl. larvae) – The immature stage between the egg and the 

pupa in insects that undergo complete metamorphosis (i.e., egg, 

larva, pupa, adult).

Mycelium – A network or mass of vegetative fungal hyphae. 

Necrosis (adj. necrotic) – Death of cells or tissues.

Nymph – The immature stage of an insect which undergoes 

incomplete metamorphosis (i.e., egg, nymph, adult).

Obovate – Inverted ovate or egg-shaped, with the wide end toward 

the tip and the narrow end toward the base.

Oviparae – Female aphids, typically wingless, which reproduce 

sexually and lay eggs.

Oviposition – The process of laying or depositing eggs. Oviposition 

wounds are damage symptoms that develop at a site where insect 

eggs are inserted directly into plant tissue.

Papillate – Having a small rounded structure (papillum); in 

Phytophthora sporangia, the papillum is located at the apex from 

which zoospores are discharged; semipapillate sporangia have a 

smaller, less distinct apical protrusion; nonpapillate sporangia lack a 

papillum, but may appear somewhat thickened apically.

Parasite – An organism that lives on or in another living organism 

(host) and obtains its food from the host without providing any 

benefit to the host.

Parthenogenic – Capable of asexual reproduction, i.e., without 

mating or male fertilization.
141

Pathogen – An organism that causes disease in another organism; 

pathogens may function as parasites or may kill host tissues and 

obtain food from the killed tissues.

Pectinate – With branches or elements like the teeth of a comb.

Perithecium (pl. perithecia) – A globose to flask-shaped ascocarp 

with a defined pore (ostiole) at the apex through which ascospores 

are disseminated.

Pocket rot – Wood decay that occurs in discrete localized zones or 

pockets, usually in the heartwood.

Pronotum – A hardened plate on the dorsal (back) surface of the 

prothorax in insects; in some insects the pronotum is enlarged or 

otherwise modified.

Prothorax – The first segment of the thorax of an insect, which 

includes the attachment points for the front pair of legs.

Pycnidium (pl. pycnidia) – A flask-shaped fungal fruiting body 

containing conidiophores that produce conidia (asexual spores).

Red/black oak subgroup – Also known as the section Lobatae 

or Erythrobalanus of the oak genus Quercus, a taxonomic group 

between genus and species containing oaks that share characteristics 

including long flower styles; acorns that mature in two years (except 

for in coast live oak), have a densely hairy inner shell, and are very 

bitter; leaf lobes, if present, are pointed; dark gray or blackish bark; 

reddish brown wood.

Resupinate (referring to a basidiocarp) – Growing flat on the 

substrate with the spore-bearing surface on the outside surface, 

lacking ancillary structures such as a stalk or a cap.

Rhizomorph – An elongate, root-like aggregation of fungal hyphae, 

often differentiated into a dark outer rind and a lighter interior.

Saprophyte – An organism that obtains food from dead organic 

material, such as dead leaves, bark, or wood; some parasitic 

organisms can also survive and grow saprophytically.

Glossary
142 A Field Guide to Insects and Diseases of California Oaks

Sooty mold – Black, superficial mycelial mats that grow on 

honeydew deposited on plant surfaces by sap-feeding insects; the 

epiphytic fungi that produce sooty mold are mostly in the family 

Capnodiaceae.

Sporangium (pl. sporangia) – A sac-like structure in which asexual 

spores are produced through the conversion of the sporangium’s 

contents into spores.

Stroma (pl. stromata) – A compact mycelial structure which 

contains or supports fungal fruiting bodies. 

Sympodial – A branching pattern in which each successive branch 

develops behind and to one side of the previous apex where growth 

has ceased.

Thorax – The middle segment of an insect body, between the head 

and the abdomen, to which the wings and legs attach.

Webbing – Silken threads produced by an insect, usually used to 

form a cocoon or other molting structure.

White oak subgroup – Also known as the section Quercus or 

Lepidobalanus of the genus Quercus, a taxonomic group between 

genus and species containing oaks that share characteristics 

including short flower styles; acorns that mature in one year, have 

a hairless inner shell, and are sweet or slightly bitter; leaf lobes, if 

present, are mostly rounded; light gray or brown bark; light brown 

or yellowish wood.

White rot – A type of fungal wood decay in which the three major 

wood components (cellulose, hemicellulose, and lignin) are decayed 

at the same rate; the wood becomes white as it decays.
143

List of plants

This list includes common and scientific names for plants 

referred to in the text. Only the genus name is noted for 

common names that include multiple species.

Common name Scientific name

acacia Acacia

alder Alnus

almond Prunus dulcis

apple Malus domestica

apricot Prunus armeniaca

ash Fraxinus

aspen Populus

avocado Persea americana

bay, California Umbellularia californica

beech Fagus

birch Betula

bitterbrush, antelope Purshia tridentata

buckeye, California Aesculus californica

ceanothus Ceanothus

chamise Adenostoma fasciculatum

cherry Prunus sp.

chestnut Castanea

chestnut, American Castanea dentata

chinquapin Chrysolepis

citrus Citrus

redwood, coast Sequoia sempervirens

coffeeberry Frangula californica 

(=Rhamnus californica)

cottonwood Populus

currant Ribes

dogwood Cornus

Douglas-fir Pseudotsuga menziesii

elm Ulmus

elm, American Ulmus americana

eucalyptus Eucalyptus

List of Plants

Continues on next pages.
144 A Field Guide to Insects and Diseases of California Oaks

Common name Scientific name

grape Vitis

hackberry Celtis

hawthorn Crataegus

hazel, hazelnut, filbert Corylus

hazelnut, California Corylus cornuta var. californica

hickory Carya

huckleberry, California Vaccinium ovatum

locust Robinia

madrone Arbutus

manzanita Arctostaphylos

maple Acer

mesquite Prosopis

oak, blue Quercus douglasii

oak, California black Quercus kelloggii

oak, canyon live Quercus chrysolepis

oak, coast live Quercus agrifolia

oak, cork Quercus suber

oak, deer Quercus sadleriana

oak, desert Quercus turbinella

oak, eastern black Quercus velutina

oak, Engelmann Quercus engelmannii

oak, English Quercus robur

oak, holly or Holm Quercus ilex

oak, huckleberry Quercus vacciniifolia

oak, interior live Quercus wislizeni

oak, island Quercus tomentella

oak, leather Quercus durata

oak, Muller Quercus cornelius-mulleri

oak, northern red Quercus rubra var. rubra

oak, Nuttall’s scrub Quercus dumosa

oak, Oregon white Quercus garryana

oak, pin Quercus palustris

oak, post Quercus stellata

Continues on next page.

Common and scientific names for plants referred to in the text 

(continued).
List of Plants 145

Common name Scientific name

oak, scarlet Quercus coccinea

oak, scrub Quercus berberidifolia

oak, Shreve Quercus parvula var. shrevei

oak, southern live Quercus virginiana

oak, southern red Quercus falcata

oak, Tucker’s Quercus john-tuckeri

oak, valley Quercus lobata

oak, water Quercus nigra

oak, willow Quercus phellos

pear Pyrus

pecan Carya illinoinensis

persimmon Diospyros

plum Prunus

poison oak Toxicodendron diversilobum

pome fruits Malus and Pyrus

poplar Populus

pricklyash Zanthoxylum

prune Prunus domestica

pyracantha Pyracantha

quince Cydonia oblonga

redbud Cercis

rose Rosa

stone fruits Prunus

sweetgum Liquidambar styraciflua

sycamore Platanus

tanoak Lithocarpus densiflorus

toyon Heteromeles arbutifolia

walnut Juglans

willow Salix

Common and scientific names for plants referred to in the 

text (continued).
146 A Field Guide to Insects and Diseases of California Oaks

References

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Ehler, L.E. 2005. Biological control of Melanaspis obscura on oaks in 

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149

Agrilus angelicus (50)

Aleuroplatus coronatus (44)

Aleuroplatus gelatinosus (44)

Ambrosia beetles (52)

Andricus spp. (28)

Aneflomorpha lineare (49)

Antron spp. (28)

Apiognomonia errabunda (76)

Archips argyrospila (16)

Archips rosana (16) 

Armillaria mellea (124)

Armillaria root rot (124)

Asterolecanium minus (40)

Asterolecanium quercicola (40)

Asterolecanium variolosum (40)

Banded alder borer (67)

Bark beetles (56)

Branch canker (88)

Brasiliomyces trina (82)

Brenneria quercina (72)

Bucculatrix albertiella (18)

California laurel borer (67)

California oakmoth (14)

California oakworm (14)

California prionus (66)

Callirhytis spp. (28)

Canker rots (96)

Carpenterworm (60)

Cercospora polytricha (76)

Choristoneura rosaceana (16)

Chrysobothris femorata (62)

Chrysobothris mali (62)

Clostridium species (122)

Colletotrichum gloeosporiodes (76)

Croesia semipurpurana (16)

Crown whitefly (44)

Cryphonectria gyrosa (90)

Cryptocline cinerescens (76)

Curculio aurivestis (10)

Curculio occidentis (10)

Curculio pardus (10)

Cydia latiferreana (12)

Cylindrosporium kelloggii (76)

Cystotheca lanestris (82)

Daedalea quercina (106)

Dicarpella bina (76)

Diplodia quercina (88)

Discula umbrinella (76)

Disholcaspis spp. (28)

Drippy nut (72)

Dros spp. (28)

Early defoliation/leaf browning/

 drought stress (74)

Endothia gyrosa (90)

Endothiella gyrosa (91)

Erineum mite (26)

Eriophyes mackiei (26)

Erwinia quercina (72)

European leafroller (16)

Filbert weevils (10)

Filbertworm (12)

Flatheaded appletree borer (62)

Flatheaded borers/Metallic wood

 boring beetles (62)

Fomitiporia robusta (111)

Forest tent caterpillar (20)

Fruit tree leafroller (16)

Gall wasps (28)

Ganoderma applanatum (128)

Ganoderma brownii (128)

Ganoderma lucidum (128)

Ganoderma root rot (128)

Gelatinous whitefly (44)

Gleosporium nervisequum (76)

Gleosporium quercinum (76)

Gleosporium quercuum (76)

Index

Index
150 A Field Guide to Insects and Diseases of California Oaks

Gnomonia errabunda (76)

Gnomonia quercina (76)

Gnomonia veneta (76)

Golden oak scale (40)

Hedgehog fungus (100)

Hericium erinaceus

 f. erinaceus (100)

Hypoxylon thouarsianum (102)

Inonotus andersonii (96)

Inonotus dryadeus (136)

Inonotus dryophilus (96)

Kuwana oak scale (32)

Kuwania quercus (32)

Laetiporus conifericola (106)

Laetiporus gilbertsonii (106)

Laetiporus sulphureus (106)

Lead cable borer (64)

Leaf spots (76) 

Malacosoma californicum (20)

Malacosoma constictum (20)

Malacosoma disstria (20)

Marssonina martinii (76)

Melanaspis obscura (38)

Microsphaera extensa curta (82)

Microsphaera penicillata (82)

Monarthrum dentiger (52)

Monarthrum scutellare (52)

Nautical borer (68)

Neoclytus conjunctus (69)

Neuroterus spp. (28)

Oak anthracnose (76)

Oak bark beetles (56) 

Oak leaf blister (80)

Oak leaf phylloxera (34)

Oak leaftier (16)

Oak lecanium scale (36)

Oak mistletoe (92)

Oak pit scale (40)

Oak ribbed casemaker (18)

Oak root fungus (124)

Oak twig borer (48)

Oak twig girdler (50)

Obliquebanded leafroller (16)

Obscure scale (38)

Omphalotus olivascens (137)

Orange hobnail canker (90)

Orgyia vetusta (24)

Pacific flatheaded borer (62)

Pacific tent caterpillar (20)

Parthenolecanium quercifex (36)

Phellinus gilvus (110)

Phellinus robustus (111)

Phoradendron villosum (92)

Phryganidia californica (14)

Phycitid oak leaftier (16)

Phyllactinia angulata (82) 

Phylloxera spp. (34)

Phytophthora cactorum (132)

Phytophthora cinnamomi (132)

Phytophthora citricola (132)

Phytophthora citrophthora (132)

Phytophthora nemorosa (112)

Phytophthora pseudosyringae (112)

Phytophthora ramorum (112)

Phytophthora ramorum canker 

(112)

Phytophthora root rot (132)

Phytophthora stem canker (112)

Pigeon tremex (65)

Pit scales (40)

Platycotis vittata (42)

Platycotis vittata quadrivittata (42)

Pleurotus species (110)

Powdery mildews (82)

Prionoxystus robiniae (60)

Prionus californicus (66)
151

Pseudomassaria agrifolia (76)

Pseudopityophthorus agrifoliae (56)

Pseudopityophthorus pruinosus (56)

Pseudopityophthorus pubipennis (56)

Rosalia funebris (67)

Roundheaded borers/

 Long-horned beetles (66)

Scobicia declivis (64)

Scobicia suturalis (49)

Septoria dryina (76)

Septoria quercicola (76)

Setiostoma fernaldella (16) 

Stanford’s whitefly (44)

Stegophylla essigi (46)

Stegophylla querci (46)

Stegophylla quercifoliae (46)

Stemonid oak leaftier (16)

Stereum species (110)

Styloxus fulleri californicus (48)

Sudden oak death (112)

Sulfur fungus (106)

Sunscald (118) 

Synanthedon mellinipennis (70)

Synanthedon resplendens (70)

Taphrina caerulescens (80)

Tent caterpillars (20)

Tetraleurodes stanfordi (44)

Trachycera caliginella (16)

Trametes species (110)

Trametes versicolor (118)

Treehoppers (42)

Tremex columba (65)

Twig blight (76)

Weeping conk (136)

Western jack o’lantern 

 fungus (137)

Western oak bark beetle (56)

Western sycamore borer (70)

Western tent caterpillar (20)

Western tussock moth (24)

Wetwood and alcoholic flux (120)

Whiteflies (44)

Woolly oak aphids (46)

Xylotrechus nauticus (68)

Index

0 1 2 3 4 5 6 7 in
Curculio aurivestis, Curculio occidentis, Curculio pardus, Cydia latiferreana, Phryganidia californica, Archips argyrospila, Bucculatrix albertiella, Malacosoma californicum, Malacosoma constictum, 

Malacosoma disstria, Orgyia vetusta, Eriophyes mackiei, Andricus spp., Antron spp., Callirhytis spp., Disholcaspis spp., Dros spp., Neuroterus spp., Kuwania quercus, Phylloxera spp., Parthenolecanium quercifex, Melanaspis obscura, Asterolecanium minus, Asterolecanium quercicola, Asterolecanium variolosum, Platycotis vittata, Platycotis vittata quadrivittata, Aleuroplatus coronatus, Aleuroplatus gelatinosus, Tetraleurodes stanfordi, Stegophylla querci, Stegophylla quercifoliae, Stegophylla essigi, Styloxus fulleri californicus, Aneflomorpha lineare, Scobicia suturalis, Agrilus angelicus, Monarthrum dentiger, Monarthrum scutellare, Pseudopityophthorus pubipennis, Pseudopityophthorus agrifoliae, Pseudopityophthorus pruinosus, Prionoxystus robiniae, Chrysobothris femorata, Chrysobothris mali, Scobicia declivis, Tremex columba, Prionus californicus, Rosalia funebris, Synanthedon resplendens, Brenneria quercina, Apiognomonia errabunda, Cryptocline cinerescens, Septoria quercicola,Taphrina caerulescens, Brasiliomyces trina, Cystotheca lanestris, Microsphaera extensa curta, Microsphaera penicillata, Phyllactinia angulata, Diplodia quercina, Cryphonectria gyrosa, Phoradendron villosum,Inonotus andersonii, Inonotus dryophilus, Hericium erinaceus f. erinaceus, Hypoxylon thouarsianum, Laetiporus gilbertsonii, Phellinus gilvus, Phellinus robustus, Phytophthora ramorum, Phytophthora nemorosa, Phytophthora pseudosyringae, Armillaria mellea, Ganoderma applanatum, Ganoderma brownii, Ganoderma lucidum, Phytophthora cinnamomi, Inonotus dryadeus, Omphalotus olivascens