PRODUCTION OF A CHLOROSIS-INDUCING, HOST-SPECIFIC, LOW-MOLECULAR-WEIGHT TOXIN BY ISOLATES OF PYRENOPHORA-TRITICI-REPENTIS, CAUSE OF TAN SPOT OF WHEAT

An extraction procedure was developed to isolate toxin from cell-free culture filtrates of Pyrenophora tritici-repentis, cause of tan spot disease of wheat (Triticum aestivum). Designated Ptr-toxin, this low-molecular weight toxin (MW 800-1800) induces the characteristic chlororis associated with tan spot disease when inoculated to healthy wheat leaves and differentially inhibits the elongation of wheat seedling coleoptiles. The sensitivity of seven wheat, one barley, and non-host (com) cultivars to Ptr-toxin corresponds with reported field reactions to the pathogen. Seven isolates of P. tritici-repentis from wheat in Oklahoma and four from smooth bromegrass (Bromus inermis, an alternative host of P. tritici-repentis), in North Dakota were evaluated for production of Ptr-toxin in vitro. Results indicate that Ptr-toxin may be involved in the expression of disease symptoms in tan spot of wheat, but is not the sole determinant of P. tritici-repentis pathogenicity. Instead, toxin production may contribute to the virulence of individual isolates of the pathogen. Isolates from the alternative host, smooth bromegrass, appear at least as toxigenic as those from wheat, with the exception of one which appeared more toxigenic than four of the wheat isolates tested. Possible relationships of Ptr-toxin to high-molecular weight toxins reported to be produced by P. tritici-repentis in vitro (Ptr-necrosis-toxin) are not understood.