March 15, 2013 - If I asked you to name the top two yield-robbing soybean pests in the US, you may be able to accurately answer Soybean Cyst Nematode and Phytophthora Root Rot. Coming up with number three on the list could be more of a challenge, but you would be correct to say Charcoal Rot. 7.1 Million bushels of production are lost annually to this disease that prospers in the presence of conditions that conjure up the words "stress" and "arid". Causal agent is Macrophomina phaseolina, a fungus with over 500 known hosts including corn and grain sorghum. But while most fungi prefer cool and damp conditions, Charcoal Rot likes it hot.
Infection takes place in the earliest growth stages 3 to 4 weeks after planting. The pathogen stays dormant until later reproductive stages when favorable conditions appear. Hot dry weather would be considered "favorable" here (soil temps of 82-95 degrees), and anything that adds stress to the plant increases the severity of the disease. Consider your foes at this point to be compaction, sandy soils, terrace tops, heavy plant populations, insect feeding, weed pressure, SCN, open canopy that allows soil moisture to escape, etc. First visible symptoms are plants that wilt by day, then somewhat recover by night. Soon these plants wilt permanently and leaves begin to yellow. Plant height, root mass and yield all can be knocked by 50% as the affected crop eventually dies. Note that these dead plants will hold on to their leaves and not defoliate. To further investigate Charcoal Rot, cut open a tap root. It will be internally streaked with black or brown discoloration. Up along the stem of the plant black dots called sclerotia form just under the epidermis and in the stem wall.
Charcoal Rot does not respond to fungicide treatments. Basic course of action involves many common sense methods for reducing stress and capturing soil moisture. First of all, check with your AgVenture seed supplier about products that show good resistance. Then use crop rotation as a means of allowing the pathogen to die off naturally. Reduce seed planted populations, irrigate if possible, utilize residue to maintain ground cover and hold soil moisture, reduce soil compaction, minimize tillage, plant in row patterns that produce a comprehensive canopy, knock off insect and weed competition early, reduce SCN populations...all of these can help alleviate the severity of Charcoal Rot by reducing stress and holding soil moisture.
Submitted by AgVenture's Product & Technology Manager Soybean/Alfalfa, Jeff Shaner.