Factors affecting red crown rot caused by Calonectria ilicicola in soybean cultivation

Red crown rot caused by Calonectria ilicicola, a soil-borne fungus, is one of the main soybean diseases in Japan. In field surveys to understand the present status of the disease in Akita, Fukushima, Niigata, Toyama, and Nagano prefectures, survey data included the cultivation history and conditions as well as the disease ratings for each field. The disease was confirmed to occur in all five prefectures, and the percentage of diseased fields ranged from 38.9 to 100%. Generalized linear mixed model analyses using the data from Akita, Fukushima, and Niigata prefectures revealed the explanatory variables conducive to the disease from the cultivation history, cropping system, and field conditions. Four factors significantly affected disease incidence and severity: past disease incidence in field, number of years with continuous soybean cultivation, sowing date, and number of intertillage ridging applications. Analysis of Niigata Prefecture data revealed additional factors affecting the disease: the use of agrochemical-treated seeds and field drainage-related variables. Early implementation of intertillage ridging was associated with greater disease severity, indicating that the disease was affected not only by the number but also by the timing of intertillage ridging application. Careful planning of cultivation and management methods to avoid these risk factors may reduce damage caused by the disease. To our knowledge, this is the first comparative analysis of the incidence and severity of red crown rot and the factors influencing the disease using a comprehensive data set from five Japanese prefectures.