Evaluation of Soybean Resistance to Sclerotinia Stem Rot Using Reciprocal Grafting

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Abstract
Sclerotinia stem rot of soybean is one of the major soybean diseases in the north central region of the United States. One disease management option is to plant cultivars that have resistance. Some sources of partial resistance have been identified, but information pertaining to the nature of resistance is limited. The objective of this study was to determine if the expression of resistance is dictated by shoots of resistant plants and if this can be altered by using resistant and susceptible soybean genotypes grafted in different shoot and rootstock combinations of self-, single-, or double-shoot grafts. After successful grafts were made, several experiments were conducted using different inoculation techniques and soybean genotypes. In one experiment, cotyledons were inoculated with a plug of fungal mycelium, plants were incubated in a mist chamber for 23 h, and plant survival was recorded over time. Based on seven grafting combinations of cross- and self-grafted plants using two soybean cultivars, grafts with NKS19-90 (partially resistant) as shoots had greater (P ≤ 0.05) plant survival at 3, 4, and 5 days after inoculation than the other graft combinations. In another experiment, a total of 17 graft combinations were generated using resistant plant introductions and two susceptible cultivars. Resistant self-grafts of the plant introductions had greater (P ≤ 0.05) plant survival (mean = 75%) than self-grafts of the susceptible cultivars (mean = 15%) at 5 days after inoculation. Inter-genotypic grafts with resistant shoots had greater (P ≤ 0.05) plant survival (mean = 65%) than those in reciprocal combinations (mean = 8%) 5 days after inoculation. A cut stem inoculation method was used to test graft combinations of one resistant and two susceptible cultivars. Grafts with susceptible shoots of cvs. Williams 82 and Asgrow 2242 had greater (P < 0.05) lesion lengths (mean = 13.2 cm) than shoots of NKS19-90 (mean = 9.2 cm) regardless of the rootstock 15 days after inoculation. In a double-graft experiment, shoots of both NKS19-90 and Williams 82 were grafted to either NKS19-90 or Williams 82 rootstocks. Regardless of the rootstock, the shoots of Williams 82 died while shoots of NKS19-90 survived. For all the experiments, resistance was greater when the grafted shoot came from a resistant source on a susceptible rootstock compared with the reciprocal combination regardless of the type of grafting technique or inoculation method.