Morphological Diagnosis of Macrophomina phaseolina, the Causal Agent of Charcoal Rot Disease in Cluster Bean [Cyamopsis tetragonoloba (L.) Taub] and a Study of its Pathogenicity

Charcoal rot of cluster beans caused by Macrophomina phaseolina (Tassi) Goid. is a significant disease issue in the cluster bean cultivation regions of Rajasthan. The first signs of charcoal rot included leaf yellowing and wilting of plants that dried out within a week. Sick plants are hard to remove. The tissue in the collar area tears and becomes pliable. Following infection, a dark brown lesion appears on the stem at the ground level, and later, black-colored structures (sclerotia) may be observed on the stem and root. Isolation was performed on infected root sections, and the pathogen was purified with hyphal tip cut methods, identified as Macrophomina phaseolina through its morphological and colony characteristics. The young hyphae of the fungus were observed to be hyaline, thin-walled light brown to dark brown and having more septa. Branches from the main hyphae are generally formed at the right angle to parent hyphae with constriction of the point of origin. Pathogenicity was also proved through Koch’s postulates techniques under pot conditions by using normal soil, autoclave soil, field soil + inoculum and autoclave soil + inoculum technique methods. Field soil + inoculum were found effective followed by autoclave soil. The maximum per cent disease incidence (62.50%) was recorded with the field soil + inoculum technique followed field soil (57.14%) techniques. The minimum per cent disease incidence (0.03%) was recorded with the autoclave soil technique. Minimum germination per cent was observed in field soil + inoculum (67.67%) followed by field soil (71.67%).