Some soybeans exhibit a symptom referred to as “Physiological Scorch”. When there is extensive chlorosis (yellowing) between the veins of the leaf, or necrosis (dead tissue) between the veins, which may occur on the top of the plant or throughout the plant, we refer to this symptom as physiological scorch. It typically occurs when the roots and vascular system aren’t effectively doing their job, such as when root and or stem pathogens restrict the vascular system when soybean is in the reproductive phase. A number of pathogens can cause this symptom. Most commonly this symptom is associated with “SDS” (sudden death syndrome or) “CBR” (cylindrocladium black root rot) of soybean. Lab and or visual analysis are needed to distinguish between the two diseases. Other diseases that may occasionally cause these symptoms include Dectes stem borer, Phytophthora root and stem rot, stem canker, and charcoal rot. Regardless of which disease is present, fungicides are unlikely to provide a remedy since these are a result of root rots or other vascular disease.
Frogeye Leaf Spot
Frogeye leafspot caused by Cercospora sojina.
Frogeye leaf spot is caused by the fungus Cercospora sojina. Most soybean varieties currently grown are resistant to this disease, and the use of resistant varieties is the preferred method of control. Although frogeye leaf spot is seed borne, it tends to be worse in fields of continuous soybean. Only newly formed leaves are susceptible to this disease, and fully expanded leaves are resistant until they start to senesce. Immature leaves become infected with periods of rain or high humidity, but infection will be limited by dry weather. So, as the soybean plants put on new layers of leaves, frogeye may be present or absent depending on weather conditions during leaf expansion. This can lead to a situation where frogeye is layered in the canopy at different levels. Frogeye has caused yield losses of 30% in some fields, so the general recommendation for susceptible varieties is the application of a strobilurin type fungicide, especially if continued wet and/or humid weather is expected. We do not have a threshold for number of spots or percent leaf area affected to justify fungicide application. If wet and/or humid weather persists as plants start to senesce, older leaves become susceptible again, and the plant may defoliate early. Early defoliation can result in smaller seeds which will translate into yield loss. Also, pod infection can cause a reduction in seed quality or contribute to seed rot.
Frogeye leaf spot resistance to strobilurin fungicides
Resistance of the frogeye leaf spot fungus (Cercospora sojina) to strobilurin fungicides (FRAC code 11; Headline, Quadris, Evito) has been reported from the Mississippi Delta and other areas, especially in the Mississippi river valley as far north as Illinois. Some growers are reporting that management of frogeye with fungicides has been poor this year. This may be a result of: 1) applications made in an untimely manner, 2) applications of a less than labeled rate, and (or) resistance of the fungus to the fungicides used. If an application of a strobilurin type fungicide has been made, then a traizole fungicide (FRAC code 3) should be used if a second application is necessary.
Article by Steve Koenning, NCSU Extension Plant Pathologist and Jim Dunphy, NCSU Extension Crop Scientist