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Role of Host-Selective Toxin in Colonization of Corn Leaves by Helminthosporium carbonum. J. C. Comstock, Graduate Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823, Present address of senior author: Department of Botany and Plant Pathology, Iowa State University, Ames 50010; R. P. Scheffer, Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48823. Phytopathology 63:24-29. Accepted for publication 5 July 1972. DOI: 10.1094/Phyto-63-24.

Conidia of Helminthosporium carbonum race 1 germinated, formed appressoria, and penetrated epidermal cells of susceptible, tolerant (intermediate), and resistant corn cultivars. Further fungal growth was rapid in susceptible, restricted in tolerant, and confined to one or two cells in resistant leaves; resistance was evident by 16 hr after inoculation. H. victoriae (an oat pathogen) and a nonpathogenic H. carbonum isolate penetrated corn leaves but were confined to one or two cells. H. carbonum-susceptible but not resistant corn leaves were colonized by H. victoriae and nonpathogenic H. carbonum in the presence of H. carbonum (HC) toxin (2.0 µg/ml). When corn leaves were inoculated with H. victoriae, and HC-toxin was added after fungal growth had stopped, tissue colonization was successful. Prior inoculation with H. victoriae did not affect development of H. carbonum in susceptible corn leaves. The data support three conclusions: (i) HC-toxin is required for colonization of susceptible corn tissue by H. carbonum; (ii) dead or seriously damaged cells are not required for successful colonization (disruptive effects were not evident for more than 20 hr after inoculation); and (iii) inhibitory compounds produced by corn cells do not account for resistance to H. carbonum or to homologous pathogens.