ABSTRACT
The fungal pathogen Phytophthora megasperma Drechs. f. sp. glycinea (Hildeb.) Kuan and Erwin (recently redesignated as P. sojae) can infect all vegetative soybean organs (Sinclair, 1982). It causes symptoms ranging from rapid pre- and postemergence seedling damping off to slowly spreading lesions on older plant tissues. The soybean-P. sojae association provides an excellent system for the study of molecular aspects of host-pathogen interactions. Resistance to the pathogen in soybean is determined by major dominant Rps genes occurring at seven loci, with several allelic forms at two of these loci. There are at least 25 known races of P. sojae which are characterized by their different specific interactions with these Rps genes (Schmitthenner, 1985). Where a given Rps gene provides effective resistance to a given race of P. sojae, the interaction between host and pathogen is termed incompatible. Where a given Rps gene provides no resistance to a given race of P. sojae, the interaction is termed compatible.
