Characterization of Macrophomina phaseolina, the charcoal rot pathogen of cluster bean, using conventional techniques and PCR-based molecular markers

Phenotypic and genetic diversity of 59 Macrophomina phaseolina isolates collected from various host species growing in or near cluster bean (Cyamopsis tetragonoloba) fields in four states of north and north-west India were characterized using RAPD and PCR-RFLPs of the ITS region. These isolates, and 11 from various hosts from culture collections, were classified into three mycelial phenotypes: dense, feathery and restricted, based on variable growth patterns on nutrient agar containing 120 mM chlorate. Pathogenicity of isolates was evaluated by measuring the length of stem lesions 21 days post-inoculation on the susceptible cluster bean genotype FS 277. Isolates showed considerable variation in aggressiveness, with the isolates from cluster bean with dense chlorate phenotype producing relatively higher lesion lengths on cluster bean plants. The results of the RAPD assay clearly distinguished the isolates on the basis of chlorate phenotype and host origin. Isolates from a single host were generally similar to each other, but differed distinctly from those from other hosts. Chlorate-sensitive isolates were distinct from chlorate-resistant isolates within a given host. A high degree of polymorphism in restriction patterns of the ITS region, including part of 25S rDNA, has been reported for the first time in the charcoal rot fungus.