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Variation in Pathogenicity of Diverse Sources of Cronartium fusiforme on Selected Slash Pine Families. G. A. Snow, Principal Plant Pathologist, Southern Forest Experiment Station, U.S. Department of Agriculture, Forest Service, Gulfport, Mississippi 39501; R. J. Dinus(2), and A. G. Kais(3). (2)(3)Principal Plant Geneticist, and Plant Pathologist, Southern Forest Experiment Station, U.S. Department of Agriculture, Forest Service, Gulfport, Mississippi 39501. Phytopathology 65:170-175. Accepted for publication 21 August 1974. DOI: 10.1094/Phyto-65-170.

In three experiments, open-pollinated slash pine families with known degrees of fusiform rust resistance were inoculated with Cronartium fusiforme basidiospores from diverse sources. In one experiment, ten families were exposed to inocula collected along a transect across Florida, Georgia, Mississippi, Louisiana, and Texas. Two families were resistant to all inocula; three were uniformly susceptible, and the remaining five were resistant to certain inocula. In a second experiment, inocula were from a north-south transect in Mississippi, and another in western Florida and Georgia. Responses of families from two resistant and two susceptible parent trees demonstrated variation in pathogenicity not only between eastern and western collections, but also among inocula from within both transects. In a third experiment, virulence of cultures from eight individual galls in the Mississippi transect was highly variable on three resistant pine families. Two cultures were more virulent than others on all families, while one family was immune to two of the cultures. These findings demonstrate great genetic diversity in both the fungus and host populations. It is clear that inoculum source and inoculum sampling methods must be considered in evaluating resistance of slash pine to southern fusiform rust. Further, proper deployment of several forms of stable resistance seems necessary for long-term control of the disease.