There have been many reports of black areas in soybean fields in Indiana this season. These areas have appeared after weather from hurricane Helene passed through the region. In the field that I have observed recently, a few things were contributing to this issue– first areas with black soybeans showed up in previous pockets of poor growth due to drought conditions (dry hill tops) and/or other disease issues caused plants to shut down earlier than the rest of the field. In Casteel trials that were side by side, an earlier maturing variety was more symptomatic with black appearance with hanging, dead leaves (drought conditions plus another causal agent) compared to a fuller variety that showed no black appearance. It is possible there is a planting date/maturity effect. Soybeans that were reaching maturity earlier and beginning to dry down (or shut down) when the hurricane weather went through are the areas that are most impacted. When affected soybeans are harvested the dust cloud is black. In general, what we are seeing in samples coming into the clinic is saprophytic fungi – secondary colonizers that are responsible for breaking down and recycling the dead plant material.
Figure 1. An example of black soybeans areas in a field. (Photo Credit: Abigail Heidenreich, Purdue Extension)
The black color on soybeans can be attributed to a number of different organisms from saprophytic fungi to Diaporthe stem blight (formally known as Phompsis), Anthracnose stem blight, or Cercospora spp. Some of these soybean diseases can greatly impact seed quality. In Indiana, the most common are Diaporthe seed decay (Phomopsis, caused by Diaporthe spp.), Cercospora purple seed stain Cercospora kikuchii); Frogeye leaf spot (Cercospora sojina); Anthracnose (Colletotrichum spp.); downy mildew (Peronospora manshurica); Soybean viruses (bean pod mottle and soybean mosaic); and various other secondary fungal invaders of injured pods including Alternaria, Fusarium, Claosporium, and Pencillium.
Figure 2. An example of some black soybeans submitted to Purdue Plant Pest Diagnostic Lab (PPDL), there are multiple fungi infecting both the stems, pods and seeds.
The table below provides several descriptive characteristics to begin the diagnostic process and choose appropriate management recommendations. It is important to note, however, that although Purple Seed Stain is easily identified by the ‘signature’ purple symptom on the seed, accurate diagnosis of most of the fungal diseases on seed requires microscopic assistance offered by the Purdue Plant and Pest Diagnostic Laboratory (PPDL). The diversity of symptoms that can be observed on diseased soybean seed is shown in the example in Figure 3. In this image, all the seed are infected with the frogeye leaf spot pathogen C. sojinia.
Figure 3. A diverse range of symptoms observed from Cercospora sojina (frogeye leaf spot) infection. (Photo Credit: Gail Ruhl, PPDL)
Disease infected seed can have reduced storability, decreased germination, loss of seed weight and reduced meal and oil quality.
Optimum storage conditions to limit fungal growth include
- Free from fungi or other pests.
- Clean seed without organic or other waste material.
- Less than 12% moisture.
- Cool uniform storage temperature.
Future management options to minimize diseases on soybean seed:
- Start with clean seed (pathogen free) and use resistant varieties when available.
- Fungicide options –
- Seed treatments can help reduced seed to seedling disease transfer.
- Foliar fungicides may help reduced pod and seed infection by reducing the disease epidemic in the field.
- Tillage and crop rotation – bury the inoculum from disease-infested residue and further reduce the inoculum by planting a non-host the next season.
- Control other pests such as bean leaf beetle, aphids and other insect vectors that injure the pod opening the door to fungal infection
Other Resources
Mueller, D. et al. An Overview of Pod and Stem Blight and Diaporthe Seed Decay. Crop Protection Network. CPN-1007. Https://doi.org/10.31274/cpn-20190620-016
