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Oxalic Acid Effects in Exudates of Sclerothinia trifoliorum and S. sclerotiorum and Potential Use in Selection

USDA-ARS, Crop Sci. Lab. Mississippi State, MS 39762-5367, P.O. Box 343, Stoneville, MS 38776

^* Corresponding author.

Sclerotinia crown and stem rot caused by Sclerotinia trifoliorum Eriks. or S. sclerotiorum (Lib.) de Bary is a serious problem on crimson clover (Trifolium incarnatum L.), berseem clover (T. alexandrinum L.), and alfalfa (Medicago sativa L.) in temperate climates. The inability to develop cultivars resistant to this disease indicates more detailed research is needed on factors causing disease by either fungus. The objectives of this study were to determine what deleterious effects fungal exudates have on the seed of these legumes and attempt to measure the importance of oxalic acid in the fungal exudates. The percent seed germination and length of germinated seedlings of four cultivars each of crimson clover, berseem clover, and alfalfa were measured in five environments. Seed was placed in a host-pathogen interaction system and exposed to exudates of S. trifoliorum or S. sclerotiorum on water agar acidified with oxalic acid to a pH of 3.3, and water agar as a control (pH 6,2). In contrast to a control, fungal exudate caused a 51 to 60% reduction in seedling length and an average of two-thirds of this reduction appeared to be caused by oxalic acid. The seedling length on oxalic acid amended agar was correlated with that of seedlings exposed to exudates from S. trifoliorum (r 0.71, 10 dr) and S.sclerotiorum (r = 0.83, 10 di~. Oxalic acid appeared to have potential value in selection of resistance to either fungus in alfalfa or resistance to S. sclerotiorum in crimson clover.

Received for publication October 19, 1992.

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