Bipolaris zeicola race 3, originally reported as a pathogen causing a leaf spot disease of maize, is also highly pathogenic to rice plants. Its spore-germination fluids (SGFs) were specifically phytotoxic to rice leaves but induced susceptibility to infection by nonpathogens on leaves of both rice and maize. Effective compounds were partially purified from the SGFs which caused leaf chlorosis only on rice and enabled spores of nonpathogenic Alternaria alternata and B. victoriae to infect leaf tissues of rice and maize, as judged by the formation of disease symptoms on the leaves. The partially purified compounds were further purified by HPLC and found to consist of at least three separate components. The host-specific phytotoxicity to rice and the susceptibility-inducing activity of the components with rice and maize were evident only when combined, but not when used singly. HPLC analysis of extracts from SGFs and culture filtrates of several isolates of Bipolaris and Helminthosporium demonstrated that the components were produced only by B. zeicola race 3. We conclude that the complex of the components may be a pathogenicity factor of B. zeicola race 3 for rice and maize plants. © 1991 Academic Press Limited.
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