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Root Colonization of Soybean Cultivars in the Field by Fusarium solani f. sp. glycines

Soybean sudden death syndrome (SDS), caused by Fusarium solani f. sp. glycines, is a problem in some soybean-growing areas in the United States. Resistance is an important control strategy. In this study, root colonization of six soybean cultivars by F. solani f. sp. glycines was determined. Cultivars included susceptible P3981, CM497, and Spencer and field resistant LS90-1920, Pharaoh, and Ripley. All cultivars were tested in field experiments at different locations in southern Illinois in 1997 and 1998. Roots were collected at six sampling times and were dried and ground to isolate and enumerate the pathogen on a selective medium. SDS foliar disease index (FDX), the area under the F. solani f. sp. glycines population curve (AUPC), the incidence of colonized roots at 45 days after planting (RCI₄₅), and the root colonization rate (RCR) were used to compare cultivars. FDX on the three resistant cultivars was significantly lower than on the three susceptible cultivars. Means of AUPC on the three resistant cultivars were significantly lower than those on the susceptible CM497 and P3981. RCI₄₅ of Pharaoh was significantly lower than those of P3981 and Spencer. RCRs of all three resistant cultivars were significantly lower than that of P3981, and RCR of Ripley was also significantly lower than that of CM497. Based on combination of all cultivars, AUPC was significantly correlated with RCI₄₅ and RCR.