The disease can develop at any growth stage but plants are most susceptible at the beginning of flowering. The symptoms are usually visible during long periods of warm, dry weather. Plants have low vigor, and they start to wilt during the hottest hours of the day, recovering partially during the night. Younger leaves start yellowing and pods remain unfilled. The rot in roots and stem is characterized by a reddish-brown grainy discoloration in the internal tissues. Randomly distributed black specks at the base of the stem is another symptom of the fungal growth.
Charcoal rot of soybean is caused by the fungus Macrophomina phaseolina. It overwinters in plant debris in the field or in the soil, and infects the plants via the root early in the season. Symptoms can remain latent until adverse environmental conditions (e.g. hot, dry weather) stress the plants. The damage caused to the root internal tissues impairs water uptake when the plants need it the most. Unlike other fungi, the activity and growth of the charcoal rot fungus are favored by dry soils (27 to 35°C).
The parasitic fungi Trichoderma spp parasitizes other fungi, among them Macrophomina phaseolina. Soil application of Trichoderma viride (5 kg enriched on 250 kg of vermicompost or FYM) at the time of sowing reduces significantly the incidence of the disease. Other methods include the use of the bacterium Rhizobium sp. to control the fungus.
Always consider an integrated approach with preventive measures together with biological treatments if available. No fungicide seed or foliar treatments offer consistent control of charcoal rot. Seed treatment with mancozeb @3 gr/kg seed can be envisaged to reduce inoculum at sowing. The application of 80 kg/ha of MOP in two splits also reduces the severity of the symptoms.