Phytophthorasojae (Kaufmann and Gerdemann) is the major pathogen of soybeans in Ohio and contributes to losses in several production regions in the United States and the world (3,8). Under saturated soil conditions, P.sojae produces motile zoospores which locate and infect soybean plants throughout the growing season. Phytophthora seed rot and damping-off are early-season diseases, while Phytophthora root and stem rot occurs mid-to-late season.
Single resistance genes, designated Rps genes, have provided
a somewhat durable disease-management strategy. Cultivars with single
Rps genes were first deployed in Ohio during the 1960s. Thirteen
Rps genes have been identified to date, of which eight
(Rps1a, Rps1b, Rps1c, Rps1k, Rps2,
Rps3a, Rps6, and Rps7) have been used in commercial
soybean cultivars. However, following deployment of each single Rps
gene, races of P.sojae were subsequently identified that had
a susceptible interaction with the Rps gene. Single Rps
genes have been effective for eight to 15 years depending on inoculum
density and environmental conditions (4). Phytophthora root rot was
identified in fields in which cultivars with the single gene,
Rps1k, were grown as early as 1990 in Ohio (5). Races of
P.sojae that elicit a susceptible interaction with Rps1k
have also been isolated from soil in Indiana (1) and plants in Iowa (9)
during 1993 and 1991, respectively. In addition, races of
P.sojae with virulence pathotypes for Rps genes that have
never been deployed have also been identified
(1, 5, 7).
New sources of resistance need to be identified in order to maintain single Rps genes as a viable tool. In order to identify novel sources of resistance, the authors selected P.sojae isolates that would have a susceptible interaction with each of the 13 Rps genes. This was followed by a series of isolates which would have susceptible interactions with specific 2 and 3 Rps gene combinations. It is not uncommon for soybean accessions to have 2 Rps genes. For example, the soybean cultivar, Tracy, that was widely planted throughout the southern United States has Rps1b and Rps3a. Each soybean plant can have a maximum of 7 Rps genes represented by genes at each locus identified to date.
The purpose of this study was to identify new sources of Rps and partial resistance to P.sojae in the USDA Soybean Germplasm collection. The purpose of this publication is to provide a complete listing of the accessions tested and their reactions. A summary report has been published in Plant Disease (v 84:1303-1308). This data is also available in a searchable format on the USDA National Plant Germplasm System, Germplasm Resources Information Network (GRIN) at http://www.ars-grin.gov/npgs/.
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