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Mapping the distribution and incidence of southwestern dwarf mistletoe on ponderosa pine in New Mexico National Forests using roadside surveys

Gregory Reynolds: US Forest Service, Forest Health Protection

<div>Southwestern dwarf mistletoe (SWDM), <em>Arceuthobium vaginatum</em> subsp. <em>cryptopodum</em>, is the most damaging pathogen on ponderosa pine, <em>Pinus ponderosa</em>, in the Southwest. Although the parasite is native to the region, fire suppression has led to overstocking that may have facilitated increased spread through stands. Incidence of SWDM was previously rated along roadsides in New Mexico National Forests in 1960 and 1987. Our objective is to map current distribution and incidence of SWDM in the State and identify changes since these previous surveys. Over 150 miles of roadways throughout three Ranger Districts in two New Mexico National Forests were traversed at 15 mph with an observer noting relative incidence of SWDM along 0.1 mile road increments to a width of 66’ into the forest (representing 0.8 acres per increment). Incidence was rated from 0-3, with 0 representing no mistletoe observed, 1 representing less than 33% incidence, 2 representing 33-66% incidence, and 3 representing greater than 66% incidence. Data were georeferenced using GPS, and maps were developed in ArcGIS. Summary statistics were developed and compared to previous surveys to identify changes in SWDM. Infection was not observed across 67% of the assessed area. Light infection was present in 18.6% of the area, moderate infection was present in 9.2% of the area, and heavy infection was present in 5.2% of the area. Overall, SWDM has spread into previously uninfected areas and increased in incidence.</div>