Genetics and mapping of adult plant rust resistance in soybean PI 587886 and PI 587880A.

Two soybean accessions, PI 587886 and PI 587880A, previously identified as having resistance to Phakospora pachyrhizi Syd. (soybean rust, SBR) were used to create two populations (POP-1 and POP-2) segregating for SBR resistance. F(2)-derived F(3) (F(2:3)) families from each population were grown in a naturally SBR-infected field in Paraguay to determine inheritance and map resistance genes. Over 6,000 plants from 178 families in POP-1 and over 5,000 plants from 160 families in POP-2 were evaluated at R5 for lesion type: immune reaction (IR), reddish-brown (RB), or tan (TAN) colored lesions. Based on the lesion type present, each F(2:3) family was rated as resistant, segregating or susceptible and this classification was used to infer the F(2)-phenotype and genotype. For both populations, the F(2) segregation ratios fit a 1:2:1 (resistant:segregating:susceptible) ratio expected for a single gene (P > 0.05). The RB lesions occurred almost exclusively in the heterozygous class, indicating incomplete dominance under the conditions of this study. Molecular markers flanking the locations of the known resistance genes were used to map the resistance gene in both populations to the Rpp1 locus. However, evaluation of PI 587886 and PI 587880A against eight P. pachyrhizi isolates indicated that the resistance allele in these two accessions was different from Rpp1. This test also demonstrated that these accessions were resistant to at least one P. pachyrhizi isolate collected in the southern US. This is the first report of using an adult plant field-screen with natural rust pressure to map SBR resistance.

Breeding for Resistance to Soybean Rust.

Soybean rust occurs in all major soybean-growing regions of the world including the North American mainland. Soybean rust, caused by Phakopsora pachyrhizi, is the most destructive foliar disease of soybean, and yield losses of over 50% are common when environmental conditions are conducive for disease development. Heavily infected plants defoliate and mature more rapidly than plants not infected with rust. P. pachyrhizi has a broad host range and can infect many other legumes including some native to Australia. A number of physiological races of the fungus have been reported on these native legumes from Australia and on soybean. In addition, four single genes for rust resistance were previously identified in four different soybean plant introductions. These sources of resistance also have been reported to be susceptible in some field locations and when challenged with certain isolates of P. pachyrhizi. Partial resistance, expressed as reduced pustule number and increased length of latent period, has also been reported but has not been widely used in breeding programs. Yield stability has been used in the past and compares percentage of yields in fungicide and nonfungicide plots. Cultivars or lines with a higher percentage of yield have greater yield stability in the presence of rust. Although soybean rust only recently was found in the continental United States, a proactive project to evaluate the USDA soybean germ plasm collection for rust resistance was initiated in 2002 at the Fort Detrick plant biocontainment facility and at six international locations. Part of this project is to discover soybean lines with greater yield stability, and additional single and partial resistance. To help minimize the impact of soybean rust, the first line of defense will be fungicides, with host resistance and yield stability augmenting the long-term management of soybean rust.