Development of the Leaf Spot Fungus, Cercospora ricinella, on Castor Leaf – an SEM Account

Abstract Structure and development of Cercospora ricinella, the causal agent of the castor leaf spot, were studied by scanning electron microscopy. The conidia started to germinate 9 h post inoculation, producing an appressorium directly from the tip of conidium or at the end of a short germ tube. A single germ tube or appressorium was formed from the apical cell of each germinating conidium. Germ tubes or appressoria did not grow towards stomata, and penetration of the leaf appeared to be by piercing the epidermis. Leaf spots appeared 7 days post inoculation (dpi), and after approximately 12 dpi, clusters of two to many conidia emerged from both surfaces of the leaf through the epidermis or through stomatal openings. Stomata were torn when a large number of conidia emerged through them, and conidia also pierced the epidermis above the guard cells of stomata. Emergence through stomata appeared to be random rather than a preference.