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In Ohio, soybean seedling damping-off and seed rot are problems routinely encountered soon after planting. Reduced tillage systems that lead to inoculum build-up combined with saturated soil conditions are ideal environments for seedling diseases, which cause large losses of soybean stand and thus yield. Prior Ohio field surveys identified multiple species of Pythium and Phytophthora that contribute to soybean seedling damping-off. Among the most common and aggressive species are Phytophthora sojae, Pythium irregulare, Pythium ultimum var. ultimum, and Pythium ultimum var. sporangiiferum. Fungicide seed treatment and host resistance are two management strategies that are used to minimize yield loss caused by these pathogens. Thus, the objectives of these studies were to: i) evaluate new active ingredients for efficacy in the lab and field, and ii) identify and characterize new sources of resistance towards the most common seedling pathogens. These are key strategies for the development of effective strategies for the management of soybean seedling disease. During 2014-2015, at two environments, ethaboxam seed treatments combined with metalaxyl on a susceptible cultivar significantly increased yield compared to other fungicide treatments containing metalaxyl or mefenoxam alone. Soybeans treated with ethaboxam plus metalaxyl had significantly higher plant populations when compared to the nontreated control at all four 2016 field locations, while one environment had significantly higher yield. In laboratory seed plate and greenhouse cup assays, ethaboxam plus metalaxyl in a commercial formulation provided equal or better protection against multiple species of Pythium when compared with other seed treatments that contained metalaxyl or mefenoxam only. These results indicate that ethaboxam with metalaxyl is effective at managing seed and rot root caused by the diverse species of Pythium and Phytophthora and provides another seed treatment fungicide available to producers which can be used in an integrated disease management program. The parents that were used to develop six nested association mapping (NAM) populations were previously identified as segregating for resistance towards Phytophthora sojae, Pythium irregulare, Pythium ultimum var. ultimum, and Pythium ultimum var. sporangiiferum. Following inoculation in a cup assay, the resistance was quantitatively inherited in each of the NAM populations towards the four seedling pathogens. In total, 33 QDRL from the six populations surpassed the genome-wide logarithm of odds (LOD) threshold and there was a large number of suggestive QDRL that surpassed the chromosomal LOD threshold. Of these 33 significant QDRL, 10 explained more than 15% of the phenotypic variation. Only four QDRL conferred resistance to more than one of the oomycete pathogens; one on chromosome 3, one on chromosome 17, and two located at separate locations on chromosome 13. This indicates that there may be multiple mechanisms for resistance to these root pathogens. Further analyses are needed to precisely map these QDRL so they may be selectively bred into highly resistant germplasm in order to manage seed and seedling damping-off. These NAM populations will serve as a rich resource for breeders to incorporate resistance into adapted soybean cultivars. |
