DIFFERENTIAL RESPONSES OF ALFALFA GENOTYPES TO STEM INOCULATIONS WITH SCLEROTINIA-SCLEROTIORUM AND S-TRIFOLIORUM

Differences in susceptibility of alfalfa genotypes to Sclerotinia sclerotiorum and S. trifoliorum were evaluated by comparing the extent of pathogenesis in individually inoculated stems of replicated clones. Stems were inoculated by applying cotton pads, saturated with 20% V-8 juice and infested with each pathogen, around tips and enclosing with tape. Plants were incubated in a saturated atmosphere for 3 days to establish infection and then in ambient air for 11 days. Pathogenesis proceeded basipetally within stems, and the length of necrosis from the inoculation point to the margin of healthy tissue was measured on each stem at 2 wk after inoculation. In replicated experiments with each Sclerotinia sp., significant differences among 19 genotypes in the extent of pathogenesis of stems were observed, but no significant differences between experiments or interactions between genotypes and experiments. Mean responses of genotypes to the two pathogens were not significantly correlated. These results indicate that the stem-inoculation technique may be used to identify consistent differences among alfalfa genotypes in susceptibility or tolerance of stem tissues to S. sclerotiorum and S. trifoliorum. The differences in susceptibility to the two pathogens appeared to occur independently among the 19 genotypes in this study.