Parasitic and nonparasitic pathogenesis of tomato plants by Pythium myriotylum

Abstract

Tomato plants grown in soil infested with Pythium myriotylum or in soil containing nonviable mycelium were severely stunted. Mycelium of P. myriotylum was produced on a glucose–glutamate–cholesterol medium, separated from the medium, lyophilized, and ground into a fine powder in a Waring blendor. Tomato plants transplanted into steam-pasteurized soil with the addition of 200 mg of the nonviable mycelium developed foliar necrosis and were severely stunted. Seedlings with roots immersed in an autoclaved aqueous suspension of the mycelium (100 μg/ml) developed laminar collapse and dehydration within 12 h and were killed within 24 h. Detached tomato leaves behaved similarly. On autoclaving, the toxin associated with the mycelium of P. myriotylum was relatively stable below pH 6 but labile at pH values above 6. Treated leaves lost more fresh weight at 35 °C than at 27 or 22 °C. Evidence for variability in toxin production within P. myriotylum was obtained. Toxin(s) in the mycelium of P. myriotylum may be involved in reduced growth and death of tomato plants. This is the first known report of toxin production by P. myriotylum.