Black Root Rot of Soybean and Fungus Gnats

We recently received a report of fields with areas of soybean that are dying from what appears to be insect injury. Further examination revealed that the stems of the unhealthy soybean plants were inhabited by fungus gnats. However, fungus gnats are more readily associated with being pests of greenhouses, nurseries, or indoor plants and are not usually associated with being pests of crops. After further examination the SDSU Plant Diagnostic Clinic determined that the soybean were also suffering from Black root rot, which is caused by Thielaviopsis basicola – a fungus attacking the roots of the soybean plant. Although this disease is rarely seen in South Dakota soybean, its presence is likely related to the cool, wet planting conditions experienced this year. These conditions are ideal for black root rot development. Infections of black root rot are generally observed during the first month after planting when soybean are in their early developmental stages. Fungus gnat larvae feed primarily on fungi, organic matter, or plant roots. In this case, the fungus gnat larvae were secondary invaders of the soybean, and were feeding on the existing black root rot infection.

Black Root Rot Symptoms
Black root rot typically causes infected plants to appear severely stunted. Infected plants may also have chlorotic spots that form along the veins of the leaves, and nodes that are stacked. The primary diagnostic characteristic of black root rot is blackened, deformed roots. The blackened roots are the result of fungal infection and colonization of cortical tissue which eventually leads to root necrosis. Soybean plants often snap off at the soil line and roots are black in color.

Black Root Rot Management
There is currently speculation that crop rotation to corn will lessen the disease incidence. It is unknown how effective fungicide seed treatments and in-furrow sprays are at preventing/managing black root rot in soybean.

Fungus Gnat Identification
Although fungus gnat larvae may be found in the soybean plants that are infected with black root rot, they are not the source of plant injury. Instead the larvae are feeding on the existing fungus, as was observed with the submitted soybean sample. Fungus gnat larvae are legless, small (less than ¼ of an inch) and have a shiny black head capsule. Their bodies are white or clear, and it is often possible to observe their internal organs. They can be distinguished from other fly pests of early stage crops based on the presence of the head capsule. For example, seedcorn maggot larvae lack a head capsule but have two small hooks that are used for feeding.

Source: Adam J. Varenhorst, South Dakota State University 

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