When scouting fields in July, look for these symptoms. They may be big problems or really no problems at all.
As we move further in the growing season and start scouting soybean fields we may see symptoms of disease, insect infestation or environmental conditions. Some are easily recognized while others can be confused with something else. Some symptoms are more uncommon and not seen every year.
Below is a guide of soybean symptoms that are starting to become common in this year of uncommon growing conditions.
Sunscald is natural phenomena often associated with a swing of approximately 30° F between day and night temperature, along with heavy dew and bright sunny days. Regardless of the daily high temperature, injury symptoms occur on the soybean leaves that were saturated with heavy dew. Symptoms usually appear on the underside of the soybean leaves. Neighboring leaves can guard other leaves from the ultraviolet light by shading the leaf. Sunscald can easily be confused with Cercospora. Expect no yield loss with sunscald.
Cercospora leaf blight is one of the yield-limiting foliar diseases in Illinois. It easily can be confused with sunscald. With cercospora, light purple spots appear on the top surface of leaves exposed to light. The discolored areas expand into a deepened color of reddish-purple or even bronze. The infected leaves appear leathery and sunburned. Foliar fungicide treatments are useful for managing this disease.
Septoria brown spot (brown spot) is a common leaf disease of soybeans in Illinois. Yield losses of 5 to 8 percent may occur under severe conditions when defoliation occurs. The first symptoms usually appear in the lower canopy (unlike bacterial blight which begins in the upper canopy) and then progress to the middle and upper canopy throughout the summer.
Symptoms are small dark brown spots. The brown spots often expand and grow together into irregular brown areas. In severely diseased areas leaves may fall off early which can cause yield loss. Internal leaf structure does not fall off and does not appear to be torn. Foliar fungicides can provide some control under conditions when an application may be warranted.
Bacterial blight has lesions that are typically found on the younger leaves of the plant, in the upper canopy. Lesions of brown spot will also have no yellow halo that are present in young lesions of bacterial blight. These lesions can expand and form areas of necrotic or dead leaf tissue that may crumble causing the leaf to have a tattered appearance. Applications of fungicides are not recommended for management of bacterial blight since it is a bacterial infection.
Spider mites are uncommon pests, not really seen since the drought of 2012. Below is a quick reference on the symptoms and treatment methods for spider mites, which could make an appearance in some areas that are beginning to experience drought conditions.
Infestations typically are first witnessed near field edges or anywhere soybeans are stressed. Examine plants at the field edge first, next to roadsides, drainage ditches or alfalfa. Note webbing on the image below. Tap infested leaves over a white sheet of paper. If spider mites are present, they will fall on the paper.
Your choices to control spider mites are limited to chlorpyrifos, dimethoate, bifenthrin or mixtures containing these ingredients. Due to removal of beneficial insects after application of a control measure, monitor the field to avoid flaring, the rapid rise of sprayed populations.
Nick Marley is an agronomist and Certified Crop Adviser. He has a B.S. in agronomy and crop science from the University of Illinois and is working on a Master’s in agronomy and crop science at Iowa State University. He is a 2017 Illinois Soybean Association CCA Soy Envoy.