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Disease Determinant of Helminthosporium maydis Race T. S. M. Lim, Assistant Professor, Department of Plant Pathology, University of Illinois, Urbana 61801; A. L. Hooker, Professor, Department of Plant Pathology, University of Illinois, Urbana 61801. Phytopathology 62:968-971. Accepted for publication 1 March 1972. DOI: 10.1094/Phyto-62-968.

Extracts of corn leaves having cms-T cytoplasm infected with Helminthosporium maydis race T exhibited a differential toxicity to resistant (normal) and susceptible (cms-T cytoplasm) corn similar to that of the pathotoxin (HmT-toxin) produced by the fungus in vitro and similar to the reaction of the two corn cytoplasm types to the fungus itself. The extracts obtained from the resistant corn leaves infected with race T, with the exception of a very susceptible inbred, did not show the selective toxicity. The toxic activity of extracts from corn inbreds with large lesions was greater than that from inbreds with small lesions. The physical and chemical characteristics of the toxic fraction extracted from infected leaves are similar to the pathotoxin produced by H. maydis race T in culture. This study indicates that the toxic fraction of the infected leaf extract contains pathotoxin, which is the disease-determinant of southern corn leaf blight.