Abstract
Soybean (Glycine max L. Merr.) white mold (SWM), caused by Sclerotinia sclerotiorum (Lib) de Barry), is a devastating fungal disease in the Upper Midwest of the United States and southern Canada. Various methods exist to evaluate for SWM resistance and many quantitative trait loci (QTL) with minor effect governing SWM resistance have been identified in prior studies. This study aimed to predict field resistance to SWM using low-cost and efficient greenhouse inoculation methods and to confirm the QTL reported in previous studies. Three related but independent studies were conducted in the field, greenhouse, and laboratory to evaluate for SWM resistance. The first study evaluated 66 soybean plant introductions (PIs) with known field resistance to SWM using the greenhouse drop-mycelium inoculation method. These 66 PIs were significantly (P < 0.043) different for resistance to SWM. However, year was highly significant (P < 0.00001), while PI x year interaction was not significant (P < 0.623). The second study compared plant mortality (PM) of 35 soybean breeding lines or varieties in greenhouse inoculation methods with disease severity index (DSI) in field evaluations. Moderate correlation (r) between PM under drop-mycelium method and DSI in field trials (r = 0.65, p < 0.0001) was obtained. The PM under spray-mycelium was also correlated significantly with DSI from field trials (r = 0.51, p < 0.0018). Likewise, significant correlation (r = 0.62, p < 0.0001) was obtained between PM across greenhouse inoculation methods and DSI across field trials. These findings suggest that greenhouse inoculation methods could predict the field resistance to SWM. The third study attempted to validate 33 QTL reported in prior studies using seven populations that comprised a total of 392 F4 : 6 lines derived from crosses involving a partially resistant cultivar “Skylla,” five partially resistant PIs, and a known susceptible cultivar “E00290.” The estimates of broad-sense heritability (h2) ranged from 0.39 to 0.66 in the populations. Of the seven populations, four had h2 estimates that were significantly different from zero (p < 0.05). Single marker analysis across populations and inoculation methods identified 11 significant SSRs (p < 0.05) corresponding to 10 QTL identified by prior studies. Thus, these five new PIs could be used as new sources of resistant alleles to develop SWM resistant commercial cultivars.
Highlights
Soybean white mold (SWM), caused by Sclerotinia sclerotiorum (Lib) de Bary, is a major soybean
The objectives of this study were to: (a) evaluate the reactions of 66 partially resistant PIs from Hoffman et al (2002) using drop-mycelium inoculation method, (b) predict field resistance to soybean white mold (SWM) using greenhouse inoculation methods, and (c) validate the 33 QTL reported in three prior studies using seven populations comprising 392 F4 : 6 lines derived from crosses involving a well-known resistant cultivar, a susceptible cultivar, and five partially resistant PIs
Due to the random nature of spraying in spray-mycelium inoculation, secondary infections at multiple points were distinctly visible on the whole plants, whereas disease progressed downward from the apical meristem in drop-mycelium evaluation
Summary
Soybean white mold (SWM), caused by Sclerotinia sclerotiorum (Lib) de Bary, is a major soybean The progress in the development of resistant cultivars is very slow due to the quantitative nature of the disease resistance (Kim and Diers, 2000; Arahana et al, 2001; Peltier et al, 2012) and lack of certainty to achieve desired SWM pressure during field evaluations of the breeding materials. Infections of soybean in the field environments are caused by ascospores that first land on the delicate plant parts such as flower petal. Irrigation, narrow plant spacing, early flowering, and thick vegetation contribute to the development and spread of SWM (Boland and Hall, 1987; Kim et al, 2000). Control through chemical approaches has the added risk of increased production costs
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