VIEW ARTICLE

Resistance

Relative Effectiveness and Stability of Different Resistance Mechanisms to White Pine Blister Rust in Sugar Pine. Bohun B. Kinloch, Jr., Geneticist, Pacific Southwest Forest and Range Experiment Station, Forest Service, U. S. Department of Agriculture, P. O. Box 245, Berkeley, CA 94701; James W. Byler, plant pathologist, Region 1, Forest Service, U. S. Department of Agriculture, P. O. Box 7669, Missoula, MT 59807. Phytopathology 71:386-391. Accepted for publication 2 September 1980. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981. DOI: 10.1094/Phyto-71-386.

In progeny and clonal tests, several heritable types of resistance to Cronartium ribicola were found in a selected population of sugar pine parents. Major gene resistance (MGR), controlled by a single dominant gene, was found in 16% of selected candidates and was highly effective. Although vulnerable to a virulent race that had been previously detected at low frequency in the rust population, MGR was highly stable for up to 14 yr. Sudden erosion of resistance was observed following 1 yr of unusually favorable conditions for inoculum production and infection. It was uncertain whether this represented only an ephemeral breakdown in resistance due to an unusually high inoculum density, or a change in frequency of the virulent race. Slow rusting, presumably under polygenic control, was indicated by differential infection rates of families from different parents. The degree of resistance exhibited for this trait was low, but relatively stable and amenable to considerable improvement in early breeding generations. Another kind of apparently quantitative resistance was expressed by large differences in infection among grafted clones. Since this variation contrasted greatly to the generally high levels of infection of seedling progenies derived from the same clones, the existence of ontogenetic factors of resistance was strongly suggested.