Host-Pathogen Interactions: XX. Biological Variation in the Protection of Soybeans from Infection by Phytophthora Megasperma F. sp. glycinea

Phytophthora megasperma f.sp. glycinea, which causes soybean (Glycine max) root and stem rot, exists as several races which differ in their ability to infect a range of soybean cultivars. A glycoprotein-rich fraction (Fraction I) isolated from fungal culture fluid protects soybean seedlings from infection with compatible races. In an early study (13), seedlings were protected only by Fraction I purified from incompatible races. In 1979, seedlings were better protected by Fraction I isolated from incompatible races than by Fraction I isolated from compatible races. In 1980, seedlings were protected equally well by Fraction I from incompatible and compatible races. Materials similar in composition to Fraction I did not protect seedlings from infection. No cause could be identified for the apparent change, during the 3-year period, in the race specificity of the protection assay. Variability in the bioassay prohibited further purification or characterization of Fraction I components that protect seedlings from infection.