Two convenient methods to evaluate soybean for resistance to Sclerotinia sclerotiorum

Several greenhouse inoculation methods are available to evaluate soybean (Glycine max (L.) Merr.) for resistance to Sclerotinia sclerotiorum (Lib.) de Bary. Most of these methods are labor intensive and often produce inconsistent results among the tests. The objective of this research was to develop a low-cost and high-efficiency greenhouse inoculation method that can generate a consistent result. We developed a spray-mycelium method in which mycelia were cultured in liquid potato dextrose broth and homogenized before spraying on the soybean leaves. We also developed an inoculation method (the "drop-mycelium" method) in which a drop of homogenized mycelium suspension was dropped on the tips of main stems. Inoculated plants were incubated in a greenhouse chamber with 60 to 80% relative humidity. Plant mortality and area under the wilt progress curve (AUWPC) were used to measure disease severity daily from 3 to 14 days after inoculation (DAI). Eighteen soybean genotypes, including partially resistant line NKS19-90 and susceptible line Resnik, were employed in this study. The spray-mycelium method and the drop-mycelium method were compared with the cut-petiole method in the greenhouse. The three experiments were a randomized complete block design. Twenty-four plants per genotype in each experiment were inoculated at V3 growth stage in the greenhouse. Significant differences (P < 0.05) in disease ratings of plant mortality and AUWPC to Sclerotinia stem rot were found among 18 tested genotypes. The results obtained with the spray-mycelium and drop-mycelium inoculation methods were significantly (R > 0.73, P < 0.01) correlated with the results obtained with the cut-petiole inoculation method for both of the plant mortality and AUWPC. Compared with the cut-petiole method, the spray-mycelium and the drop-mycelium methods used less inoculation time and are less expensive in terms of materials. Both of these new methods are low cost, efficient, and reliable and they can be valuable for large-scale evaluation of germ plasm and breeding lines for resistance to Sclerotinia stem rot in a greenhouse or other similar facilities.