SOYBEAN FLAVONOID EFFECTS ON AND METABOLISM BY PHYTOPHTHORA-SOJAE

Various soybean flavonoids were examined for their effects on the growth and development of Phytophthora sojae and for their metabolism by P. sojae. Three classes of molecules were identified based on their effects on growth. Coumestrol, biochanin A, genistein, naringenin and isorhamnetin were inhibitory at concentrations of 60-120 muM and were fungicidal at 240 muM. Quercetin and its 3-0-beta-D-glucoside, isoquercitrin, caused significantly prolonged lags in P. sojae growth at 60-240 muM, but were not fungicidal at any of these concentrations. Daidzein, formononetin, kaempferol, apigenin, chrysin and rutin were not inhibitory over this range. P. sojae rapidly hydrolysed all flavonoid glycosides tested and metabolized several flavonoids to non-aromatic products. However, the aglycones of the most inhibitory compounds were not signficantly degraded. Metabolism of the compounds by P. sojae was a very early event and appeared to be associated predominantly with the hyphal tips. Several compounds had potentially interesting effects on fungal morphology and development. For example, genistein and its conjugates caused a marked swelling of the hyphal tip and an increase in the number of oogonia formed.