USDA Forest Service Res. Note RMRS–RN–6. 1999 1

A Preliminary Hazard Model of White Pine

Blister Rust for the Sacramento Ranger

District, Lincoln National Forest

Brian W. Geils1, David A. Conklin2, and Eugene P. Van Arsdel3

United States

Department of

Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS–RN–6

August 1999

Abstract—Blister rust, caused by the introduced

fungus Cronartium ribicola, is a serious disease of

white pines in North America. Since about 1970, an

outbreak has been increasing in the Sacramento

Mountains of southern New Mexico and threatens

southwestern white pine. To help determine the

expected extent and impact of blister rust, we propose a preliminary hazard model for the Sacramento Ranger District. The model is based on field

observations and experience. We assume blister

rust incidence and severity on white pine varies

with microclimate and proximity to telial hosts

(certain species of Ribes). We identify the sites at

risk and rank them into three relative hazard classes

based on elevation, plant association, and topographic position. Information is currently available to provisionally identify hazard for blister rust

on 35% of the district area at risk. Of the area rated,

12% is low hazard, 43% is moderate hazard, and

45% is high hazard. Average rust incidence level

for plots rated as low, moderate, or high hazard

was 6%, 45%, and 47%, respectively; average severity was 0.1, 2.5, and 4.5 cankers per tree. Studies are

underway to test and refine the model.

Keywords: Cronartium ribicola, southwestern white

pine, Pinus strobiformis

Introduction

White pine blister rust, Cronartium ribicola J. C.

Fisch., is an introduced pathogen to North America

where it causes serious economic and ecological

damage to white pines (Ziller 1974). The fungus

has a complex life cycle of alternating spore stages

between white pines (section Strobus, aecial hosts),

where it causes perennial cankers, and currents or

gooseberries (Ribes, telial hosts), where it causes a

foliage disease.

Since the initial report of the disease in the

Sacramento Mountains of southern New Mexico

(Hawksworth 1990), the rust has been found

throughout most of the range of southwestern white

pine (Pinus strobiformis Engelm.) in the Sacramento

and adjoining White Mountains. This area includes

the Sacramento Ranger District (RD), much of the

Smokey Bear RD, and the Mescalero Apache Indian Reservation. Based on the apparent age of

cankers, the rust first became established around

1970 on the west side of the Sacramento Mountains.

After remaining at relatively low levels for several

years, a major expansion occurred around 1985

(Hawksworth and Conklin 1990, Conklin 1994,

Van Arsdel and others 1998). Since then, there have

been several years in which significant numbers of

new infections have occurred on pine. In 1994,

infected pines were found in the Capitan Mountains (Smokey Bear RD), an isolated range about 30

miles north of the main outbreak area (Conklin and

Schultz, 1999, personal communication). In 1999,

two infected white pines were found on Gallinas

Peak (Cibola National Forest), about 50 miles north

of the Capitan Mountains (Van Arsdel and Conklin,

1999, personal communication). Other nearby populations of white pine at risk are on the San Mateo

and Magdalena Mountains.

1 Research Plant Pathologist with the Rocky Mountain

Research Station, Flagstaff, Arizona.

2 Plant Pathologist with the Southwestern Region, Albuquerque, New Mexico.

3 Retired Plant Pathologist and volunteer with the Rocky

Mountain Research Station, Flagstaff, Arizona.
2 USDA Forest Service Res. Note RMRS–RN–6. 1999

Peterson and Jewell (1968) suggested that conditions in the Southwest were suitable for an “explosion” of blister rust. Especially in the Sacramento

Mountains, the abundance of susceptible hosts and

favorable climate permits development of a serious

epidemic. Southwestern white pine is present

throughout much of the forest, and Ribes pinetorum

Greene (a telial host that supports abundant rust

production) is common in many locations (Conklin

1994, Van Arsdel and others 1998). July and August

rains favor rust dispersal and development

(Kimmey and Wagener 1961). If a severe outbreak

of blister rust reduced or eliminated southwestern

white pine, this would have negative impacts on

biodiversity, forest health, and wildlife habitat

(Conklin 1994, Dahms and Geils 1997).

Although the blister rust outbreak in the Southwest is recent, several efforts have described its

status and planned appropriate research and management (Geils 1993, Conklin 1994, Van Arsdel and

others 1998). In 1997, the Special Technology Development Program (USDA Forest Service, Forest

Health Protection and Vegetation Management and

Protection Research) funded a project to determine

the expected effects of white pine blister rust on the

distribution and abundance of white pine in the

Southwest. The description and work plan are available on line (The Peridium 1999). A major task of that

project is to develop, test, and revise a model that

can be used to map hazard for blister rust across the

Sacramento RD using data already available in the

Rocky Mountain Resource Information System

(RMRIS) and geographic information system (GIS).

This task is preliminary to construction of a more

detailed hazard model requiring site-specific data

on Ribes distribution and microclimatic patterns

(McDonald and others 1990). Hazard maps provide a tool for management planning at forest and

project scales, a basis to stratifying landscapes for

damage and impact surveys, and a foundation for

additional epidemiological and ecological studies.

Hazard Rating

Forest disease outbreaks are described in the

epidemiological terms of rates of spread and intensification, incidence, severity, damage, and ecological impacts. Hazard rating provides a means

for combining these attributes into a relative index

or classification for comparing disease potential

among sites. Rust epidemiology is strongly influenced by genetics, abundance, and proximity of

hosts and by microclimate. Because these factors

vary in time and space, rust behavior and effects

are difficult to characterize. Direct measurement of

spread and intensification requires either years of

observation or a reliable dating method. Incidence

and damage surveys that only describe present

conditions have limited predictive value when the

outbreak is still increasing. But a knowledge of rust

epidemiology combined with information on forest conditions can be used to produce a hazard

model for mapping disease potential. Hazard models may project percent of trees infected, number of

cankers per tree, volume lost and killed, or changes

in various ecological indicators. In the preliminary

hazard model presented here, we rank forest sites

with regard to existing and/or expected rust infection on white pine. In combination with a damage

or impact model, this information can be used to

project resource losses or ecological effects.

A Preliminary Model

Our observations in the Sacramento Mountains

and experience with older rust outbreaks (Van

Arsdel and others 1961, Van Arsdel and Krebill

1995) identify two primary factors that affect infection rate: microclimate and proximity to a telial

host. These factors, however, are not easily determined; and their relationship to infection rate is

complex. Elevation, plant association (i.e., potential vegetation), and topographic position are standard elements used to describe forest sites (note: a

site or land unit is the smallest area mapped in the

GIS and described in the RMRIS). These features

serve in our preliminary model as surrogates for

microclimate and proximity. We are focusing on

the Sacramento RD for initial model development

since it includes the greater proportion of the outbreak area and the largest concentration of white

pines in the Southwest. However, the model may

be applicable in other portions of the outbreak area

as well. We are also attempting to determine relative hazard in other ranges in New Mexico and

Arizona that contain white pines.

The landform of the Sacramento Mountains is a

tilted, block uplift with a steep west escarpment

and gentle east slope dissected by broad canyons

(Alexander and others 1984). The general landform

determines regional precipitation and vegetation;

local topography modifies these at the site level.
USDA Forest Service Res. Note RMRS–RN–6. 1999 3

Annual precipitation is greatest along the escarpment ridge (over 25 inches above 8,000 feet);

amounts decrease sharply to the west and gradually to the east (under 20 inches below 7,500 feet).

Plant associations are broadly zonal with elevation; but cooler, wetter aspects and positions allow

associations to occur at lower elevations. Southwestern white pine and Ribes species have broad

ecological requirements. They are more common

on cool, mesic sites; they generally occur above

7,500 feet on the western escarpment and above

7,000 feet on the eastern slope. The habitat types

(Alexander and others 1984) that comprise the mesic

mixed conifer plant associations (and rare sprucefir types) occur on the coolest, wettest sites where

white fir (Abies concolor (Gord. & Glend.) Hilde.) is

the dominant climax tree species. On warmer and

drier sites of the xeric mixed conifer plant associations, Douglas-fir (Pseudotsuga menziesii (Mirb.)

Franco) is the dominant climax tree species. On the

warmest and driest forested sites, ponderosa pine

(Pinus ponderosa Doug. ex Laws. & Laws.) is the

dominant climax species. Southwestern white pine

and Ribes are rare or absent in other plant associations. Topographic position not only affects a site’s

annual climate and vegetation, it also influences

diurnal temperature-moisture patterns. Cool, moist

air descending from upper slopes and settling into

valley bottoms during the critical stage of rust

dispersal to pine creates favorable conditions for

infection (Van Arsdel and others 1961).

We define 27 strata from three levels of the three

factors (elevation, plant association, and topographic position). These strata are used to rank

sites for hazard and to allocate sampling effort for

testing the model. Sites are ranked for increasing

hazard as follows:

• elevation—sites below 7,500 feet, 7,500-

8,000 feet, and above 8,000 feet;

• plant association—pine, xeric mixed conifer, and mesic mixed conifer; and

• topographic position—sites on ridges and

upper slopes, mid-slopes, and lower slopes.

Although these factors may be confounded and

could be weighted and combined various ways, we

use a simple, additive model for reducing 27 strata

to three hazard levels. Each stratum is assigned a

score of 0, 1, or 2 for increasing hazard from each

factor, scores are summed, and levels are assigned.

Sites with a composite score of 0, 1, or 2 are low

hazard; sites with a score of 3 or 4 are moderate

hazard; and sites with a score of 5 or 6 are high

hazard. Sites at risk of blister rust infestation lay

between the 7,500-foot contour on the west and the

7,000-foot contour on the east (the approximate

range of southwestern white pine). The preliminary hazard map (see page 5) is compiled by scoring sites with data from the RMRIS database (USDA

Forest Service 1998); sites are mapped with location data from the GIS. The hazard class area is

obtained by summation (table 1). The model is

tested in six study areas that are distributed across

the district and include the sample strata (map on

page 5 and see Future Model Development).

For that portion of the Sacramento RD (federal

lands) considered at risk, the preliminary model

classifies 35% of the area into three rust hazard

levels (map on page 5 and table 1). Of the 97,234

acres that were classified, 12% was identified as

low hazard, 43% as moderate, and 45% as high.

Sixty-five percent of the area at risk was not classified because information for either ownership, elevation, plant association, or slope position was

not available in the RMRIS database. Information

may be missing because data have not been updated since the last fire or silvicultural treatment.

Considering the landform and distribution of vegetation, we suspect much of the unclassified area

would be ranked as moderate to high hazard. Classifying these additional areas could increase the

extent of moderate and high hazard by 50 to 100%.

Because data are still being collected on rust

distribution and history, the statistical significance

of the model rankings have not been tested and

alternative formulations of the model have not

been assessed. However, available data (Van Arsdel

Table 1. White pine blister rust hazard on the Sacramento District, Lincoln National Forest. Hazard area is

determined for federal lands at risk by application of a

preliminary hazard model to site data extracted from

the RMRIS database, compiled 5/3/1999. The area of

missing information includes areas for which data on

ownership, elevation, plant association, or topographic

position were not available.

Hazard zone Area, acres

Low hazard 11,214

Moderate hazard 42,066

High hazard 43,954

Missing information 183,682
4 USDA Forest Service Res. Note RMRS–RN–6. 1999

acteristics, future refinements could include factors such as current vegetation, stocking, or crown

density, which are affected by silvicultural treatment. This second generation hazard model could

be used to assess how vegetation management

projects might reduce the impacts of the blister rust

outbreak.

Acknowledgments

We are grateful for the valuable work of the

many individuals who contributed to this project.

John Popp, Rocky Mountain Research Station, participated in various early surveys of the district,

assembled information, and helped design the sampling procedure. Brytten Nelson, Rocky Mountain

Research Station, compiled the hazard map using

GIS data from Linda Cole, Lincoln National Forest,

and RMRIS data from Pat Jackson, Southwestern

Region. This study is funded by the Special Technology Development Program as Project 3-97-1,

Hazard Rating White Pine Blister Rust in the Southwest.

Table 2. Incidence and severity of blister rust for

selected plots sorted by a preliminary hazard model.

Adapted from Van Arsdel and others (1998).

Hazard Percent Cankers

Plot location rating infected per tree

Lower Fence Low 2 0.02

Poison Spring Low 8 0.1

Sixteen Springs Low 9 0.1

James Ridge Moderate 16 0.4

Bonito Lake Moderate 27 0.4

Hoosier Moderate 63 3.9

Little Apache Moderate 72 5.2

Monjeau High 4 0.04

Carisa Lookout High 26 0.4

Wills Canyon High 58 2.6

Upper Fence High 52 3.5

Silver Springs High 95 15.9

and others 1998) can be used to characterize rust

incidence and severity by hazard class. The plots

established by Conklin (average 46 white pine trees

per plot) are classified by the preliminary model

into three plots of low hazard, four plots of moderate hazard, and five plots of high hazard. The

average incidence of blister rust infected white

pine trees is 6%, 45%, and 47%, respectively. The

average number of cankers per tree is 0.1, 2.5, and

4.5, respectively (table 2). Ribes pinetorum is found

to be common at the Hoosier and Little Apache

plots but not found at the Monjeau and Carisa

Lookout plots. The preliminary model appears reasonable but discrimination may be improved by

including information on proximity to Ribes

pinetorum.

Future Model Development

Our observations suggest that white pine blister

rust is spreading across the district in the early

phase of exponential expansion (Van Arsdel and

others 1998). We have visited numerous mesic sites

close to susceptible Ribes bushes where the white

pine trees bear only a few young cankers. In time,

rust incidence and severity on these sites is expected to increase. Our projections of the extent of

rust hazard (map and table 1) are therefore provisional. The model is being tested and refined with

data from studies described by Van Arsdel and

others (1998). Plots on six study areas provide

detailed information on microclimate, rust severity

and history, and Ribes by strata for elevation, plant

association, and topographic position (see The Peridium).

A hazard model based on the fundamental land

management units (sites) and standard inventory

elements (elevation, plant association, and topographic position) is readily integrated into various

planning and monitoring activities. The model

could be used at the forest planning level to assess

where significant change in forest species composition is expected. Information sources in addition to

the RMRIS database may be used to extend the map

hazard over a greater area. Although the preliminary model is constructed using physical site char-

6 USDA Forest Service Res. Note RMRS–RN–6. 1999

Literature Cited

Alexander, B.G., Jr.; Ronco, F., Jr.; Fitzhugh, E.L.; Ludwig, J.A.

1984. A classification of forest habitat types of the Lincoln

National Forest, New Mexico. Gen. Tech. Rep. RM-104. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service,

Rocky Mountain Forest and Range Experiment Station.

Conklin, D.A. 1994. White pine blister rust outbreak on the

Lincoln National Forest and Mescalero-Apache Indian Reservation, New Mexico. Report R3-94-2. Albuquerque, NM:

U.S. Department of Agriculture, Forest Service, Southwest

Region. 12 p.

Conklin, D.A. 1999. [Letter to B. W. Geils]. May 6. 1 leaf. On file

at: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Research Station, Southwest Forest Science Complex, Flagstaff, AZ; BWG files.

Dahms, C.W.; Geils, B.W., tech. eds. 1997. An assessment of

forest ecosystem health in the Southwest. Gen. Tech. Rep.

RM-GTR-295. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range

Experiment Station. 97 p.

Geils, B.W. 1993. An outline for proposed research and cooperative studies on blister rust of southwestern white pine. In:

Allison, J., comp. Proc. 41st Annual Western Forest Disease

Work Conference; 1992 September 13–17; Boise, ID. San

Bernadino, CA: U.S. Department of Agriculture, Forest Service, San Bernadino National Forest: 103–111.

Hawksworth, F.G. 1990. White pine blister rust in southern

New Mexico. Plant Disease. 74: 938.

Hawksworth, F.G.; Conklin, D.A. 1990. White pine blister rust

in New Mexico. In: Hoffman, J.; Spiegel, L.H., comps. Proc.

38th Western Forest Disease Work Conference; 1990 September 17–21; Redding CA. Boise, ID: U.S. Department of Agriculture, Forest Service, Forest Pest Management: 43–44.

Hoff, R.J.; Bingham, R.T.; McDonald, G.I. 1980. Relative blister

rust resistance of white pines. European Journal Forest

Pathology. 10: 307–316.

Kimmey, J.W.; Wagener, W.W. 1961. Spread of white pine

blister rust from Ribes to sugar pine in California and Oregon. Tech. Bull. 1251. U.S. Department of Agriculture,

Forest Service, Pacific Southwest Research Station. 71 p.

McDonald, G.I.; Geils, B.W.; Scripter, M.W. 1990. Integration of

GIS and ecosystem process models: Site-specific mapping of

rust hazard. In: Proc. IUFRO XIX World Congress; 1990

August 5–11; Montreal: 205.

Peterson, R.S.; Jewell, F.F. 1968. Status of American stem rusts

of pine. Annual Review of Phytopathology. 6: 23–40.

The Peridium. (1999, February 1—last update). [Homepage of

Brian W. Geils], [Online]. Available: http://

www.rms.nau.edu/rust/ [1999, May 3, 1999].

USDA Forest Service. 1998. Rocky Mountain Resource Information System. Unpublished data dictionary and appendices indices by U.S. Department of Agriculture, Forest Service, [Southwestern Region, Albuquerque, NM], version

March 1998.

Van Arsdel, E. P. 1999. [email to Vogler, Conklin, and Geils;

subject: Blister rust and Ribes 5/17]. May 21. Copy on file at:

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Southwest Forest Science Complex,

Flagstaff, AZ; BWG files.

Van Arsdel, E.P.; Conklin, D.A.; Popp, J.B.; Geils, B.W. 1998. The

distribution of white pine blister rust in the Sacramento

Mountains of New Mexico. In: Jalkanen, R.; Crane, P.E.;

Walla, J.A.; Aalto, T., eds. Proc. 1st IUFRO Rusts of Forest

Trees Working Party Conference; 1998 August 2–7;

Saariselkä, Finland. Res. Pap. 712. Rovaniemi, Finland: Finnish Forest Research Institute: 275–283.

Van Arsdel, E.P.; Krebill, R.G. 1995. Climatic distribution of

blister rust on pinyon and white pine in the USA. In: Kaneko,

S.; Katsuya, K.; Kakishima, M.; Ono, Y., eds. Proc. 4th IUFRO

Rust of Pines Working Party Conference; 1994 October 2–7;

Tsukuba, Japan. Kukizaki, Japan: Forestry and Forest Products Research Institute, Forest Microbiology Section: 127–

133.

Van Arsdel, E.P.; Riker, A.J.; Kouba, T.F.; Suomi, V.E.; Bryson,

R.A. 1961. The climatic distribution of blister rust on white

pine in Wisconsin. Stn. Pap. 39. St. Paul, MN: U.S. Department of Agriculture, Forest Service, Lake States Forest Experiment Station. 34 p.

Ziller, W.G. 1974. The tree rusts of western Canada. Pub. 1329.

Victoria, BC: Department of the Environment, Canadian

Forestry Service. 272 p.