Kinloch, B. B., Jr., Libby, W. J., and Vogler, D. R. 1987.
U.S.D.A., Forest Service, Institute of Forest Genetics,
Berkeley, CA 94701, and
Departments of Natural Resources and Plant Pathology, University of California,
Berkeley, CA 94720.
Abstr. in Proceedings of the Western Forest Geneticist's
Prince George, British Columbia. (1987)
A remarkable degree of variation in susceptibility to natural infection by western gall rust (Peridermium harknessii, syn. Endocronartium harknessii) was observed between two species, and among varieties, populations, and individual trees of California closed-cone pines growing in a common garden near Berkeley, California. Average infection of Monterey pine (Pinus radiata) was nearly twice that of Bishop pine (P. muricata) - 85% vs. 46% - and variation among populations of either species ranged from 65 to 100% in Monterey pine and from 9 to 100% in Bishop pine. There was no obvious geographic pattern in relative susceptibility of populations of either species, except that in Monterey pine the two island varieties were least susceptible (var. binata, Guadalupe Island, 65%; var. Cedroensis, Cedros Island, 80%). The most pronounced variation was expressed among individual trees within populations, especially those of Monterey pine. A substantial proportion of trees with no or few infections were present in all populations, while in some the number of galls per tree ranged up to 100 or more. In striking contrast to this diversity among and within host populations, genetic variation among individual rust galls in the common garden population, as measured by over 20 isozyme loci, was essentially nil. The same result also was obtained within and among populations parasitizing the three mainland Monterey pine stands, as well as selected isolates from other pine host species in diverse environments. The implications of these results for variation in pathogen virulence and host resistance are discussed.