INVITRO ASSAY FOR EVALUATION OF PHYTOPHTHORA ROT ON SOYBEAN TAPROOTS

Root lesions indicate soybean [Glycine max (L.) Merr.] cultivar resistance to phytophthora rot, caused by the soilborne fungus Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin (Pmg). The type and degree of resistance to Pmg was rapidly, decisively gauged by monitoring lesion expansion and branch root proliferation in a microculture assay. 'Williams 82' soybean, containing the resistance gene Rps1-k, exhibited a dark, discrete lesion on taproots within 12 h after inoculation. In the absence of an effective Rps gene, light-brown water-soaked lesions on taproots continued to expand with time. Cultivars with different levels of rate-reducing resistance could be distinguished by measuring lesion size and rate of expansion.