The infection process of Colletotrichum fuscum on oregano leaves and stems

Abstract

Anthracnose, caused by Colletotrichum fuscum, produces regular necrotic spots on oregano leaves and stems, causing severe crop losses. In this study, Koch’s postulates were fulfilled and infection process was investigated using scanning electron microscopy. Leaves and stems of Origanum vulgare were inoculated and incubated at 24°C in wet chambers under high relative humidity. Pathogenicity experiments demonstrated that all tested C. fuscum isolates had infected stems and leaves of oregano. Of all inoculation methods, direct placement of colonized agar plugs on injured epidermis and soaking plant organs in conidial suspension were the most effective. The behavior of the conidia deposited on the oregano leaves was investigated at different time intervals after inoculation: at 12, 18, 32, 48, 67 and 98 h. Conidia produced an appressoria of varying shapes which has been formed at the end of germ tubes of different lengths. Penetration to host tissue through stomata was observed. Acervuli formed on the leaves surface after 98 h after inoculation, typically with sharp pointed setoses.