Resistance of Ponderosa Pine to Western Dwarf Mistletoe in Central Oregon

Ponderosa pines with little or no dwarf mistletoe in infested stands on the Deschutes, Ochoco, and Rogue River National Forests in Oregon were tested for resistance to dwarf mistletoe (Arceuthobium campylopodum). Small trees produced by grafting scions from the resistant and susceptible candidates onto seedling rootstock were planted in 1967-69 beneath a heavily infested stand of ponderosa pine in central Oregon. Between 12 and 15 grafted trees from each of 30 resistant parent selections and about the same number from each of eight susceptible parents were included in a replicated, randomized planting. Also included as controls were about 125 ungrafted seedlings. In 1989, fewer than half of the resistant grafts from selections from the Deschutes and Ochoco National Forests were infected, and the average level of infection per tree was very low. Mortality among the resistant selections was also low over the test period. About 85 percent of all Rogue River grafts, the susceptible grafts, and the seedlings were infected during the test period. Few Rogue River grafts died, but 52 percent of the susceptible grafts and seedlings died in comparison to 24 percent of the resistant trees. Size of the test trees, foliar habit, andcrown characteristics were not found to be correlated with resistance. High levels of resistance to dwarf mistletoe are present in some native ponderosa pines in central Oregon. Cover: The tallest ponderosa pine in the center is only lightly infected with dwarf mistletoe, even though adjacent pines were heavily infected. Grafts made from this tall pine and exposed to heavily infected overstory also showed low levels of infection after 20 years in stands in northern California.