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Genetic and Environmental Effects on Production of Inhibitory Compounds in Corn Resistant to Helminthosporium turcicum¹

By utilizing the leaf drop diffusate technique, seedling leaves of corn (Zea mays L.) were inoculated with a spore drop suspension of Helminthosporium turcicum Pass, in a humid chamber at 21 C and light intensity of 3,230 lux. The drop suspensions were then withdrawn from 1 to 7 days after inoculation and centrifuged. The supernatant or diffusate, now free of spores and germ tubes, was mixed with a drop of fresh spore suspension. Diffusates which inhibited spore germination or delayed growth of the germ tube were assumed to contain phytoalexin or other inhibitory substances.

The four homozygous combinations of the genes Ht and Bx were bioassayed plus the heterozygote HthtBxBx. The HtHtBxBx genotype, treated with drops of distilled water only, served as control. Greatest to least inhibition was produced by the double dominant HtHtBxBx, followed by the single dominant genotypes and then the double recessive hthtbxbx. Both Ht and Bx had a significant inhibitory effect. A significant amount of inhibition occurred within 24 hours after inoculation, increased to a maximum on the second or third day following inoculation, and continued at this high level until the sixth day. Inhibition was decreased significantly by leaf detachment, low spore concentration, and high preconditioning temperature.

Key Words: Northern corn leaf blight • Bioassay • Phytoalexin • Diffusate

² Research Associate, Agronomy Department, University of Nebraska, Professors of Plant Science and former Graduate Research Assistant, Plant Science Department, University of New Hampshire, Durham, NH 03824. The authors express their appreciation to A. L. Hooker, University of Illinois, and R. H. Hamilton, The Pennsylvania State University, for the corn inbreds.

Received for publication March 8, 1973.