Characterization of Sclerotinia sclerotiorum isolates from USA and South America soybean fields for aggressiveness and mycelial compatibility grouping
J. J. Steadman (1), T. J. MIORINI (2), R. Jhala (1), R. Higgins (1). (1) University of Nebraska-Lincoln, Lincoln, NE, U.S.A.; (2) Sao Paulo State University, Sao Paulo, Brazil
Sclerotinia stem rot (SSR) on soybean, caused by <i>Sclerotinia sclerotiorum</i>, is an important yield-reducing disease in the USA and South America. The use of disease resistance is the lowest cost control for growers. To screen for SSR resistance, pathogen isolates need to be determined. However, pathogen variation for soybean isolates is not understood. Thus, variability of 55 <i>S. sclerotiorum</i> isolates from USA/Nebraska, Argentina and Brazil soybean fields was analyzed using clonality (mycelial compatibility groups or MCGs) and a detached leaf test. All isolates were paired in all possible combinations and formation of a barrage line indicated incompatibility. From 55 isolates, 23 MCGs were identified with 9 MCGs composed of a single unique isolate. Of the 14 MCGs with more than one isolate, 7 MCGs were composed of isolates within a single field site, while 7 MCGs had isolates collected from multiple field locations. For aggressiveness, leaves of a resistant soybean line, NKS 19-90, were inoculated with mycelial plugs and SSR diseased leaf area was measured. In the aggressiveness test with Brazilian isolates diseased leaf area at 48 hours was correlated with Area Under Disease Progress Curve (AUDPC). Most isolates were similar in aggressiveness, but one had a significantly lower disease area on leaves. The information on isolate relatedness and aggressiveness will be used to select relevant isolates to screen for broad resistance to Sclerotinia stem rot.
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