Phytophthora sojae is effectively managed through soybean cultivars with single resistance genes (Rps) and partial resistance inherited quantitatively. There are over 50 described races of Phytophthora sojae, but many more pathotypes have been reported. Many of the isolates recently collected are more complex, causing disease on plants with 3 or more Rps genes. This diagnostic guide provides some standard assays to assist in the identification of virulence types of P. sojae and genetic resistance in soybean. Accepted for publication 4 October 2007. Published 18 January 2008.
Soybean [Glycine max (L.) Merr.] PI 391589B, a selection from PI 391589A was recently identified as a new source of resistance to Sclerotinia sclerotiorum (Lib.) deBary, which causes Sclerotinia stem rot. The objective of this study was to identify the quantitative trait loci (QTLs) associated with resistance to S. sclerotiorum in PIs 391589A and 391589B. BC1F4:5 and BC1F4:6 populations from a cross of ‘Kottman’(2) × PI 391589A and a population of F2‐derived lines from a cross of PI 391589B × IA2053 were evaluated for resistance to S. sclerotiorum in the field and in the greenhouse from 2003 to 2005 and genotyped with simple sequence repeat markers. Single factor analysis identified 18 markers on nine linkage groups (LGs) significantly (P < 0.05) associated with resistance to S. sclerotiorum in the two populations. Four regions on LGs E, F, M, and O were significantly associated with the disease resistance in both populations. The four regions are between Satt411 (12.9 cM) and Satt369 (56.2 cM) on LG E, between Satt269 (11.4 cM) and AW186493 (21.0 cM) on LG F, between Satt463 (50.1 cM) and Satt323 (60.1 cM) on LG M, and between Satt581 (106.0 cM) and Satt153 (118.14 cM) on LG O on the soybean composite map developed by Song and others in 2004. Composite interval mapping identified seven QTLs (P < 0.10), each explaining 6.0 to 15.7% of the phenotypic variance. A QTL on LG M near marker Satt463 (50.1 cM) is unique to PI 391589A and B. Therefore, PIs 391589A and 391589B offer breeders a new allele for resistance to the disease.
Pythium spp. were baited with corn and soybean seed from soils collected from three locations in Ohio where soybean and corn stand establishment was a concern. Five species, P. catenulatum, P. irregulare, P. paroecandrum, P. splendens, and P. torulosum, were recovered and a subset of these isolates was then tested for pathogenicity on corn and soybean seed and sensitivity to the seed treatment fungicide metalaxyl. There was a range of both pathogenicity and sensitivity to metalaxyl within and among the Pythium spp. recovered from the three locations. A more thorough evaluation of the Pythium populations that exist in grain production fields in the north central region of the US is needed to facilitate the development and deployment of broader based seed treatment products. Accepted for publication 12 January 2004. Published 2 February 2004.
Phytophthora sojae, which causes Phytophthora root and stem rot of soybean, is a serious disease worldwide and is managed primarily by deploying cultivars with resistance. Thirty-two soybean plant introductions (PIs), all but three of which were from South Korea, were proposed as new sources of single-gene resistance to P. sojae. The objective of this study was to characterize the inheritance of resistance to P. sojae in these PIs. Twenty-two soybean populations from crosses of these PIs and the susceptible cv. Williams were inoculated with P. sojae OH17 (vir 1b, 1d, 2, 3a, 3b, 3c, 4, 5, 6, 7), and OH25 (vir 1a, 1b, 1c, 1k, 7). These isolates were selected because they are virulent on soybeans with all known Rps genes and many Rps gene combinations. Thirteen of the twenty-two populations had consistent segregation responses following inoculations between the two generations. In two PIs, resistance was conferred by two genes to OH17 and three genes to OH25. Resistance to both isolates was conferred by a single gene in PI 398440 although the individual families were not resistant to the same isolates. The data suggest that six of the populations have three-Rps gene combinations as previously proposed, while another four may have either a novel Rps gene or a four-Rps gene combination. Based on this phenotypic analysis, novel and uncharacterized Rps genes may be present in this material. More importantly, these PIs may serve as sources of novel Rps genes that can be used to more effectively manage Phytophthora root and stem rot.
During the spring of 2004, corn seedlings with symptoms of wilting and stunting were observed in corn fields with emergence problems in Madison and Brown counties, Ohio. Phytophthora isolates were recovered from sections of root tissue of diseased seedlings placed on dilute V8 media amended with pentachloronitrobenzene, iprodione, benlate, neomycin sulfate, and chloramphenicol. Colonies were rosaceous on potato dextrose agar, with a growth rate of 5 mm per day. Homothallic isolates with paragynous antheridia were observed on lima bean agar (LBA); oogonia were 35 to 50 μm in diameter. Sporangia were ovoid to obpyriform, nonpapillate, with an average size of 49 × 30 μm. Pathogenicity was tested on corn seeds using a petri dish assay with 3-day-old cultures on LBA and a sand-cornmeal cup test amended with inoculum from 7-day-old cultures on LBA (1). After 1 week in the petri dish assay, the seeds failed to germinate completely and were covered with white, fungal-like, aerial mycelia and the pathogen was recovered from brown discolored radicle roots. In the cup assay, 2-week-old seedlings developed the same symptoms observed in the field; the pathogen was also isolated from brown discolored roots. In both assays, no symptoms developed in the noninoculated controls. Both pathogenicity tests were repeated two times. Genomic DNA was extracted from mycelia of two isolates and the internal transcribed spacer (ITS) region was amplified and sequenced using ITS6/ITS4 primers (2). Both isolates had identical ITS sequences (GenBank Accession No. GQ853880). A BLAST search of the NCBI database showed 100% homology with the sequence of the haplotype isolate of Phytophthora sansomeana (Accession No. EU925375). P. sansomeana is a new species characterized principally by a large oogonial diameter (37 to 45 μm), rapid growth rate (7 to 10 mm/day), and an ITS sequence falling in Cooke's clade 8 (4). Pathogenicity tests, morphological characteristics, and the ITS sequence analysis indicate that P. samsomena is the causal agent of the symptoms observed on corn seedlings. P. sansomeana has been reported as a pathogen of soybean in Indiana, Douglas-fir in Oregon, and weeds in alfalfa fields in New York (4). To our knowledge, this is the first report of P. sansomeana infecting corn in Ohio, albeit other isolates have previously been recovered from soybean in the state. There are four previous reports of Phytophthora spp. affecting corn in the United States and Mexico (3). Crop rotation will have little effect in management of this pathogen since corn and soybean are produced in the same fields continuously throughout the state. References: (1) K. E. Broders et al. Plant. Dis. 91:727, 2007. (2) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (3) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN. 1989. (4) E. M. Hansen et al. Mycologia 101:129, 2009.
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