Stomatal penetration of cowpea (Vigna unguiculata) leaves by a Colletotrichum species causing latent anthracnose

Abstract

The infection process of a Colletotrichum species causing latent infection and anthracnose in cowpea (Vigna unguiculata) was studied in seedlings by light arid confocal microscopy. Leaf surfaces were extensively colonized by an anastomosing network of germ-tubes and conidia. This epiphytic mycelium produced abundant Secondary conidia on short conidiophores. Although melanized appressoria were developed, the host surface was not penetrated directly. The fungus only gained ingress into leaves through stomatal openings, by means of undifferentiated germ-tubes, and slowly colonized the mesophyll by intercellular hyphae, without initially Producing visible symptoms. Anthracnose lesions with multisetate acervuli appeared on senescent leaves after a prolonged symptomless period of host colonization lasting >2 weeks. Analysis of the nucleotide sequences of the amplified D2 and ITS-2 regions of rDNA revealed close similarities (95-96%)between this cowpea pathogen and isolates of C. gloeosporioides from Aeschynomene virginica, Stylosanthes scabra and Mangifera indica. These results, in addition to other morphological and growth attributes, identify this endophytic anthracnose pathogen of cowpea as a Colletotrichum species distinct from C. capsici and C. destructivum.