Use of subepidermal discoloration as a disease index for red crown rot of soybean

A new method for field evaluation of soybean red crown rot was developed based on the percentage of reddish to black discolored subepidermal area on a root peel (ca 6 cm long, 0.58–0.90 mm thick) from two sites on the primary root (near the soil surface) per plant that had turned. The size of the discolored area on the peel from 61 plants in 2022 and 74 plants in 2023 was positively correlated with the amount of DNA of the pathogen (Calonectria ilicicola) in the peel; thus, the proportion of discolored area was an indicator of the degree of colonization. More than 50% of the peel was discolored on severely colonized plants. We propose a new severity index: No discoloration means no disease, up to 50% discolored means mild disease, and more than 50% means severe disease.