EVIDENCE OF A HOST-SPECIFIC CHLOROSIS TOXIN FROM PYRENOPHORA-TRITICI-REPENTIS, THE CAUSAL AGENT OF TAN SPOT OF WHEAT

Spore germination fluid, cell-free culture filtrate, and intercellular washing fluid from race 5 of Pyrenophora tritici-repentis contained a host-specific toxin which elicited extensive chlorosis on Katepwa and 6B662, characteristic of the chlorotic symptom induced by the fungus on the same wheat genotypes. These three sources of toxin showed equiv alent host specificity. Other races (including races 1 and 3) which ar e known to induce chlorosis in different wheat genotypes than does rac e 5 did not produce a detectable level of a chlorosis toxin, as assess ed by bioassay, when grown in a still liquid culture or during spore g ermination. Of the 15 plant species tested, only wheat (genotypes 6B66 2, Katepwa, and Neepawa) and triticale (cv. Banjo) developed distinct chlorosis when inoculated with race 5 or infiltrated with its spore ge rmination fluid or partially purified culture filtrate. F-2 progenies from a cross between race 5-susceptible and race 5-resistant wheat gen otypes were evaluated. Susceptibility of the seedlings to race 5 and t heir sensitivity to its toxin cosegregated. A ratio of 1:3 (resistant/ susceptible) was observed, suggesting the involvement of a single, dom inant locus controlling the reaction to the fungus and the toxin. The chlorosis toxin appeared to be a pathogenicity factor and was designat ed as Ptrchlorosis toxin.