VARIATION IN PATHOGENICITY AND VIRULENCE OF PHYTOPHTHORA-CLANDESTINA ON 4 CULTIVARS OF SUBTERRANEAN CLOVER

The pathogenicity of 6 isolates of Phytophthora clandestina on seedlin gs of 4 cultivars of subterranean clover was studied. There was a high ly significant isolate x cultivar interaction in pathogenicity tests o n axenic seedlings and seedlings grown in pasteurised potting mix or u ntreated sandy loam soil, indicating the existence of race-cultivar sp ecificity. The isolates showed differences in virulence against the cu ltivars. Three isolates (race 0) caused severe disease only on Woogene llup; 2 isolates (race 1) caused severe disease on Larisa, Trikkala, a nd Woogenellup, but not on Meteora; one isolate (race 2) caused severe disease on Meteora and Woogenellup, but not on Larisa and Trikkala. A s well as differing in virulence (the ability of a race to attack a ra nge of cultivars), the races also differed in their aggressiveness on Woogenellup, with race 2 being the most, and race 0 the least, pathoge nic. The isolates varied in their growth rate on agar medium, but this was not related to virulence or aggressiveness.