Developing soybean varieties with genetic resistance toPhomopsis spp

Phomopsis longicolla Hobbs andP. sojae Lehman are the principal causal organisms ofPhomopsis seed decay. This disease can reduce germination and quality of soybean. Production of mycotoxins byPhomopsis spp. has been reported. No commercial cultivars are resistant toPhomopsis seed decay. However, the plant introduction Pl‐417479 is a source of genetic resistance. When grown under field conditions favorable for infection byPhomopsis spp., Pl‐417479 was free of seed infection in two tests and had 3% infection in another. In the same environments, the cultivar “Williams 82” had 25 to 59% infection. Inheritance of the trait was determined to provide information for efficient transfer of the resistance to improved cultivars. Crosses were made between Pl‐417479 and two susceptible genotypes. Five generations were developed for each cross and tested at two locations. Plots were artificially inoculated to enhance infection. Seeds from plants that showed various degrees of infection in the first season were progeny‐tested. Environment strongly influenced disease incidence, but results indicated that resistance toPhomopsis seed decay is controlled by two complementary dominant nuclear genes. Information developed in these studies will facilitate development of resistant cultivars.