Abstract

Alternaria cassiae is the causal pathogen of a new, destructive foliar disease of cowpea (Vigna unguiculata). Cowpea leaves were inoculated with conidia of A. cassiae. Conidium germination, appressorium formation, penetration and colonisation of the plant surface were studied using light and scanning electron microscopy. Multiple germ-tubes developed randomly from each conidium and grew in any direction across the leaf surface. Terminal (71%) or intercalary (29%) appressoria were formed above epidermal cells or over stomata. Penetration of the plant surface, whether directly through the epidermis or via stomata, occurred with or without the formation of appressoria. Following penetration through a stoma, bulbous primary hyphae developed within the sub-stomatal cavity. Secondary hyphae developed from the primary hyphae and grew within the intercellular spaces, penetrating epidermal and mesophyll cells. Conidiophores emerged directly through the epidermis or through stomata.