Soybean resistance to white mold: evaluation of soybean germplasm under different conditions and validation of QTL

Authors: KANDEL, RAMKRISHNA - University Of Florida; CHEN, CHARLES - Auburn University; GRAU, CRAIG - University Of Wisconsin; DORRANCE, ANN - The Ohio State University; LIU, JEAN - Pioneer Hi-Bred International; WANG, YANG - Jilin Agricultural University; WANG, DECHUN - Michigan State University

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2018
Publication Date: 4/20/2018
Citation: Kandel, R., Chen, C.Y., Grau, C.R., Dorrance, A.E., Liu, J.Q., Wang, Y., Wang, D. 2018. Soybean resistance to white mold: evaluation of soybean germplasm under different conditions and validation of QTL. Frontiers in Plant Science. 9:505. https://doi.org/10.3389/fpls.2018.00505.
DOI: https://doi.org/10.3389/fpls.2018.00505

Interpretive Summary: Soybean white mold, caused by Sclerotinia sclerotiorum, is a devastating fungal disease in the Upper Midwest of the United States and southern Canada that can result in severe soybean yield losses. Plant breeders have worked to identify soybean genes that provide resistance to this disease using studies conducted in field environments. However, these studies can be expensive, time-consuming, and methodologically challenging. In particular, it can be difficult to get adequate disease pressure on all soybean lines across a field plot, which makes resistance testing of diverse soybean lines difficult. Therefore, we studied a greenhouse inoculation method as a potentially efficient, low-cost alternative to field trials. We tested the method using several soybean lines that had previously been determined to be resistant or susceptible to white mold disease. We showed that our techniques could reliably replicate the more time-consuming and space-demanding field studies. The greenhouse inoculation method can now be used to test more soybean lines and to speed up breeding efforts directed towards gene discovery and the development of improved soybean varieties that show resistance to white mold.

Technical 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 from0.39 to 0.66 in the populations. Of the seven populations, four had h2 estimates that were significantly different from zero Kandel et al. Soybean White Mold QTL Validation (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.