Infection process and identity of the hemibiotrophic anthracnose fungus (Colletotrichum destructivum) from cowpea (Vigna unguiculata)

The infection process of a hemibiotrophic Colletotrichum species causing anthracnose disease in cowpea ( Vigna unguiculata ) was studied by light and electron microscopy. During the biotrophic phase, the fungus produced unusual, large, multilobed, multiseptate infection vesicles with elongated neck regions, which remained confined within the initially-infected epidermal cell. Biotrophy lasted for the first 72 h of the host-pathogen interaction. The necrotrophic phase was characterized by the rapid development of invasive secondary hyphae which radiated from the multilobed vesicles into the surrounding tissues, producing water-soaked lesions on the surface of infected organs. Eventually, acervuli erupted in abundance on the surface of colonized tissues 120 h after inoculation. Each acervulus possessed, characteristically, one septate melanized seta. Multilobed vesicles were successfully isolated from homogenates of infected leaves by isopycnic centrifugation and were shown to lack cross-reactivity to monoclonal antibodies raised against infection structures of C. lindemuthianum . Analysis of the nucleotide sequences of the amplified D2 and ITS-2 regions of rDNA revealed very close similarities (97–99%) between the cowpea isolate and three isolates of C. destructivum obtained from lucerne ( Medicago sativa ). The results presented here, in addition to other morphological and growth attributes, suggest that the hemibiotrophic cowpea anthracnose pathogen must be considered as a form of C. destructivum .