Phomopsis seed decay (PSD) is part of a major soybean [Glycine max (L.) Merr.] fungal disease complex found in the USA and other areas of the world. The predominant disease organism, Phomopsis longicolla T.W. Hobbs, can cause reductions in seed quality and germination. While some chemical and cultural control measures exist to reduce losses due to this disease, genetic resistance would be desirable. An extensive screening project has identified PI 417479 as resistant to PSD. To most efficiently transfer the disease resistance found in PI 417479 to improved cultivars, knowledge of the inheritance of the trait is needed. Crosses were made between PI 417479 and each of two PSD‐susceptible genotypes, ‘Agripro 350’ and PI 91113. Five generations (F1, F2, F3, B1, and B2, in which B1, represents a backcross between the F1 and the resistant parent and B2 represents backcross between the F1 and the susceptible parent) were generated for each cross. These were tested in 1990 in the field at two locations near Columbi, MO. A sample of seeds from each plant was bioassayed to determine percent PSD incidence. Seeds from plants showing various degrees of infection in 1990 were progeny tested in 1991. Although environment was found to strongly influence disease incidence, results indicate that the PSD resistance found in PI 417479 is controlled by two complementary dominant nuclear genes. We concluded that the resistance genes can be transferred using a backcross procedure.
Phomopsis iongicolla Hobbs and P. sojae Lehman are the principal causal organisms of Phomopsis seed decay. This disease can reduce germination and quality of soybean. Production of mycotoxins by Phomopsis spp. has been reported. No commercial cultivars are resistant to Phomopsis seed decay. However, the plant introduction PI-417479 is a source of genetic resistance. When grown under field conditions favorable for infection by Phomopsis spp., PI-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 cuitivars. Crosses were made between PI-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 progenytested. Environment strongly influenced disease incidence, but results indicated that resistance to Phomopsis seed decay is controlled by two complementary dominant nuclear genes. Information developed in these studies will facilitate development of resistant cultivars.]AOCS 72, 1431-1434 (1995).
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