Purification of a Cultivar-Specific Toxin from Pyrenophora tritici-repentis, Causal Agent of Tan Spot of Wheat. A. Tomas. Departments of Plant Pathology Kansas State University, Manhattan 66506–5502 U.S.A. G. H. Feng(2), G. R. Reeck(2), W. W. Bockus(1), and J. E. Leach(1). Departments of (1)Plant Pathology and (2)Biochemistry, Kansas State University, Manhattan 66506-5502 U.S.A. MPMI 3:221-224. Accepted 29 January 1990. Copyright 1990 The American Phytopathological Society.

Pyrenophora tritici-repentis, causal agent of tan spot of wheat, produces a toxin in culture that induces necrosis only on wheat cultivars susceptible to the pathogen. The toxic compound, named Ptr toxin, was purified by gel filtration chromatography and ion exchange chromatography. Unlike most other fungal phytotoxins, Ptr toxin is a low molecular weight protein of 14,700. It has a high content of aspartate/asparagine, serine, and glycine, and a low content of histidine, methionine, and Iysine. Reduction with dithiothreitol abolished toxic activity indicating that cysteine is present and that a disulfide bond is required for activity. Concentrations of 90 nM Ptr toxin caused symptoms in leaves of a susceptible, but not a resistant wheat cultivar using the leaf infiltration bioassay described in this study.