Primary cause of cotyledonous bacterial blight on soybean

Abstract

One of the main causes of the worldwide decrease of sowing quality of soybean seeds is a cotyledonous bacterial blight in the form of necrotic decaying spots on the outer or inner side of cotyledons. We hypothesize that there is a primary non-bacterial cause for the development of necrotic spots on cotyledons of soybean seeds. We carried out the research in 2016-2020 at V.S. Pustovoit All-Russian Research Institute of Oil Crops on soybean seeds. Cotyledonous bacterial blight is never recorded on immature seeds of healthy soybean plants at the stages of a complete filling and the beginning of physiological maturation. But the secondary (rain) moistening of mature seeds on unharvested plants leads to the development of cotyledonous necroses and a decrease of soybean seed germination due to the subsequent necrotization of over-moistened and repeatedly dried tissues of cotyledons. The physiological mechanism of cotyledon necrosis formation is based on the secondary moistening of individual areas or whole mature seeds to a critical moisture level that initiates nucleic acid synthesis. This irreversibly activates the cell division processes of the over-moistened areas of cotyledons. Subsequent drying of seeds with over-moistened areas of cotyledons leads to their necrosis. The isolation of bacterial pathogens of various species and families in the necrotic areas of cotyledons can be explained by the secondary saprophytic infection of the over-moistened tissues already dead after secondary drying.