A laboratory procedure for determining relative levels of field resistance in soybeans to Phytophthora megasperma f.sp. Glycinea

Abstract

A reliable laboratory assay is described which allows rapid determination of relative levels of field resistance in soybeans to Phytophthora megasperma fsp, glycinea. The expression of field resistance depended on both the inoculum level and the age of seedlings at time of saturation of the potting mix with water. At inoculum levels in the range 0.018-0.576 g dry weight of mycelium per kg dry weight of potting mix, ready differentiation in the resistance levels of the cultivars Davis (field resistant) and Semstar (field susceptible) could be made after 7 days of continuous saturation of the mix, which commenced 3 days after sowing. When inoculated by the conventional hypocotyl puncture technique, both cultivars gave susceptible reactions. The seedling assay was used to screen a range of soybean cultivars with known levels of field resistance to the fungus. Disease assessments made in the laboratory assay were similar to those expected under natural conditions of infection in the field. Possible use of the method in future soybean breeding programs is discussed.