Variation in aggressiveness of Phaeoisariopsis griseola and angular leaf spot development in common bean
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Aggressiveness of fifteen isolates of Phaeoisariopsis griseola and variations in angular leaf spot symptom development in common bean were studied. The isolates were selected based on their virulence and genetic differences and represented Andean, Afro-Andean and Mesoamerican groups of P. griseola. Aggressiveness was determined based on length of incubation period, rate of lesion expansion, lesion size and density, rate of sporulation, disease severity and area under disease progress curve (AUDPC). There was a wide variation in aggressiveness and the type of symptoms induced by the different isolates of the pathogen. Incubation period varied significantly (P < 0.05) among isolates and ranged from six to 15 days. Mesoamerican isolates had significantly shorter incubation period than Andean and Afro-Andean isolates. Disease severity, AUDPC and rate of sporulation differed significantly (P < 0.05) among the isolates. Lesions induced by the different isolates varied significantly in size, density and rate of expansion. Mesoamerican isolates induced significantly more lesions than Andean and Afro-Andean isolates. Disease severity was negatively correlated to incubation period and positively correlated to lesion density, lesion size and rate of sporulation. The significant correlations between disease severity, AUDPC, incubation period, lesion density and rate of sporulation indicate that all these parameters are important measures of aggressiveness in P. griseola. The significant variations in aggressiveness between isolates, virulence and genetically defined groups of P. griseola indicate that this parameter can be used to characterise isolates of the pathogen. Symptoms induced by isolates of P. griseola in different bean cultivars varied extensively in size, shape and time of appearance. These variations could, however, not be attributed to any particular cultivar or isolate and can, therefore, not be used to characterise the pathogen.