White mold caused by Sclerotinia sclerotiorum (Lib.) de Bary can be an important cause of yield loss in soybean. Only partial resistance to this disease has been found to date, and little is known about the loci contributing to this resistance. The objectives of this study were to identify quantitative trait loci (QTL) associated with partial resistance to white mold progression on the main stem. One hundred eighty F4–derived lines from a cross between a partially resistant cultivar, Maple Donovan, and a susceptible cultivar, OAC Bayfield, were tested for resistance to white mold under field conditions with (2003 and 2004) or without irrigation (2005 and 2006). The resistance of the lines was assessed by measuring the length of lesions 7 d after inoculation with a mycelial suspension. These lines were genotyped with 128 simple sequence repeat markers, and three QTLs associated with lesion length were detected consistently (in at least three of the four trials). Two of the QTLs were located on LG C2 (chromosome 6) and the third was on LGI (chromosome 20). In these genomic regions, the favorable alleles came from Maple Donovan and contributed to a decrease in lesion length. Together, they accounted for 30.7 to 50.9% of lesion length variation across year trials. Selective phenotyping of 26 lines carrying contrasting alleles at these QTLs in four additional environments resulted in significant phenotypic contrasts between the two genotypic classes.
Soybean (Glycine max) seedlings with symptoms of Pythium damping-off were collected in northeastern Iowa soybean fields and processed for isolation of the causal agents on both potato dextrose agar (PDA) and pimaricin-, ampicillin-, rifampicin-, and pentachloronitrobenzene (PARP)-containing media. Isolates were identified based on morphological characteristics, growth rates, along with sequence data for the nuclear rDNA ITS1-5.8S-ITS2 region (ITS barcode). Nine isolates were identified via NCBI BLASTn search of sequences available in GenBank: one isolate of Pythium orthogonon; three isolates of P. inflatum; two isolates of P. ultimum var. ultimum; one isolate of P. torulosum; and two isolates of P. ultimum var. ultimum or P. ultimum var. sporangiferum. Pathogenicity of all the nine isolates, along with a positive control (P. irregulare), was tested in greenhouse conditions on soybean variety Pioneer 22T61R. Soybean seeds were planted in potting mixture inoculated with Pythium inoculum fermented on sterilized proso millet grains. The Pythium spp. were subsequently re-isolated from symptomatic plants. Average incidence of Pythium damping-off across isolates was 27.4% but varied among isolates, ranging from 1.2 to 79.8%. Among the Pythium spp. collected in this single location, the most aggressive isolate was selected to test the efficacy of seed treatments using foliar fungicides in artificially-inoculated field conditions. Out of the eight tested foliar fungicides, six of them significantly suppressed damping-off compared with the untreated control. The average yield advantage of foliar fungicides as seed treatments was 0.23 mt (metric ton)/ha (ranged from 0.15 to 0.31 mt/ha
Soybean Sclerotinia stem rot or white mold caused by Sclerotinia sclerotiorum is a major disease in commercial production, particularly in intensive production regions of the United States, Argentina, and northern China. The occurrence of white mold (Figure 1) depends on various seasonal factors such as cool temperatures <85°F, good soil moisture during flowering or early pod formation stage, canopy closure, narrow row spacing, and germination of sclerotia producing apothecia during flowering. White mold management measures typically are a preventative program, which includes application of fungicides and also herbicides like Cobra. It is known that Cobra can cause phytotoxic effects on the canopy and yield responses may be variable. Fungicide use in soybean has gone up from <1 percent of the soybean planted acreage in 20 program states
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