Editor’s note: The following was written by Mandy D. Bish, Pierce Taylor and Peng Tian with the University of Missouri for its Integrated Pest Management website June 8.
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.
Even so, the MU Plant Diagnostic Clinic has received multiple samples of soybean seedlings with seedling disease-like symptoms over the last week. 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.
Symptoms were reported as sporadic in some fields to nearly 75% of a field being damaged. Many, but not all, of the samples have come from central and west central Missouri.
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 this spring. Symptoms can occur in seeds and seedlings and include lack of emergence, wilting and premature leaf death.
Tiny black fungal structures called microsclerotia 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.
Contrast this with Pythium and Phytophthora infections, which require saturated soils and water to move the spores (called zoospores) towards roots for infection. 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 suggests it 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.
