Evidence of a Host-Specific Chlorosis Toxin fromPyrenophora tritici-repentis,the Causal Agent of Tan Spot of Wheat

Abstract

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 equivalent host specificity. Other races (including races I and 3) which are known to induce chlorosis in different wheat genotypes than does race 5 did not produce a detectable level of a chlorosis toxin, as assessed by bioassay, when grown in a still liquid culture or during spore germination. Of the 15 plant species tested, only wheat (genotypes 6B662, Katepwa, and Neepawa) and triticale (cv. Banjo) developed distinct chlorosis when inoculated with race 5 or infiltrated with its spore germination fluid or partially purified culture filtrate. F 2 progenies from a cross between race 5-susceptible and race 5-resistant wheat genotypes were evaluated. Susceptibility of the seedlings to race 5 and their sensitivity to its toxin cosegregated. A ratio of 1 :3 (resistant/susceptible) was observed, suggesting the involvement of a single, dominant locus controlling the reaction to the fungus and the toxin. The chlorosis toxin appeared to be a pathogenicity factor and was designated as Ptr-chlorosis toxin.