When we think soybean seedling diseases, we often think Pythium or Phytophthora infection. Both of which require wet conditions for optimal infection. Weather conditions that we obviously have not experienced much of, if any, during this growing season (Figure 1).

Figure 1 Rainfall data collected from a central Missouri mesonet station.

Figure 2 Example of a sample submitted to MU Plant Diagnostic Clinic this month.

Even so, the MU Plant Diagnostic Clinic has received multiple samples of soybean seedlings with seedling disease-like symptoms over the last week (Figures 2). A subset of samples had no disease detected and are likely suffering from environmental effects such as soil compaction enhanced by the drought. Some tested positive for Fusarium species or for the fungal pathogen associated with Charcoal Rot (Macrophomina phaseolina) or both (Figure 3). One sample tested positive for Fusarium, Macrophomina, and Rhizoctonia (Figure 4). No Pythium spp. or Phytophthora spp. have been detected from samples so far.

Figure 3 Cultures started from infected soybean crown tissue at the MU Plant Diagnostic Clinic. Fusarium has a pink color and Macrophomina, which causes Charcoal Rot, is black.

Figure 4 These seedlings submitted to the MU Plant Diagnostic Clinic tested positive for Fusarium, Charcoal Rot, and Rhizoctonia infection.

Symptoms were reported as sporadic in some fields to circle patterns in other fields to nearly 75% of a field being damaged. Many, but not all, of the samples have come from central and west central Missouri.

The best way to determine if a soil-born pathogen is present and to differentiate among them is to submit physical samples to the MU Plant Diagnostic Clinic for analysis. Knowing which pathogens are present can assist with replant and crop rotation decisions in subsequent years.

We typically think of Charcoal Rot as a late season disease that affects soybean nearer to the pod fill stage. However, this disease is favored by warm soils and dry conditions, like those observed over much of Missouri this spring (Figures 1 and 5). Symptoms can occur in seeds and seedlings and include lack of emergence, wilting, and premature leaf death.

Figure 5 2-inch soil temperatures data collected from a central Missouri mesonet station.

Tiny black fungal structures called microsclerotia (Figure 6) are produced in roots and stems of plants with Charcoal Rot disease. These microsclerotia fall off mature plants at harvest, return to the soil, and can infect crops in subsequent seasons. Microsclerotia survive best in low soil moisture. Warm and dry conditions favor colonization of the fungi on soybean roots and stems following infection. As the fungus grows, it moves into the plant's vascular system and disrupts water and nutrient transport to the upper parts of the plant. This results in wilting and premature leaf death. Damage is confounded by drought. Disease symptoms typically appear first in the driest regions of the field, such as field edges, hillsides, and areas of compacted soils.

Figure 6 Image of microsclerotia on an infected soybean plant courtesy of Dr Kaitlyn Bissonnette.

Contrast this with Pythium and Phytophthora infections, which require saturated soils and water to move the spores (called zoospores) towards roots for infection. Symptoms of these diseases are more common in poorly drained soils or low-lying areas prone to flooding. Fusarium and Rhizoctonia infections of seedlings are favored in wet and cool soils. However, their disease symptoms may become more pronounced later in the season during hot and dry conditions.

If replanting in a field where Charcoal Rot has been confirmed, consider:

• Replanting may lead to the same result if the lack of rain continues as both drought conditions and the pathogen will still be present.
• There are soybean cultivars with partial resistance to Charcoal Rot available in Maturity Group IV and later.
• Irrigation can reduce overall disease severity by reducing water stress. (Research suggest irrigation does not reduce infection.)

If replanting is imminent and the partially resistant soybean cultivars are not at hand, consider using a different combination of soybean variety and seed treatment. Understand this may very well lead to the same outcome, but you already know what to expect with the combination used for the initial planting if drought conditions persist.

Going forward consider wheat as a nonhost crop. A rotation with wheat can reduce levels of inoculum in the field. (Corn and grain sorghum are host). Also consider management of soybean cyst nematode (SCN), which can enhance disease severity in infected fields. Soil samples for SCN analysis can be submitted to MU's SCN Diagnostics.

For more information and images on these soil-borne diseases visit the Crop Protection Network. To submit samples to the MU Plant Diagnostic Clinic visit the sample submission website or call the clinic at 573-882-3019.