Expression of Inoculum and Family Specific Responses in the Ponderosa Pine-Western Gall Rust Pathosystem.

Host-pathogen interactions in the ponderosa pine-western gall rust pathosystem were studied using seedlings from eight open-pollinated mother-tree families and Peridermium harknessii aeciospores from two geographically separate sources. Pregall symptoms occurred on seedlings by 4 days after inoculation (DAI). Gall occurrence was essentially complete by 230 DAI. Of three measured pigments, light red and dark red pigments on the needles developed most rapidly. Light red pigment on the base of the needles between 21 and 66 DAI was the pregall symptom most often (i) affected by inoculum source and host family, and (ii) correlated to gall width. Pre- and postgall symptoms varied significantly (i) between inocula, and (ii) among mother-tree families. No relationship was found between field resistance ratings of the mother trees and the expression of resistance in their progeny. The inocula varied in pathogenicity, and the seedling families varied in response to infection, as shown by differences in level of incidence and site of development of pigmentation and gall size on inoculated seedlings.