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Leaf Bioassay for Helminthosporium carbonum Toxin – Search for Phytoalexin. Sigmar E. Hoffmann, Department of Agronomy and Range Science, University of California, Davis 95616; F. P. Zscheile, Jr., Department of Agronomy and Range Science, University of California, Davis 95616. Phytopathology 63:729-734. Accepted for publication 20 December 1972. DOI: 10.1094/Phyto-63-729.

Resistance mechanisms in the Helminthosporium leaf spot disease of maize were investigated. Leaf extracts, obtained by boiling leaf-tissue homogenate or by centrifugation of intact leaf sections, were assayed for spore-germination inhibitory and mycelial-growth limiting substances. No such phytoalexin-like substances were found in extracts of inoculated leaves and no fungal nutrition deficiencies could be demonstrated. Applications of indole-3-acetic acid and H. carbonum-toxin influenced resistant host-pathogen interactions by increasing the size of lesions on resistant hosts, but did not overcome resistance. Double inoculation produced no cross-protection. A leaf bioassay for H. carbonum-toxin was developed which is ca. 10% as sensitive as the root bioassay but expresses a more meaningful relation to the disease symptoms. Differences in tolerance between susceptible and resistant maize leaves were demonstrated.