5 Scouting for Diseases

Every time a field is scouted for weeds or insects, the scout should also watch for diseases caused by fungi and viruses. (Bacteria are not of economic importance in North Carolina.) Some diseases have characteristic symptoms that make them easy to identify. Some symptoms are not specific and may be mistaken for weather or chemical damage. Plant samples (including roots and soil) should be taken to the local county Extension office if a disease is suspected.

Several diseases can cause significant yield loss to soybeans. Look for plants that are stunted or discolored. Diseases caused by viruses and fungi can be detected most readily. It is critical to note these types of diseases during the present growing season because the diagnosis is important in developing controls for the next crop, particularly if the following crop is soybeans.

Environmental conditions and crop susceptibility will determine which diseases are important. Those that are commonly of concern are damping-off, phytophthora root and stem rot, southern stem blight, charcoal rot, brown spot, pod and stem blight, bean pod mottle, tobacco ringspot, and soybean mosaic. Identifying diseases in the current crop is the best way to plan a disease control program for following years.

Damping-Off

Death of young seedlings and poor stands are characteristic of seedling diseases. Symptoms vary from a reddish brown decay of the outer cortical layer to a water-soaked appearance on the stems. Roots are generally discolored. Several species of fungi can cause damping-off. Poor quality seed is a major factor contribution to seedling diseases. Pesticides and fertilizers that have been applied improperly can cause similar damage.

Phytophthora Root and Stem Rot

This disease can occur on any soil type but is more likely to occur in poorly drained soils. Plants of any age can be affected. Seedlings turn brown, die, and remain erect. On older plants the tissue between the veins becomes chlorotic and then dies. The lower part of the stem may be discolored and usually has dark brown lesions. The roots are normally rotted, especially in severe cases.

Southern Blight

The stems are girdled near the soil line. A dense, white, cottony growth and tiny seed-like structures on the stem are the distinguishing features of this disease.
 

Charcoal Rot

Charcoal rot is primarily a disease of the roots and basal stem caused by a fungus. Tiny black specks that look like pulverized charcoal beneath the epidermis on the roots and lower stem are the signs of this disease.
 

Brown Spot

Reddish brown, angular spots from pinpoint size to 0.2 inch wide occur first on the primary leaves, then may appear progressively higher on the plant.
 

Pod and Stem Blight

Symptoms are most evident late in the growing season when plants are senescing or mature. Poor seed quality is often the result of this disease. Under severe conditions the seeds may shrivel or become moldy. The most characteristic symptom is the linear rows of the fungus's tiny black fruiting structures on the stems.
 

Bean Pod Mottle

Expanding or newly expanded leaves are mottled. The mottling disappears on mature leaves. Plants infected with soybean mosaic virus and bean pod mottle virus are stunted.
 

Soybean Mosaic Virus

Leaves are distorted and narrower than normal. The margins tend to turn downward. Leaves of severely infected plants are puckered along the veins, similar to the effects of 2,4-D injury. Seeds may be mottled.
 

Tobacco Ringspot Virus

Symptoms of tobacco ringspot virus are not generally noticed until the plant matures. Infected plants remain green because few seeds develop. Large clusters of tiny pods frequently occur instead of normal pods. The terminal buds may become brown, curved downward, and dry.