Methods for Evaluating Soybean Cultivars for Resistance toSclerotinia sclerotiorum

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Abstract
In several areas of Illinois [USA], stem rot or white mold caused by S. sclerotiorum has become a problem in soybean (G. max) fields previously cropped to snap beans (Phaseolus vulgaris). The disease reaction of soybean cultivars from maturity groups II-IV was evaluated by 3 inoculation methods. Plants in full bloom (R2) were inoculated by spraying an ascospore suspension (1-5 .times. 105 ascospores/ml) to runoff. Four-week-old prebloom plants (V4-V5) were inoculated by placing pieces of autoclaved carrot colonized by the fungus directly onto the leaf surface. After inoculation with either ascospores or colonized carrot pieces, plants were placed in a mist chamber for 7-12 days at 20-25.degree. C with a 12-h photoperiod. Reactions of all cultivars tested by these 2 inoculation methods were highly susceptible. Ascospores readily infected plants in the blossom stage and only infected 4-wk-old plants when an exogenous nutrient source was added or when tissues were injured. Limited-term inoculation, the 3rd method, was accomplished by attaching autoclaved celery pieces colonized by the fungus to the nodes of 4-wk-old plants (V4-V5) for 24 h. Differences in susceptibility were detected among 10 soybean cultivars evaluated by this method.

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