NATURE OF ENHANCED SEVERITY OF ANTHURIUM ROOT-ROT BY DIURON TREATMENTFOR WEED-CONTROL

Diuron treatment for weed control greatly increased anthurium root rot caused by Pythium splendens, P. spinosum, P. vexans and Calonectria c rotalariae. Diuron in agar medium was inhibitory to the growth of myce lium, formation and germination of sporangia of P. splendens. Sporangi a of P. splendens produced in diuron-amended medium did not differ in pathogenicity to anthurium roots from those produced in diuron-free me dium. When diuron was applied to kill weeds in the planting medium, th e population of P. splendens in it was not decreased during the lest. Diuron was inhibitory to a number of micro-organisms in the planting m edium. Exudation of anthurium roots was not increased by diuron treatm ent. Increase in severity of anthurium root rot by diuron treatment wa s similar whether the experiments were performed in the presence or ab sence of planting medium, suggesting that the enhancing effect of diur on on root rot is mainly due to an increase in susceptibility of the h ost plants.