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Techniques

Use of a Host-Pathogen Interaction System to Test Whether Oxalic Acid is the Sole Pathogenic Determinant in the Exudate of Sclerotinia trifoliorum. Franklin E. Callahan, Cotton Host Plant Resistance Research Unit and Forage Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Crop Science Research Laboratory, P.O. Box 5367, Mississippi State 39762-5367; Dennis E. Rowe, Cotton Host Plant Resistance Research Unit and Forage Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Crop Science Research Laboratory, P.O. Box 5367, Mississippi State 39762-5367. Phytopathology 81:1546-1550. Accepted for publication 30 July 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/Phyto-81-1546.

A host-pathogen interaction system was used to test the hypothesis that oxalic acid is the sole pathogenic determinant in the exudate of Sclerotinia trifoliorum. The system allowed exposure of germinating alfalfa seedlings (Medicago sativa) to continuously produced fungal exudate without physical contact with the fungus. Blockage of diffusion of macromolecular components (>3,500 mol wt) of the exudate without alteration of oxalic acid levels reduced the observed inhibition of alfalfa radicle length by 40–50%. Such results indicate that in this system oxalic acid is not the sole inhibitory factor and that other, yet unidentified, macromolecular components share a codeterminant role in the observed inhibitory effect. The techniques described should be generally applicable to characterization of exudates of other pathogenic fungi or bacteria that are cultured on solid media, while allowing flexibility in design of pertinent bioassays.