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Phytotoxins Produced by Germinating Spores of Bipolaris oryzae. J. Z. Xiao, Ph.D. graduate student, Laboratory of Plant Pathology, faculty of agriculture, Nagoya University, Nagoya 464, Japan; M. Tsuda(2), N. Doke(3), and S. Nishimura(4). (2)Associate professor, Pesticide Research Institute, Kyoto University, Kyoto 606, Japan; (3)(4)Professor, and professor, respectively, Laboratory of Plant Pathology, faculty of agriculture, Nagoya University, Nagoya 464, Japan. Phytopathology 81:58-64. Accepted for publication 9 May 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-58.

Bipolaris oryzae produces ophiobolin A and ophiobolin B during spore germination. Ophiobolin A, 6-epiophiobolin A, anhydroophiobolin A, 6-epianhydroophiobolin A, and ophiobolin I are produced during culture in a nutrient medium. Each compound showed nonselective phytotoxicity to host and nonhost plants in assays for root elongation and induction of leaf chlorosis. Ophiobolin A was most toxic, with an ID50 value of 5 ?g/ml, followed by 6-epiophiobolin A, with an ID50 value of 10 ?g/ml for inhibition of root elongation. Others were toxic only at levels of more than 100 ?g/ml. At a concentration of approximately 3 ?g/ml, ophiobolin A also induced susceptibility of rice leaf tissues to a non-pathogenic isolate of Alternaria alternata. However, ophiobolins are not likely to function as disease determinants of B. oryzae, because their concentrations in spore-germination fluids were only about one one-thousandth of that required for biological activity. The spore-germination fluids nevertheless exhibited extensive susceptibility-inducing activity and phytotoxicity. These activities were host selective for rice and were detectable soon after spore germination, when ophiobolins were not detectable at effective concentrations.