MECHANISMS OF RESISTANCE TO SCLEROTINIA MINOR IN SELECTED PEANUT GENOTYPES

Sclerotinia minor (Jagger) Kohn is a serious and increasingly prevalent pathogen of peanut (Arachis hypogaea L.). Peanut stem tissues were reported to differ, in their resistance to S. minor, but field performance is not always correlated with laboratory evaluations of resistance to Sclerotinia diseases in other crops. Differences in genotype performance in field and laboratory results may reflect differences in mechanisms of resistance. The objective of this study was to characterize mechanisms of resistance to S. minor in selected peanut genotypes by using agar culture tests, wounded and nonwounded stem inoculations, and field trials. For the culture test, sap was expressed from five genotypes with different levels of held-resistance to S. minor. Each extract was incorporated into an agar medium, which was overlaid with a dialysis membrane. The fungus produced distinctive infection hyphae on the media. Genotype extracts differentially affected size of terminal and secondary hyphae and the number of hyphae per organized cluster. Nine genotypes were evaluated for resistance to S. minor in two stem inoculation tests. Inoculation sites were wounded in the first method, and were not wounded in the second method. Significant differences in lesion size were found with both methods, but more differences were found among genotypes in the nonwounded inoculation. Genotype performance in culture and stem inoculation tests was not correlated with performance in the field. These studies demonstrated that although some genotypes had resistance to stem colonization by S. minor, other mechanisms account for most of the resistance expressed in the field.