Over the past three weeks there have been increasing reports of soybean sudden death syndrome (SDS) in full season soybeans. SDS is one of the top ten most important diseases affecting soybeans, but is still a minor player in the mid-Atlantic. Yield loss can occur when foliar symptoms show up before R5. In Delaware and Maryland, we see SDS in small patches, often 15 yards across or less. Many times affected areas are found in low lying areas that hold more water.
Infection of soybeans typically occur early in the growing season, when conditions are cool and wet and plants are in the vegetative stage of growth. The fungus responsible for SDS first infects the roots, and gradually infects the lower portion of the stem. When plants reach the later reproductive stages, the fungus can produce a toxin that moves with the water conducting tissues into the foliage, where it accumulates. Accumulation of the toxin results in the development of foliar symptoms. Leaves of infected plants develop yellow spots, which expand, while the leaf veins remain green. Roots will often be rotted, and the lower stem, when split, will be brown on the outside while the inner tissue will remain unaffected. This stem symptom can help differentiate between SDS and brown stem rot (BSR). BSR will rot the entire central stem column, resulting in a brown, dry-rotted appearance. Foliar symptoms alone are not diagnostic of SDS. Occasionally blue fungal growth can be observed on the root or soil surface, especially after heavy rains.
Management of SDS starts with crop rotation to a poor host such as corn for at least two seasons. Unfortunately the pathogen can still overwinter and survive on corn and other residues for quite some time, so in severe cases, you will need at least three years out of soybeans. Thusfar, I have yet to see a field in DE or MD where infection was severe enough to warrant this approach. When rotating, even short term, rotations with snap or lima beans should be avoided.
If you have soybean fields with known SDS issues, select a variety with tolerance to the disease to minimize potential yield losses. Tolerant varieties are much less likely to suffer severe yield losses if the disease does occur.
Although the seed treatment iLeVO has been shown to help reduce SDS, it may not be economically sound to plant entire fields with this seed treatment if only sections or small portions of the field are affected. If considering iLeVO for SDS management, one potential route would be to scout and map out affected areas and plant treated seed in the field sections or areas where SDS is problematic. Then, plant your untreated or regularly treated seed in unaffected areas of the field.
The pathogen infects when it is cool and wet. Therefore, double cropped soybeans are less likely be infected by the SDS pathogen. They can still develop the disease, especially if persistent cool wet weather occurs during the vegetative stages, but the likelihood for this is far less than in full season beans.
Finally, if you have a field with SDS and are working the ground, ensure that you clean the equipment before utilizing it in another field or till / harvest SDS-affected fields last. This will minimize potential movement of the pathogen to additional fields.