By Daren Mueller, Department of Plant Pathology
Over the past week, we have observed yellow patches developing in soybean fields across Iowa (Figure 1). There are several different causes, including spider mites and soybean cyst nematode. However, we will look at three additional problems that are causing these patches: sudden death syndrome, charcoal rot and top dieback.
Figure 1 — Yellow patch in a soybean field.
For all three diseases, the symptoms can be very similar, but there are a few clues to distinguish them from each other. All three can have yellowing leaves in the upper canopy. For top dieback (Figure 2), the yellowing occurs on the outer margins of the leaves in the top of the canopy. It has been associated with potassium deficiency, but there is no clear-cut situation that precludes disease development.
Figure 2 — Top dieback in soybean.
Charcoal rot has been reported in the past, but this season it is showing up earlier than usual. The pathogen causing charcoal rot can survive many years in the soil. Conditions experienced in 2012 — hot and dry — are conducive for development of this disease. Interveinal yellowing of the leaves, an early symptom, can look a bit like sudden death syndrome (see Figure 3).
To distinguish charcoal rot from sudden death syndrome, there are a couple of things that can be done. First, you can look at the lower stem for microsclerotia (Figure 4). These can be found in the outer or inner stem tissue. A second way to tell the diseases apart is to wait until imminent death occurs. Plants with sudden death syndrome drop leaves but the petioles remain attached to the stem. Charcoal rot-infected plants that die may have leaves attached to the plants.
Figure 3 — Early symptoms of charcoal rot.
Figure 4. Microsclerotia in the soybean stem.
Figure 5 — Advanced symptoms of charcoal rot (photo courtesy of Carl Bradley).
There are no management strategies for this season for all three of these diseases. But knowing what is causing these spots may affect management in future years. For example, if you have top dieback, you may consider checking soil for potassium levels or soybean cyst nematode. Management of SCN or applications of potassium may alleviate this problem. Selecting resistant cultivars will help with sudden death syndrome. Finding ways to alleviate stress to future soybean crops can reduce charcoal rot.
Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at (515) 460-8000 or by email at firstname.lastname@example.org.