Complex Interactions Shape Red Crown Rot of Soybean (caused by Calonectria ilicicola) Development Under Controlled Conditions.

Red crown rot (RCR) of soybean (Glycine max), caused by Calonectria ilicicola, has been increasingly reported across Illinois in recent years, yet little is known about factors shaping disease expression under controlled conditions. This study compared two representative Illinois isolates (CC1 and MAD) using in vitro, growth chamber, and greenhouse assays to quantify aggressiveness in response to inoculum density and soil substrate, and evaluated the performance of traditional and image-based measurements for assessing disease severity. In vitro CC1 exhibited greater mycelial growth. Under growth chamber conditions, aggressiveness differed between isolates and increased with inoculum density, but the magnitude of these effects varied by parameters, including root length, shoot length, root area, and root necrosis. Image-derived root area measurements highlighted treatment differences that were not captured by biomass or length measurements, including isolate separation at intermediate inoculum densities. Necrosis percentage and disease rate were also consistently responsive indicators. Greenhouse experiments demonstrated strong substrate effects: torpedo sand mix enhanced foliar symptom expression but reduced seed weight and pod number, whereas general-purpose mix supported higher grain yield but displayed variable foliar symptoms. Across both substrates, inoculation reduced seed weight in the greenhouse by up to 48%. Notably, substantial yield losses occurred even in plants exhibiting little or no visible foliar symptoms, suggesting that early root colonization and belowground damage can impair plant performance without clear aboveground symptoms. Together, these results highlight the complex interactions among isolate, inoculum density, and substrate, emphasize the value of image-derived metrics, and underscore the potential need to account for hidden yield penalties when evaluating management strategies for RCR.