Effects of Exogenous Nutrients and Inoculum Quantity on the Virulence ofPythium ultimumto Cotton Hypocotyls

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Abstract
Cotton hypocotyls were inoculated with P. ultimum grown on media 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-N and K, but not by Mg sulfate, Cl or phosphate. P. ultimum inocula grown on media without sucrose or N caused only minor symptoms on cotton hypocotyls but became progressively more virulent when grown on media with increasingly higher concentrations of sucrose or N. Lower concentrations of those nutrients were required for maximum growth in culture than for maximum virulence. High concentrations of both sucrose and N 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-N. At all nutritional levels of sucrose or N, virulence increased when larger quantities of inoculum were applied to hypocotyls. Larger quantities of inoculum grown on low-nutrient media resulted in disease symptoms similar to or more severe than those produced by smaller quantities grown on high-nutrient media.

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