Occurrence of Phytophthora clandestina in Trifolium subterraneum paddocks in Australia

In 1995-96, a survey of soil samples from subterranean clover (Trifolium su bterraneum L.) paddocks was conducted across Victoria, South Australia, New South Wales and Western Australia, to determine the distribution and the p revalence of races of Phytophthora clandestina (as determined by the develo pment of root rot on differential cultivars), and the association of its oc currence with paddock variables. In all states, there was a weak but signif icant association between P. clandestina detected in soil samples and subse quent root rot susceptibility of differential cultivars grown in these soil samples. Phytophthora clandestina was found in 38% of the sampled sites, w ith a significantly lower prevalence in South Australia (27%). There were s ignificant positive associations between P. clandestina detection and incre ased soil salinity (Western Australia), early growth stages of subterranean clover (Victoria), mature subterranean clover (South Australia), recently sown subterranean clover (South Australia), paddocks with higher subterrane an clover content (Victoria), where herbicides were not applied (South Aust ralia), irrigation (New South Wales and Victoria), cattle grazing (South Au stralia and Victoria), early sampling dates (Victoria and New South Wales), sampling shortly after the autumn break or first irrigation (Victoria), sh orter soil storage time (Victoria) and farmer's perception of root rot bein g present (Victoria and New South Wales). Only 29% of P. clandestina isolat es could be classified under the 5 known races. Some of the unknown races w ere virulent on cv. Seaton Park LF (most resistant) and others were avirule nt on cv. Woogenellup (most susceptible). Race 1 was significantly less pre valent in South Australia than Victoria and race 0 was significantly less p revalent in New South Wales than in South Australia and Western Australia. This study revealed extremely wide variation in the virulence of P. clandes tina. The potential importance of the results on programs to breed for resi stance to root rot are discussed.