STUDIES ON PHYTOPHTHORA-CLANDESTINA, THE CAUSE OF TAPROOT ROT IN SUBTERRANEAN CLOVER .1. EVIDENCE FOR PHYSIOLOGICAL SPECIALIZATION IN PHYTOPHTHORA-CLANDESTINA

Root rot caused by Phytophthora clandestina Taylor, Pascoe a Greenhalg h is a serious disease of subterranean clover (Trifolium subterraneum) , an important leguminous component of Australia's temperate pastures. Previous reports have shown that P. clandestina causes severe taproot rot in cv. Woogenellup, while cvv. Larisa and Trikkala remain highly resistant. In this study, 3 greenhouse experiments were conducted to d etermine the effects of P. clandestina isolates collected between 1983 and 1990 on different subterranean clover cultivars. Woogenellup was consistently susceptible while Meteora was less affected by ail isolat es tested. Trikkala, a previously known root rot resistant cultivar, w as as susceptible as Woogenellup when tested with a recently collected isolate from Katamatite, northern Victoria. This study provides the f irst evidence for variation in virulence between isolates of P. clande stina and for physiological specialisation in this species.