Comprehensive Sclerotinia Stem Rot Screening of Soybean Germplasm Requires Multiple Isolates of Sclerotinia sclerotiorum

Abstract

Sclerotinia sclerotiorum population variability directly affects Sclerotinia stem rot (SSR) resistance breeding programs. In the north-central United States, however, soybean germplasm selection has often involved only a single isolate. Forty-four S. sclerotiorum isolates from Illinois, Michigan, Minnesota, Nebraska, Wisconsin, Poland, and across 11 different host species were evaluated for variation in isolate in vitro growth, in vitro oxalate production, and in planta aggressiveness on the susceptible soybean ‘Williams 82’. Significant differences (P < 0.0001) were detected in isolate in planta aggressiveness, in vitro growth, and in vitro oxalate production. Furthermore, diverse isolate characteristics were observed within all hosts and locations of collection. Aggressiveness was not correlated to colony growth and was only weakly correlated (r = 0.26, P < 0.0001) to isolate oxalate production. In addition, the host or location of collection did not explain isolate aggressiveness. Isolate oxalic acid production, however, may be partially explained by the host (P < 0.05) and location (P < 0.01) of collection. Using a representative subset of nine S. sclerotiorum isolates and soybean genotypes exhibiting susceptible or resistant responses (determined using a single isolate), a significant interaction (P = 0.04) was detected between isolates and genotypes when SSR severity was evaluated. Our findings suggest that screening of S. sclerotiorum-resistant soybean germplasm should be performed with multiple isolates to account for the overall diversity of S. sclerotiorum isolates found throughout the soybean-growing regions of the United States.

Partial stem resistance in Brassica napus to highly aggressive and genetically diverse Sclerotinia sclerotiorum isolates from Australia

“…Taylor et al (2015) demonstrated that isolates collected from diverse plant species in the UK exhibited differences in aggressiveness when inoculated onto a set of B. napus varieties. Differences in aggressiveness have also been shown on other hosts, including dandelion (Taraxacum officinale L.) (Riddle et al, 1991), sunflower (Davar et al, 2011), peanut (Arachis hypogaea L.) (Lujan et al, 2016), eggplant (Solanum melongena) (Tok et al, 2016) and soybean (Willbur et al, 2017).…”

“…Differences in aggressiveness of S. sclerotiorum isolates have also been demonstrated for soybean (Glycine max (L.) Merr.) (Willbur et al, 2017). Careful considerations should therefore be made when selecting S. sclerotiorum isolates for resistance screening, since weakly aggressive isolates can increase the risk of false positive phenotypes.…”

Development and Evaluation of Glycine max Germplasm Lines with Quantitative Resistance to Sclerotinia sclerotiorum

“…Soybean plants were inoculated using the cut petiole technique ( Peltier and Grau, 2008 ) with aggressive S. sclerotiorum isolate 25 ( Willbur et al, 2017 ). A 1.5-cm-thick agar core was collected from the leading edge of mycelia on each inoculum plate with a 1,000 μl pipet tip (Fisher Scientific).…”

“…Three soybean seeds per pot were planted approximately 4 cm deep in 15.25 cm diameter peat pots of moist potting mix (Premier Pro-Mix HP BioFungicide + Mycorrhizae). Nine S. sclerotiorum isolates, of 44 previously characterized isolates with varying degrees of aggressiveness ( Willbur et al, 2017 ), were used for evaluations. As described by Willbur et al (2017) , lines were inoculated at the V4 growth stage, and lesions were measured with digital calipers (Thermo Fisher Scientific) 5, 12, and 14 DPI.…”

“…Furthermore, isolates of S. sclerotiorum have been found to differ in aggressiveness. Willbur et al (2017) highlighted the importance of using a representative panel of mildly to strongly aggressive isolates for screening soybean lines due to disparate interactions between isolates and lines which may be attributable to varying abilities of isolates to overcome host resistance mechanisms on certain genotypes. Additional efforts are needed to evaluate physiological resistance to ascertain related QTL and to breed for resistance to a wide range of isolates and environments.…”