Nature of Enhanced Severity of Anthurium Root Rot by Diuron Treatment for Weed Control

Diuron treatment for weed control greatly increased anthurium root rot caused by Pythium splendens, P. .spinosum, P. vexans and Cahnectria crotalariae. Diuron in agar medium was inhibitory to the growth of mycelium, formation and germination of sporangia of P. splendens. Sporangia of P. splendens produced in diuron-amended medium did not differ in pathogenicity to anthurium roots from those produced in diuron-free medium. When diuron was applied to kill weeds in the planting medium, the population of P. sptendens in it was not decreased during the test. Diuron was inhibitory to a number of micro-organisms in the planting medium. Exudation of anthunum roots was not increased by djuron treatment. Increase in severity of anthurium root rot by diuron treatment was similar whether the experiments were performed in the presence or absence of planting medium, suggesting that the enhancmg effect of diuron on root rot is mainly due to an increase m susceptibility of the host plants