As the soybean crop continues to grow and get closer to harvest there are a few crop diseases to keep an eye out for.
Stem Canker: Stem canker is a disease in soybeans that can commonly be confused with Phytophthora if not properly identified. Symptoms are typically present during early reproductive stages with small red-brown lesions forming at the base of branches or leaf petioles. Lesions may elongate, become sunken, and girdle the stem. A diagnostic symptom of this disease is that green stem tissue will typically be present both above and below the canker on the stem, and these symptoms will develop higher on the stem compared to Phytophthora lesions. Leaves may become yellow and brown in between the leaf veins, termed interveinal chlorosis and necrosis. Leaves wilt but will remain attached to the leaf petioles and stem. Top dieback may also occur where the upper leaves curl into a “shepherd’s crook.” Plant death is usually imminent once symptoms appear as water and nutrients are cut off from stem canker. This disease can overwinter in infected stems or in infected seed. At this time of year, options are quite limited. After harvest, tillage may help in managing infected residue. Next year if you plan to plant soybeans, talk with your seed dealer to determine which soybean cultivars carry resistance to stem canker. Seed treatments may help in reducing infection, but will not eliminate it in the field. Crop rotation may help in rotating to a cereal crop, such as corn or small grains, but this also will not eliminate the disease. If you decide to implement crop rotation, do not rotate to alfalfa next year, as alfalfa is a host for the stem canker pathogen and could increase inoculum levels in the field.
Soybean Stem Borer Larvae: Several plants coming into the clinic late this summer have been shown to have soybean stem borer damage. The adults are typically active in June, but they can continue to lay eggs well into August. Once their eggs are laid, larvae hatch and start tunneling down through the stem towards the base of the plant, near the soil line. Infested plants are detected by wilting or dying leaf tissue above this point. By splitting the stem open, you may be able to find the larvae tunneling its way down the plant. Larvae are ½–5/8 inch long, cream colored, without legs, have a very wide head, segmented bodies, and a narrow tail end. The larvae cause the inside of the stem to girdle, making a protective cell for them to overwinter in the field. Tunneling and girdling from larvae feeding cause stems to weaken, break, or lodge as plants get closer to harvest. Research has shown that girdling is more of an issue in earlier maturing varieties whereas lodging is most severe in soybeans that were planted earlier in the season. If you see plants wilting in the field and their symptoms don’t align with Phytophthora or stem canker, you may want to split the stems and look for tunneling. Fields with high pest pressure and stem tunneling are at higher risk of lodging and yield loss at harvest. Try to harvest those fields first, if possible, to minimize harvest loss.