I seem to be getting several calls every day regarding soybean plants dying in spots within the field. Although not always the case, most fields are exhibiting symptoms of interveinal chlorosis and necrosis. In the worst areas, the leaves are either burning up and dying. In some cases, the leaves are falling off of the petiole. The symptoms look similar to, but usually worse than, various nutrient deficiencies. Actually, what is occurring is very similar to a nutrient deficiency in that something is restricting the roots and/or vascular system from moving water and nutrients through the plant.
A common concern is that this is sudden death syndrome (SDS). We tend to hear a lot about this disease in the media, in a seed catalog ratings, and throughout many extension bulletins in the Midwest. And the name makes the disease sound like the entire field is getting ready to die. SDS is neither devastating in most cases nor widespread in Virginia. The disease rarely comes on suddenly but is building up slowly as the season progresses. The infection actually took place much earlier in the year. Furthermore, it’ll rarely result in widespread death of the soybean crop. Instead, you’ll see in in spots and patches in the field. Worth noting however is that SDS is commonly associated with soybean cyst nematode infestations. So, if your field has been diagnosed with SDS, then you may want to sample for nematodes.
In general, we’ve seen very little SDS in Virginia soybean over the years. More likely, the above symptoms are association with brown stem rot (BSR), which has been and continued to be the most common root/stem disease in Virginia. Most of the plant samples that I’ve brought back to our plant pathology lab for Drs. Mehl and/or Phipps to examine have been diagnosed as BSR. Like SDS, it usually doesn’t cause widespread death and it usually limited to small spots or patches in the field. In general, it will not continue to spread over the rest of the field. However, the patches will tend to enlarge over time. Yield loss is usually minimum and restricted to the infected areas.
Another symptom usually evident with BSR is a brown pith (center) of the stem and taproot right at the soil level. If you see the above ground symptoms shown above, start splitting the stems. If you see a white pith, then the disease could be SDS. Another diagnostic tool is to look at the leaves. If the leaflets fall off but leave the petiole attached to the stem, it is likely SDS. if the leaves don’t fall, it’s likely BSR. Finally, you’ll seed more rotted roots with SDS. But, to find the rotted roots, you’ll need to dig them up and wash the soil from the roots. Jerking the plant out of the ground will likely strip off most of the roots.
The last disease that could be a problem and exhibit the same symptoms is red crown rot (in peanut, this is called cylindrocladium black rot or CBR). This disease can however be recognized by red fruiting bodies found at the base of the stem. Red crown rot was more common when Virginia’s peanut acreage was greater and soybean and peanut were being rotated with each other.
Regardless of the disease, what can you do about it? Unfortunately, there is nothing that can be done this year. Foliar fungicides will not control a disease that is inside the stem and roots. But, in the future, rotate out of soybean for one or more years. Also, you may want to select a variety with resistance to that disease when you plant soybean in the field. Finally, as mentioned earlier, take a nematode sample. Just because you’re seeing SDS or BSR in your field this year doesn’t mean that you have nematodes. But anything that is restricting root growth could aggravate SDS or BSR; therefore the symptoms would be more evident in fields infested with nematodes.