Relative importance of root rots of subterranean clover caused by Aphanomyces euteiches and Phytophthora clandestina

In a growth-cabinet experiment, Aphanomyces euteiches caused more severe root disease and greater reductions in root and shoot weights of subterranean clover cv. Mount Barker than Phytophthora clandestina in pasteurized sandy loam flooded with water for a 24-h period each week. A. euteiches also reduced plant growth more than P. clandestina in untreated sandy loam. In a similar experiment, both fungi caused the same amount of disease and reduction in growth of cv. Yarloop in pasteurized clay loam flooded for 24 h each week, and P. clandestina caused more disease and a greater decrease in plant growth than A. euteiches when the soil was flooded for 4 h each week. The pathogens did not interact positively in either soil. In an irrigated pasture, soil drenches with the fungicides metalaxyl and fenaminosulf together reduced both root rots to low levels and increased subterranean clover dry matter by 1 96, 0.50 and 1 20 t/ha in the autumn, winter and spring of 1985, respectively, and by 0.59 t/ha in the autumn of 1986. Results suggest that P. clandestina caused most of these losses in yield.