VIEW ARTICLE

Nutritional and Inhibitory Factors in the Resistance of Zea mays to Puccinia graminis. K. T. Leath, Former Research Technician, Crops Research Division, ARS, USDA, Cooperative Rust Laboratory, Department of Plant Pathology, University of Minnesota, St. Paul 55101, Present address of senior author: U.S. Regional Pasture Research Laboratory, University Park, Pennsylvania 16802; J. B. Rowell, Research Plant Pathologist, Crops Research Division, ARS, USDA, Cooperative Rust Laboratory, Department of Plant Pathology, University of Minnesota, St. Paul 55101. Phytopathology 60:1097-1100. Accepted for publication 18 February 1970. DOI: 10.1094/Phyto-60-1097.

The nature of fungal inhibition was investigated in the immune host-pathogen combination of Zea mays and Puccinia graminis. In untreated, mature leaf tissue the fungus was inhibited at the infection-hyphal stage of development and did not form haustoria. In detached tissue, in tissue from plants treated with maleic hydrazide, and in senescent tissue, fungal development was the same as in untreated, mature tissue. Treatment of seedlings with chloroform vapor before or after inoculation delayed fungal inhibition and increased fungal development, including the formation of haustoria. Development of the fungus in immature leaf tissue was similar to that which occurred in the narcotized tissue. The growth of P. sorghi, a corn pathogen, was inhibited in leaf tissue infiltrated before or after inoculation with cell-free exudate from germination uredospores of P. graminis. The exudate from P. graminis did not inhibit the germination of P. sorghi spores in vitro. Thus, the limited growth of P. graminis in corn apparently results from the action of a phytoalexin rather than from a nutritional deficiency. Observations of the rust pathogens in corn tissue suggest that the phytoalexin is induced by fungal products released during uredospore germination, reaches inhibitory concentrations within 24 hr after inoculation, and is fungistatic in its action. Attempts to demonstrate a phytoalexin with the drop-diffusion method were unsuccessful.