CARBON-TO-NITROGEN RATIO, CARBON CONCENTRATION, AND AMINO-ACID-COMPOSITION OF GROWTH MEDIA INFLUENCE CONIDIATION OF HELMINTHOSPORIUM-SOLANI

Silver scurf, caused by Helminthosporium solani, is an important stora ge disease of potatoes. Experiments designed to evaluate control alter natives are limited by difficulty in producing conidial inoculum. In a n effort to better understand how this difficulty could be overcome th is study evaluated the influence of various carbon-to-nitrogen (C:N) r atios, carbon concentrations, and amino acids on conidial germination, colony diameter, and conidiation of Helminthosporium solani grown on solid-phase basal salts media. Under the conditions tested, the highes t concentrations of conidia were produced with 1.25 to 2.5 g of carbon /L at a C:N ratio of 10:1. Higher C:N ratios or higher carbon concentr ations reduced conidiation. Total conidia production was improved by u se of tyrosine or arginine as the sole nitrogen source. Use of leucine , lysine, methionine, phenylalanine, or threonine severely inhibited H . solani conidia production. Use of a nitrogen source containing a mix ture of amino acids resulted in a defined medium that permitted conidi ation and growth of H. solani that was similar to or better than that obtained with Standard V8 Juice medium.