Studies on Alternaria porri and Stemphylium vesicarium on Allium spp.
During surveys in South Africa, Alternaria porri (Ellis) cif. and Stemphylium vesicarium (Wallr.) E. Simmons were found to be destructive seed-borne pathogens of onion (Allium cepa L. ). These two pathogens are also reported on garlic (Allium sativum L.) in South Africa for the first time. The development and morphology of conidiophores and conidia of the two pathogens on the onion leaf surface were examined using scanning electron microscopy. In both pathogens, solitary or fasciculate conidiophores emerged through the epidermis. Bud-like conidial initials were produced singly at the apex of conidiophores. As conidia of S. vesicarium matured, they became oblong to ovoid and densely verrucose. Those of A. porri showed slight growth in width but pronounced elongation. Conidial germination, formation of pre-penetration structures, penetration of the onion leaf surface by A. porri and S. vesicarium, and the subsequent infection process by A. porri, were studied using light, scanning electron and transmission electron microscopy. Conidia of both pathogens usually germinated within 24 h of inoculation, forming several germ-tubes which often terminated in bulbous appressoria produced directly on the epidermal cells or on stomata. Following direct penetration of the outer epidermal cell wall or the stoma, bulbous primary hyphae developed below the appressoria. Secondary hyphae of A. porri developed from primary hyphae and grew within the intercellular spaces, penetrating mesophyll cells. The changes in ultrastructure of infected cells, and of cells in close proximity to secondary hyphae, are described. six fungicides, anilazine, benomyl, carbendazim/flusilazol mixture, procymidone, tebuconazole and thiram, as well as a hotwater soak (50 C for 20 min) and sodium hypochlorite treatment, were evaluated for their efficacy in reducing both pathogens on seed and in culture. The effect of the various treatments on seed germination, and seedling emergence and growth, was determined. None of the treatments eradicated A. porri and S. vesicarium from onion seeds. The hot-water soak proved to be the best treatment for reducing these pathogens, although percentage germination and emergence of onion seeds were reduced when compared to the control.