Nonparasitic pathogenesis of germinating tomato by Pythium myriotylum

Abstract

Surface-disinfested seed of pepper, cowpea, sorghum, and three cultivais of tomato were placed onto petri dishes containing water agar amended with powdered, lyophilized mycelium of Pythium myriotylum cultured on a liquid, glucose–glutamate medium. Only the germinating seed of tomato cultivais were adversely affected. Hypocotyl length and root length of seedlings were reduced more than 42 and 74%, respectively, when grown on water agar amended with 25 μg/mL of P. myriotylum mycelium. At 250 μg/mL of mycelium, root tips and hypocotyls became necrotic, roots were discolored, and seedlings were killed. When we autoclaved the mycelium-amended water agar at pH values at or above 6.0, the activity of the mycelial toxin(s) was significantly decreased. Roots were affected more than hypocotyls by the toxin(s). Mycelial suspensions which were sterilized by sonication were more toxic than suspensions that were autoclaved. The data support the report that P. myriotylum produces toxin(s) active on tomato and suggest that P. myriotylum mycelium contains toxin(s) that may play a role in preemergence damping-off of tomato. Toxin(s) from soil-borne fungi may be involved in poor crop performance.