THE EXPRESSION OF RESISTANCE OF SUBTERRANEAN CLOVER CULTIVARS TO RACES OF PHYTOPHTHORA-CLANDESTINA

Infection and sporulation of races of Phytophthora clandestina in susc eptible and resistant cultivars of subterranean clover and the effect of the races on growth of the cultivars in pots were studied using a t otal of five races of the pathogen and eight host cultivars. The patho gen caused severe disease where races and cultivars were compatible, a lthough a significant reduction in root and shoot dry weight occurred only on Woogenellup inoculated with race 2. There was little increase in disease severity after 1 wk from the time of inoculation. Sporangia and oospores were formed at high frequency in tap and lateral roots i n all compatible combinations, except in Woogenellup in which sporangi a were produced inconsistently. The pathogen caused negligible disease and produced very few sporangia in incompatible combinations. Plant a ge had a strong effect on disease development. Disease severity caused by virulent races on cultivars Gosse, Trikkala, Meteora and Woogenell up was less on older than on younger plants. A further experiment usin g aggressive isolates of races 0, 1, 2, 3 and 4 revealed that cultivar Gosse showed no reduction in root and shoot weight, despite it being susceptible to races 1, 3 and 4. This provided evidence of the existen ce of race-non-specific or horizontal resistance to P. clandestina in cultivar Gosse.