CONTROL OF PHYTOPHTHORA ROOT-ROT OF IRRIGATED SUBTERRANEAN CLOVER WITH POTASSIUM PHOSPHONATE IN VICTORIA, AUSTRALIA

Phosphonic acid was more effective in inhibiting the production of spo rangia of Phytophthora clandestina in sterile pond water (ED(50) = 1.4 p.p.m) than it was in inhibiting the growth of mycelium of the fungus on either corn meal agar (ED(50) = 13.8 p.p.m.) or lima bean agar (ED (50) = 236 p.p.m.). Experiments under controlled environmental conditi ons showed that better control of tap root rot of subterranean clover caused by P. clandestina was achieved by application of potassium phos phonate to a pasteurized soil mixture than by a spray on the foliage a lone. However, in a soil from a pasture, foliar sprays were more effec tive than soil treatments. Application of superphosphate at a rate of 250 kg/ha or higher to the soil reduced the effectiveness of sprays. I n three experiments in irrigated pastures a spray of potassium phospho nate at 300 or 313 ml/ha, applied to cotyledons of subterranean clover and to soil, reduced severity of Phytophthora root rot and increased the annual production of dry matter of the legume by 1.96 to 5.11 t/ha in comparison with untreated controls.