VIEW ARTICLE

Ecology and Epidemiology

Effects of Exogenous Nutrients and Inoculum Quantity on the Virulence of Pythium ultimum to Cotton Hypocotyls. L. F. Johnson, Professor, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37916; Chin-Chu Hsieh(2), and E. D. Sutherland(3). (2)(3)Graduate research assistants, respectively, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37916, (2)(3)Present address: 10 Navajo Road, East Brunswick, NJ 08816; and Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Phytopathology 71:629-632. Accepted for publication 20 November 1980. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981. DOI: 10.1094/Phyto-71-629.

Cotton hypocotyls were inoculated with Pythium ultimum grown on media prepared with different nutritional levels. Virulence of the fungus decreased with age on potato-dextrose agar (PDA) and as concentrations of nutrients in PDA were decreased. On a basal mineral salts-carbohydrate medium in which concentrations of nutrients were adjusted individually, growth and virulence of the fungus were affected by sucrose, ammonium-and nitrate-nitrogen, and potassium, but not by magnesium, sulfate, chlorine, or phosphate. P. ultimum inocula grown on media without sucrose or nitrogen caused only minor symptoms on cotton hypocotyls, but became progressively more virulent when grown on media with increasingly higher concentrations of sucrose or nitrogen. Lower concentrations of those nutrients were required for maximum growth in culture than for maximum virulence. High concentrations of both sucrose and nitrogen were required for maximum disease development following initial infection. The fungus was more virulent when grown on media containing nitrate than on media with ammonium-nitrogen. At all nutritional levels of sucrose or nitrogen, virulence increased when larger quantities of inoculum were applied to hypocotyls. Larger quantities of inoculum grown on low-nutrient media resulted in disease symptoms either similar to or more severe than those produced by smaller quantities grown on high-nutrient media.