Partial resistance to Phytophthora sojae Kauffmann and Gerdemann in soybean [Glycine max (L.) Merr.] is expressed as a reduced level of root rot and is effective against all populations of the pathogen. The objective of this study was to identify simple sequence repeat (SSR) markers associated with putative quantitative trait loci (QTLs) for partial resistance to P. sojae in the soybean ‘Conrad’. Three recombinant inbred soybean populations, Conrad × ‘Sloan’, Conrad × ‘Harosoy’, and Conrad × ‘Williams’, were evaluated for root lesion growth rate in growth chamber experiments following inoculation with P. sojae and with SSR markers to identify putative QTLs. The three populations segregated for root lesion growth rate after root inoculations. Two putative QTLs donated by Conrad were identified in all three populations and were positioned on soybean molecular linkage groups (MLGs) F and D1b+W. The QTL on MLG F explained 32.4, 35.0, and 21.4% of the genotypic variation for Conrad × Sloan, Conrad × Harosoy, and Conrad × Williams populations, respectively. The QTL on MLG D1b+W explained 10.6, 15.9, and 20.7% of the genotypic variation for the same three populations, respectively. The QTL on MLG F appears to be of more value based on the percentage of genotypic variation explained. Because the results indicate that QTLs for partial resistance to P. sojae map to different regions in soybean compared with the known Rps genes poses a challenge to soybean breeders. Marker‐assisted selection may expedite the process of combining both Rps genes with partial resistance into high‐yielding cultivars.

No abstract

bean that contains over 800 SSR markers (Cregan et al., 1999) facilitates the selection of polymerase chain Genetic diversity is low among elite Northern American soybean reaction (PCR) based markers to survey genetic diver-[Glycine max (L.) Merr.] breeding populations. Fewer than 20 soybean cultivars are responsible for 80% of the genes in public soybean sity in populations for which little data are available. cultivars released in recent years. Diversifying the soybean germplasm Single sequence repeat markers are advantageous bebase could introduce new genes for agronomic diversity as well. Recause of single locus inheritance and the ability to detect cently, soybean plant introductions (PIs) have been identified as addimultiple alleles. Consequently, SSR markers were used tional sources of both partial and specific resistance to Phytophthora in this study to determine genetic relatedness among soysojae M.J. Kaufmann & J.W. Gerdemann (syn. P. megasperma bean lines. Drechs. f. sp. glycinea T. Kuan & D.C. Erwin). The objective of thisResistance to the soybean pathogen P. sojae is constudy was to compare the genetic diversity present among soybean trolled by two mechanisms. The first is partial resistance, PIs resistant to P. sojae in relation to cultivars and breeding lines which involves multiple genes and limited damage to that represent U.S. breeding germplasm, and to develop guidelines the plant (Schmitthenner, 1985). The second is single for the genetic study and practical use of these resistant resources in applied breeding. Ninety-three accessions from South Korea, China, gene resistance, in which P. sojae interacts with Rps and Japan and 15 genotypes from the USA were evaluated for 52 genes in a gene for gene system (as defined by Flor, simple sequence repeat (SSR) primer pairs. The South Korean mate-1955) preventing disease development in the plant. Thus rial included 32 P. sojae resistant, 49 partially resistant, and 7 susceptifar, seven Rps genes have been identified in soybean, ble accessions. The SSR data were used to compute Nei's distance some of which appear to have more than one resistance estimates. Clustering and multivariate analysis of Nei's distance estiallele (Diers et al., 1992; Anderson and Buzzell, 1991). mates demonstrated that accessions from South Korea are genetically Phytophthora sojae populations have been identified in different from U.S. germplasm. These results indicate that the South Ohio and other Midwestern states which have a compat-Korean germplasm may contain alleles not present in U.S. cultivars, ible interaction, leading to susceptibility, of the host such as new alleles for P. sojae resistance. Additionally, this study plant with all of the commonly used Rps genes (Abney identifies SSR markers that can be used to begin mapping P. sojae resistance alleles in South Korean germplasm. et al., 1997). Consequently, new sources of Rps genes are needed (Schmitthenner et al., 1994).Recently, potential new sources of resistance to P.

Phytophthora root and stem rot caused by Phytophthora sojae M.J. Kaufmann & J.W. Gerdemann (=Phytophthora megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin) is the second leading cause of yield loss in soybean [Glycine max (L.) Merr.] in the USA. Currently, seven loci for resistance with 13 alleles have been identified in soybean. Populations of P. sojae exist in many soybean production regions that cause disease on plants with many, if not all, of these genes. Consequently, the need for new resistance loci is great. A number of plant introductions (PIs) from South Korea were identified in an earlier study that may contain novel resistance loci. The objectives of this project were to identify the allele(s) for resistance to P. sojae present in PI399073 and to map these allele(s) to a linkage group. PI399073 was crossed with ‘Williams’ and ‘S 19‐90’ to develop populations. These populations were assayed for disease resistance, single sequence repeats (SSR), restriction fragment length polymorphisms (RFLP), and isozyme markers. A new Phytophthora resistance locus, Rps8, was mapped to MLG (major linkage group) A2 in two different crosses with PI399073. This is the first locus for Phytophthora resistance that has been identified on MLG A2.

Genetic diversity is low among elite Northern American soybean [Glycine max (L.) Merr.] breeding populations. Fewer than 20 soybean cultivars are responsible for 80% of the genes in public soybean cultivars released in recent years. Diversifying the soybean germplasm base could introduce new genes for agronomic diversity as well. Recently, soybean plant introductions (PIs) have been identified as additional sources of both partial and specific resistance to Phytophthora sojae M.J. Kaufmann & J.W. Gerdemann (syn. P megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin). The objective of this study was to compare the genetic diversity present among soybean PIs resistant to P sojae in relation to cultivars and breeding lines that represent U.S. breeding germplasm, and to develop guidelines for the genetic study and practical use of these resistant resources in applied breeding. Ninety‐three accessions from South Korea, China, and Japan and 15 genotypes from the USA were evaluated for 52 simple sequence repeat (SSR) primer pairs. The South Korean material included 32 P sojae resistant, 49 partially resistant, and 7 susceptible accessions. The SSR data were used to compute Nei's distance estimates. Clustering and multivariate analysis of Nei's distance estimates demonstrated that accessions from South Korea are genetically different from U.S. germplasm. These results indicate that the South Korean germplasm may contain alleles not present in U.S. cultivars, such as new alleles for P sojae resistance. Additionally, this study identifies SSR markers that can be used to begin mapping P sojae resistance alleles in South Korean germplasm.