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Host-Specific Toxin Production by Helminthosporium oryzae. P. Vidhyasekaran, Senior research fellow, Department of Plant Pathology, International Rice Research Institute, P.O. Box 933, Manila, Philippines; E. S. Borromeo(2), and T. W. Mew(3). (2)(3)Research assistant and plant pathologist and head, Department of Plant Pathology, International Rice Research Institute, P.O. Box 933, Manila, Philippines. Phytopathology 76:261-266. Accepted for publication 16 September 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1986. DOI: 10.1094/Phyto-76-261.

Culture filtrates of Helminthosporium oryzae contained a toxin which elicited the characteristic brown spot symptoms on rice leaves. Procedures for isolation and purification of the toxin, based on solvent extractions, absorption on charcoal, alumina adsorption chromatography, gel filtration, and thin-layer chromatography are described. The Rf value and UV and IR adsorption spectra of the purified toxin revealed that it is a new toxin different from the three previously reported phytotoxins produced by H. oryzae. Rice cultivars susceptible to the pathogen were more sensitive to the toxin than the resistant cultivar. Sensitivity of rice cultivars and other plant species to toxin preparations corresponded to their relative susceptibility to the pathogen. When the virulent isolate lost its ability to produce toxin during repeated subculturing, it became nonpathogenic. Toxin added to the infection drops of the nonpathogenic isolate allowed invasion and colonization of rice leaves by the fungus. When the susceptible cultivars were sprayed with ferric chloride (10-2 M), they became resistant to both the pathogen and its toxin. The toxin could be isolated from the infected tissues. This toxin satisfies the criteria for consideration as a host-specific toxin and as a primary determinant of the disease.