Role of Aphanomyces trifolii and nutrition in root disease of Trifolium subterraneum

[Truncated abstract] Subterranean clover (Trifolium subterraneum) is the most important pasture legume across southern Australia, including Western Australia where some 6 million had been sown by the mid 1970s (Gladstone 1975). Decline in subterranean clover pastures manifests as a decrease in the composition of desirable species, especially the legumes, and increased weed species. It was first recognized as a problem in Western Australia during the 1960s. Root disease is the main contributing factor to pasture decline. Despite its significant impact on pasture productivity, a detailed survey of root disease on subterranean clover has not been carried out in south-west Western Australia for at least two decades. While the impact of root disease has been examined on seedlings, it has not been examined on mature subterranean clover. The management of root diseases in subterranean clover based pastures is still a major challenge, as there are no completely resistant cultivars to the spectrum of pathogens that cause root disease. Nutrient applications have been recommended for optimal subterranean clover pasture growth throughout the south-west of Western Australia, but the relationship between mineral nutrition and root disease has not been previously examined. To date, the importance of Aphanomyces as a causal agent of root disease of subterranean clover has not been examined in Western Australia. For this reason, the aims of my PhD were to: First, conduct a field survey of tap and lateral root disease on mature subterranean clover in pastures in the south-west of Western Australia. Secondly, determine if nutrient applications can be used to manage tap and lateral root disease of subterranean clover in south-west Western Australia. Thirdly, isolate, describe and determine the pathogenicity and role of the novel Aphanomyces sp. causing root disease on subterranean clover in south-west Western Australia. Fourthly, screen subterranean clover germplasm to identify