Assessment of Charcoal Rot (Macrophomina phaseolina) Incidence on Soybean in Adamawa State, North East Nigeria

Soybean (Glycine max) cultivation in Adamawa State has expanded in response to growing commercial demand for food-grade soy and feed processing industries. Charcoal rot, caused by the soilborne pathogen Macrophomina phaseolina, is a late-season disease that causes extensive stalk and root rot, particularly under drought and heat stress conditions. This study assessed the incidence and severity of charcoal rot on soybean in five local government areas of Adamawa State during the 2023 growing season. Fifty farms were surveyed at crop maturity, with disease assessment based on the proportion of plants with charcoal rot crown and root symptoms and the characteristic black microsclerotia embedded in rotted tissue. M. phaseolina was confirmed as the causal agent through isolation, morphological identification, and pathogenicity testing. Overall charcoal rot incidence was 38 percent, with the highest severity in plots that experienced mid-season drought stress. Sandy loam soils and high pre-harvest temperatures above 32 degrees Celsius were consistently associated with elevated disease pressure. Plots with crop residue incorporation showed lower disease incidence, suggesting that previous crop debris management influenced soilborne inoculum levels. The study recommends adoption of drought-tolerant varieties, residue management strategies, and irrigation supplementation during pod fill to reduce charcoal rot losses in Adamawa State soybean farms. Keywords: soybean, charcoal rot, Macrophomina phaseolina, Adamawa, disease incidence.