Scouting soybean fields in late July and into August is important to help identify potential disease issues. Analyzing field health images in the Climate FieldView™ platform is a great way to observe variation in field conditions (Figure 1). It is very possible that finding variation on a field health map may be normal for that field and not be indicative of any issues. Or, it may be caused by variation in any number of things like elevation, soil type, fungicide application, nutrients, insect damage, excess or lack of moisture, herbicide damage, or diseases. Either way, field health images can provide insight into where to begin scouting in the field.
Regarding diseases, there are several to scout for midseason to late season in Midwestern soybean fields:
Sudden Death Syndrome (SDS) tends to appear first in compacted areas. Infection occurs early in the growing season and is greatest during wet and cool conditions. Foliar symptoms appear as yellow to brown areas between healthy leaf veins (Figure 2). As with most diseases, potential yield loss is greatest when foliar symptoms appear earlier in the life cycle. While foliar SDS symptoms can be confused with brown stem rot, SDS does not create discoloration in the plant stem, so splitting the stem is the best way to help determine which disease is present. The use of resistant seed products is one source of protection from SDS. ILeVO® soybean fungicide seed treatment also provides SDS protection. Channel® products, with a rating of 3 or better for SDS tolerance, are the best defense against fields where SDS is prevalent.
White Mold favors wet, humid and cool conditions. The disease can be a serious problem in some soybean fields. White mold is often referred to as a disease of “high yield potential.” Good fertility, quick canopy closure, high seeding rates, narrow rows, etc. all favor white mold development.
White, fluffy, cotton-like growths on soybean stems are the easiest way to identify this disease (Figure 3). This disease can lead to complete leaf death and production of black sclerotia on both the inside and outside of the plant stem. These sclerotia can remain viable in the soil for up to 10-15 years and can lead to the infection of future crops. White mold can have a major impact on yield potential when severe. While some fungicides are labeled for white mold, timing and coverage are critical to their effectiveness; therefore, the best defense against white mold remains crop rotation and planting resistant products.
Charcoal Rot is generally seen in soybean fields that are under drought and heat stress; however, any stress can favor charcoal rot development including root rot from too much water, insect feeding, other diseases, etc. Charcoal rot generally appears visually as drought stress symptoms, such as wilted leaves. In severe cases, whole plants may die before dropping leaves. Charcoal rot is best identified by the appearance of tiny, black specks or grains resembling pepper embedded in the lower stem tissues. Outer stem tissues shred easily and internal tissues may have areas of graying resembling pencil shading. The best way to identify charcoal rot is to split the stem and look for the black specks on the inside (Figure 4). Like many other soybean diseases, resistant products are the best way to fight known areas of infection.
Iron Deficiency Chlorosis (IDC) is more of an early season (nondisease) issue that is most prominent in the vegetative growth stages, especially in fields with a pH of 7.5 or higher. It is first seen in low, wet areas of a field. Younger, upper leaves are the first to show symptoms of interveinal chlorosis, and under severe conditions these leaves may die (Figure 5). IDC issues may lead to yield loss. The best method of protection from IDC is to plant more tolerant products.
Pod and Stem Blight produces dark fungal bodies on the stem that often appear in rows. This disease is encouraged by wet, warm weather and typically shows up late in the growing season and when the plants reach maturity. Like most diseases, crop rotation and residue management can be management techniques for this disease. Fungicide applications typically don’t impact yield potential in plants affected with pod and stem blight.
Septoria Brown Spot is a common soybean disease that appears almost every year and can occur throughout the growing season. Symptoms are small, brown spots that start on leaves lower in the canopy and work their way up later in the season (Figure 6). These symptoms resemble and may be confused with natural senesce. While this is a very common disease, it often has little to no negative yield impact.
For more help identifying soybean diseases contact your local seedsman, DSM or agronomist.
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