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Disease-Induced Toxin Production in Helminthosporium oryzae. G. D. Lindberg, Professor, Department of Plant Pathology, Louisiana State University, Agricultural Experiment Station, Baton Rouge 70803; Phytopathology 61:420-424. Accepted for publication 16 November 1970. DOI: 10.1094/Phyto-61-420.

The pathogenicity of a healthy and two diseased isolates of Helminthosporium oryzae was compared on Nato rice seedlings. Healthy fungus grew vigorously on sterilized soil, and caused typical Helminthosporium seedling blight. The diseased isolates grew poorly on soil, but caused pronounced inhibition of seedling growth and more severe disease than that caused by typical H. oryzae. Roots of rice seedlings were only slightly inhibited in undiluted culture filtrates of healthy H. oryzae and uninhibited at dilution 1:5. Roots were inhibited completely at dilution 1:10 of culture filtrates of the diseased isolates, and inhibited 80% at dilution 1:100. Fifty per cent root inhibition occurred between dilutions 1:100 and 1:1,000 of toxin concentrate solutions. Dilutions made in vials after removal of the toxin concentrates caused root inhibition greater than 50% at 10^–8. Culture filtrates of diseased H. oryzae sprayed on flowering rice panicles caused marked discoloration and a mean sterility of 32%, whereas culture filtrates of healthy H. oryzae caused almost no discoloration of panicles and a mean sterility of 17%.