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Effects of Genetic Background on Monogenic Resistance to Helminthosporium turcicum in Maize (Zea mays L.)¹

A "drop-diffusion" technique was used to determine the effects of genetic background upon the production of phytoalexin or other inhibitory substances in corn (Zea mays L.) with monogenic resistance (Ht/Ht) to Helminthosporium turcicum Pass. Seedling leaves were inoculated with drops of an aqueous suspension of spores of H. turcicum in a humid chamber at 21C, 100% RH, and a light intensity of 3230 lux for up to 6 days. Drops of the suspension were then withdrawn from the leaf surfaces 1 to 6 days after inoculation and centrifuged. The supernatant or diffusate, then free of spores and germ tubes, was mixed with a drop of fresh spore suspension. Diffusates that inhibited spore germination or delayed the growth of the germ tube was assumed to contain phytoalexin or other inhibitory substance. Five standard susceptible inbred lines (ht/ht), their nearly isogenic resistant counterparts (Ht/Ht), and the five F₁ hybrids (Ht/ht) of paired isogenic lines were bioassayed.

The incorporation of the Ht gene into any of the five inbred lines significantly increased the amount of inhibitory substances. Homozygous resistant inbred lines (Ht/Ht) produced significantly more phytoalexin or other inhibitory substances than the heterozygotes (Ht/ht) which in turn produced significantly more inhibition than susceptible inbred lines (ht/ht). The genetic background affected the amount and to a certain extent the tune of production of inhibitory compounds. Bioassay tests and percent leaf infection following standard inoculation were not significantly correlated.

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

² Research Associate, Agronomy Department, University of Nebraska, and Professors of Plant Science, Plant Science Department, University of New Hampshire, Durham, N.H. 03824. The authors express their appreciation to A. L. Hooker, University of Illinois, for the inbred lines.

Received for publication April 14, 1973.