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White Pine Blister Rust Epidemiology in Widely Dispersed Populations of Five-Needle Pines in the Intermountain West Region of the United States

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White Pine Blister Rust Epidemiology in Widely Dispersed Populations of Five-Needle Pines in the Intermountain West Region of the United States WHITE PINE BLISTER RUST EPIDEMIOLOGY IN WIDELY DISPERSED POPULATIONS OF FIVE-NEEDLE PINES IN THE INTERMOUNTAIN WEST REGION OF THE UNITED STATES James T. Hoffman1 and Jonathan P. Smith2 ABSTRACT: In 1990 white pine blister rust (Cronartium ribicola) was found on Pinus strobiformis in southern New Mexico, more than 900 km from known rust populations. One hypothesis for the long- distance spread was natural migration of the disease via widely dispersed populations of five-needle pine trees found in the Intermountain West. Surveys conducted in the 1960’s suggested that predominantly arid conditions, and lack of close geographic continuity between potential host stands, would limit further disease spread though the region. We initiated surveys in 1995 to determine current white pine blister rust epidemic characteristics (disease incidence, intensity, and mortality). Analyses of data of over 5,400 trees in 127survey plot-transects indicate white pine blister rust incidence and intensity have increased since survey estimates in 1967. Mortality rates caused by blister rust disease are very low in the Intermountain West compared to other infected areas in the country. Since the earlier surveys southward expansion of the disease remains stationary, however, infected stands were found for the first time in western Nevada. Absence of white pine blister rust in Utah and most of Nevada suggests no connectivity of the disease from long-known infection sites in Idaho and Montana to the New Mexico infection sites. Keywords: Cronartium ribicola, Pinus, white pines, white pine blister rust, Rocky Mountain forests. INTRODUCTION In 1990, white pine blister rust (Cronartium ribicola) was found infecting southwestern white pine (Pinus strobiformis) in southern New Mexico (Hawksworth 1990). The nearest known infection sites were in southern Wyoming (900 km) or California (1500 km). In the middle of both these infection sources is the Intermountain West area of the United States. This region is a generally arid landscape of isolated, mostly parallel mountain ranges separated by broad valleys (Figure 1). To early pioneers the area was known as the “Great Desert.” The vast area includes most of southern Idaho and southwestern Wyoming, western Colorado, all of Utah, and Nevada, and portions of eastern California and Oregon. White pine seedlings infected with the blister rust fungus were brought into western North America from a nursery in France in 1910. Radiating out from the initial infection site in Vancouver, British Columbia, Canada, the blister rust disease quickly spread throughout the range of western white pine (Pinus monticola) down the Cascade Mountains and into the Sierra Nevada mountains in California (Hummer 2000). By 1950 the disease had spread throughout most of the ranges of western white pine, and whitebark pine (Pinus albicaulis), and through the northern range of limber pine (Pinus flexilis) in Wyoming. Blister rust disease surveys were conducted in some of the area in the 1960’s. Results suggested the southward spread of the fungus had been slowed or even stopped at the northern and western edges of the Intermountain West by predominantly arid conditions, and lack of close geographic continuity between potential host stands. However, a comprehensive survey of the area was needed to test this assertion. There are five species of white pines (5-needle pines) in the Intermountain West and all are susceptible to white pine blister rust infection (Hoff et al. 1980). Western white pine and sugar pine (Pinus lambertiana) have the most limited ranges in the study area, occurring only in extreme western Nevada near Lake Tahoe in the foothills of the Sierra Mountains. Great Basin bristlecone pine (Pinus longaeva) occurs in fewer than 50 high-elevation (>3,200m) locations scattered throughout the states of Utah and Nevada. Whitebark pine 1 Plant Pathologist, US Dept. of Agriculture, Forest Service, Forest Health Protection, Boise, Idaho, USA 2 Research Associate, Northern Arizona University, School of Forestry, Flagstaff, Arizona, USA is also a high-elevation subalpine species that generally grows north of the same habitats as bristlecone pine. Aside from populations of whitebark pine that extend as far south as the 37th north parallel latitude along the Sierra Nevada Mountains in California, the Intermountain Region represents the southern and eastern extent of the range of this species in North America. The most widely distributed white pine in the Intermountain Region, and indeed in the western United States, is limber pine (Little 1971) undoubtedly because of its ability to withstand dry conditions and calcareous substrates (Steele, et al. 1983). Limber pine has a wide elevational range of distribution growing between 1,500m and 3,050m.within the study area, however, the actual range of habitats that the pine dominates is narrow. In many habitats limber pine occurs as widely scattered individuals rather than a communal setting. Limber pine rarely co-occurs with whitebark pine and there is no evidence of competitive exclusion between the species (Steele, et al. 1983). Although white pines in the Intermountain West region are limited in their distribution and frequency, they are important because they provide stability for hydrologic functions, habitat and food for numerous birds and mammals, and aesthetic values for humans. Figure 1. Intermountain West region in relation to western United States. METHODOLOGY During the summers of 1995 through 1997 we conducted a white pine blister rust survey of white pines in the Intermountain West to document current disease epidemic characteristics, and to see if a continuous, or even broken corridor of infected pines occurs from populations of the rust in the California, southern Idaho, or Wyoming to the New Mexico infection site. The total study area measured approximately 50-million hectares with white pines occupying less than 0.01% of the area. We tried using United States Department of Agriculture, Forest Service timber inventories to find locations of white pines, however the limber pine species group is classified as non- commercial timber and rarely was reported. Consequently our best source of white pine locations was the personal recollections of forest workers. To further complicate the blister rust disease survey, white pines are found in remote, mountainous locations that often limited accessibility to lengthy foot travel. Despite major problems with access, we surveyed white pines in 127 subalpine locations during the summers of 1995–1997. To facilitate collection of survey data from widely dispersed populations of subject trees, we used a strip transect survey plot design. Starting from an arbitrary point in a stand of white pines, 4.6 m wide strip transects were oriented either along or perpendicular to the contour of a slope, depending upon the slope incline. An established azimuth was followed until at least 50-white pines greater than 1.3 m had been evaluated, or there was a change in continuity and consistency of stand and site conditions. In this case, crews would off-set 4.6 m, establish a new starting point, and traverse in the opposite direction of the first azimuth toward the starting point until 50 trees had been sampled. Given the uneven distribution of white pines it was sometimes impossible to find 50 sample trees in an area or mountain range to survey. In this case, crews evaluated the proximal distance to other survey plots, weighing in the time it took to get to the potential plot location. If the site filled a void in the overall geographic map of the Intermountain West, a plot was established. Site data collected included aspect, slope, elevation, topographic position, and horizontal canopy structure. Tree data collected included: live or dead condition, diameter at breast height (1.37 m), crown vigor, and tree damage (biotic and abiotic causes). We developed the following definitions for disease incidence, intensity, and damage. Blister rust disease incidence is the number of infected sample locations divided by the total number of stands sampled. Intensity is the percentage of live trees in a sampled stand that had a least one infection. Damage is based on the presence of stem cankers or on the distance of branch cankers to the main stem. Potentially lethal infections include all stem cankers and branch cankers within 15 cm of the stem. Binoculars were used for observation and assessment of foliar and stem disease signs and symptoms of blister rust infections. RESULTS AND DISCUSSION We surveyed 5,472 white pines in 127 stands for blister rust disease in the Intermountain Region during the summers of 1995 to 1997 (Table 1). The number of trees per plot ranged from 4 to 89-trees and averaged 43 trees per plot. White pine blister rust was found in 74 of the 127 (59% incidence) white pine surveyed stands. The average percentage of infected trees (disease intensity) within these stands was 36%. Potentially lethal cankers, those on the stem or within 15 cm of it, were found on 61% of infected live trees. Only 452 dead trees (8% of total) were tallied. There were 154 trees categorized as recent dead (within 5- years) and cause of death of 34 of these (22%) was attributed to blister rust disease. Specific plot locations with latitude and longitude coordinates were listed in a previous report (Smith and Hoffman 2000). Table 1. Incidence or white pine blister rust cankers, potentially lethal cankers, and intensity in formal white pine sample stands and in all sample stands
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