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

Abstract

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.