Germination, penetration and sporulation of Alternaria ricini (Yoshii) Hansf. on castor leaf

Abstract

The infection process of the leaf blight fungus Alternaria ricini was studied on castor leaves using scanning electron microscopy. Conidia germinated 9 h post-inoculation producing one to many germ tubes. The germ tubes were of various lengths, branched or unbranched, and terminated in bulbous appressoria on leaf cuticle, or penetrated the leaf through stomata. Appressoria were not formed where the germ tubes penetrated stomata. As the disease symptom appeared as small spots on leaves 11 days post inoculation, abundant conidia emerged from both surfaces of the leaf through epidermis and through stomatal openings, formed in chains on unbranched conidiophores. Conidiophores were solitary or in small clusters of 2 – 4. The emergence of conidiophores through stomata appeared to be random, as conidiophores pierced guard cells of stomata, where there was no emergence through those stomatal openings. The conidia were cylindrical while young and became bottle-shaped with terminal beaks when matured. Each conidium was born singly on a short unbranched conidiophore, and each conidiophore developed from a cushion-shaped base on the leaf surface.