COLONIZATION OF ROOTS OF SUBTERRANEAN CLOVER CULTIVARS BY VIRULENT AND AVIRULENT RACES OF PHYTOPHTHORA-CLANDESTINA

The colonization of the roots of four cultivars of subterranean clover by isolates representing four races of Phytophthora clandestina was s tudied. There was a highly significant race X cultivar interaction in the growth of inoculated tap roots and the degree of colonization of r oots by the pathogen. While all races were able to infect the roots of all cultivars tested, roots of the susceptible cultivars were coloniz ed more rapidly and extensively than those of the resistant cultivars. Zn compatible combinations, fungal colonization extended for a few ce ntimetres in the tap root and lateral roots in the moderately suscepti ble cultivars Trikkala and Meteora, or throughout the whole root syste m leading to the death of the host in the very susceptible cultivar Wo ogenellup. In contrast, limited fungal colonization of the tap root an d lack of extension of the fungus into lateral roots was typical of in compatible combinations. In all cultivars, lateral roots were as susce ptible to infection as tap roots. The number of lateral roots of Wooge nellup was significantly reduced by infection. However, neither the ra te of lateral root formation nor the total number of lateral roots of Seaton Park, Meteora and Trikkala was reduced by infection with virule nt or avirulent races of the pathogen.