Sudden death syndrome or brown stem rot
A large percentage of the local soybean fields are showing classic symptoms of sudden death syndrome (SDS). Early symptoms are mottling and mosaic of the leaves. Later, leaf tissue between the major veins turns yellow, then dies and turns brown. Soon after the leaflets die and shrivel. In severe cases, the leaflets will drop off, leaving the petioles attached. At present there are fields exhibiting all stages of this disease and in some fields the soybeans have died. In examining fields throughout northern Missouri, these symptoms are wide spread. Opinions from various producers and experts is that this is the worse case of SDS that they have ever seen.
There is also evidence that SDS may not be the total problem. Several samples of plants exhibiting symptoms of SDS were actually brown stem rot (BSR). The leaf symptoms produced by SDS are also the same leaf symptoms produced by BSR and stem canker. In order to definitely determine which disease is in your field you need to pull plants, split the lower part of the stem and examine the inside of the stem. This is the only way to differentiate which disease is present. The key traits to differentiate each disease are:
SDS -- When split, the lower stem and taproot of a plant infected with SDS will exhibit a slightly tan to light-brown discoloration compared to a healthy plant. The pith will remain white or slightly cream-colored.
BSR -- Brown stem rot darkens the pith, but there is little discoloration of the cortex.
Stem Canker -- Stem canker will produce lesions at the nodes on the lower portion of the plant.
The critical thing to do now is to determine for sure which disease is causing the problem in your fields. By verifying the cause of the problem, you will be able to make better variety choices for next year. There are varieties that are less sensitive to SDS. There are varieties that are claimed to be resistant to BSR. I have not seen enough stem canker in this area for it to appear to be a concern. There are no soybean varieties that are highly resistant to SDS. Disease resistance is very difficult to screen for and measure. There was one producer that indicated that the rating for his soybean variety was 8.8, but his field was 80 percent covered with SDS. Planting date, soil temperature and moisture have a large influence on the infection and development of SDS. Look at your neighbor's fields and if they are clean, try to determine what factors may have prevented the disease.
The bad news is that there is nothing you can do to control the disease now. It is also likely that the scope of the disease will increase for the rest of the season. The diseased and dying plants will also be more susceptible to pod and stem blight, especially if we see regular moisture. The damaged plants will have smaller seed and be more prone to shattering loss. If your fields have significant disease damaged plants you may want to harvest earlier to prevent seed loss and maintain seed quality.
If you want any help in identifying the disease or have questions, contact your local extension office.