A fresh weight-based method for evaluating soybean resistance to red crown rot

Soybean red crown rot (RCR) caused by Calonectria ilicicola is a serious soil-borne disease affecting soybean production and quality. The current visual necrosis-based method for the measurement of RCR severity is prone to subjectivity as well as time consuming and laborious as it requires digging out and washing the roots to remove adhering soil prior to the visual scoring. Using cultivar Enrei, we show that, upon C. ilicicola infection, relative fresh weights (RFW; fresh weights relative to non-inoculated control plants) showed a significant negative correlation with visual RCR severity in apical shoot (trifoliate and above, R2 = 0.96), shoot (unifoliate and above, R2 = 0.82) and roots (R2 = 0.89). Furthermore, apical shoot RFW efficiently correlated with varying levels of C. ilicicola resistance in two test sets containing 37 soybean cultivars and three wild soybean accessions, exhibiting a significant correlation with visual severity (R2 = 0.72 and 0.79, p < 0.01). Taken together, our results suggest that RFW can serve as an index of soybean RCR severity, providing a simple, rapid, consistent, and cost-effective method for evaluating C. ilicicola resistance in soybeans.