Figure 1. Sudden Dealth Syndrome of Soybean
In last week’s Field Crop News, we discussed how environmental conditions were favorable for white mold in parts of Pennsylvania. Another soybean disease that is dependent on favorable environmental conditions like cool and damp conditions at planting that is starting “show up” in soybean fields is Sudden death syndrome or SDS (Figure 1). The reason to write “show up” is the fact that what we are now seeing is the expression of foliar symptoms representing an infection that most likely occurred shortly after planting, typically in areas of high compaction and poor drainage. It is important to note though that infection by the pathogen that causes SDS can occur at other times during the growing season.
Sudden death syndrome is a fungal disease caused by Fusarium virguliforme, and can easily be mistaken for several different soybean diseases if care is not taken to identify correctly the different symptoms. Consult Table 1 of CPN-1011 for an excellent guide to differentiate soybean diseases from nutrient or fungicide injury.
Specifically for SDS, foliar symptoms are similar to Brown stem rot (BSR), another important soybean disease, and includes yellow to brown discoloration of the leaves around the veins. These symptoms initially begin as small, circular spots (see Figure 2 where you can see these on several of the leaves). In order to differentiate SDS from BSR, we need to examine the roots and stems. SDS-infected soybean plants can have roots that are black and rotted, and when there is reproduction of the fungus on the root surface, a bluish to purple hue can also be observed. A key symptom that can further separate the two diseases can be seen by splitting the stem since BSR will have a brown discoloration in the center of the stem (pith), while for SDS-infected plants, the pith tissue remains white.
Figure 2. Sudden death syndrome of soybean showing early, circular chlorotic spots and the later yellow to brown discoloration between veins.
Once we have identified SDS, it will be important to note the overall distribution of the disease in the field in order to make appropriate management decisions in the future, especially related to improving compaction and drainage in those areas. SDS does not occur every year due to different environmental conditions, but we need to consider an integrated approach to further reduce the risk of this disease. Start by working with your seed rep on variety selection, as there are materials that have better resistance to SDS.
Given that the pathogen can survive for long periods of time in the field and several research studies have shown that F. virguliforme has a broad host range, crop rotation may not be the most effective management tool. Foliar fungicides are not effective, while not all fungicide seed treatments have shown to have efficacy against this disease, although there are several products with F. virguliforme on its label. Delaying the planting date may only be warranted for those fields where SDS was very severe since it is known that delayed planting can decrease yield potential. Finally, while nematodes have not been a problem in Pennsylvania, it may be worthwhile to consider sending a soil sample for testing since in several studies a strong link between soybean cyst nematode and SDS has been documented, although both occur in the absence of the other.