1 USDA Forest Service Res. Note RMRS–RN–6. 1999 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 pro- pose a preliminary hazard model for the Sacra- mento 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 topo- graphic position. Information is currently avail- able 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 sever- ity 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 throughoutmostoftherangeofsouthwesternwhite 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 In- dian 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 Moun- tains (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,personalcommunication).Othernearbypopu- lations 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, Albu- querque, New Mexico. 3 Retired Plant Pathologist and volunteer with the Rocky Mountain Research Station, Flagstaff, Arizona.