Dwarf Mistletoe Damage to Small Ponderosa Pines

Ponderosa pine saplings developed from grafts with mistletoe-infected scions were compared with saplings developed from grafts with healthy scions. Healthy and diseased scions were from the same source trees. After 12 years, 33 percent of the mistletoe-infected trees had been killedand the remainder were only half as tall as the uninfected. The endophytic system of the mistletoe bridged the graft union in 30 percent of the infected trees. In a second experiment, small saplings were repeatedly inoculated with mistletoe seed. Fifteen years after the first inoculationsall trees had 2 or more main-stem infections and additional branch infections, but the gradual increase in plant number with time allowed a fuller crown development and correspondingly less severe damage than occurred with graft-infected trees. Mean height growth of infected saplings underthree levels of competitive suppression was reduced 0.25 inch per year per mistletoe plant. Pathogenic suppression of growth was greatest where suppression by other trees was heaviest. Forest Sci. 17:373-380.