Infection Process of Olive Fruits by Colletotrichum acutatum and the Protective Role of the Cuticle and Epidermis

Colletotrichum acutatum is a serious concern for the Portuguese oliviculture and food industry, due to both olive oil yield and quality decrease. While the organism’s phytopathogenic potential has been well documented, the pathogen adhesion and colonization process in olives remain poorly understood. This paper reports experiments conducted on C. acutatum-susceptible, C. acutatum-moderately resistant and C. acutatum-resistant olive fruits during infection process, in two consecutively years, to identify the physical differences related to the host cuticle promoting resistance and susceptibility to C. acutatum. Cuticle thickness, perimeter and area of epidermal cells of ‘Galega’ (susceptible), ‘Cobrançosa’ (moderate-resistant) and ‘Picual’ (resistant) fruits were measured. Here, we also describe the colonization process of olive fruits by C. acutatum. To achieve this goal olive fruits from ‘Galega’, ‘Cobrançosa’ and ‘Picual’ were inoculated in a field trial with an aqueous suspension of C. acutatum (1x106 spores ml-1) under suitable humidity and temperature conditions. Light, fluorescent and scanning electron microscopy were used to view olive fruit sections. Significant differences were observed among the parameters studied- cuticle thickness, perimeter and area of olive fruit epidermal cells. The C. acutatum-resistant (‘Picual’) fruits showed highest mean values for each parameter in both years. Pathogen ultrastructures, such as spores, germ tube, and appressoria were clearly observed using different microscopy techniques. Acervuli sporulation was observed only 192 hours after inoculation in C. acutatum-susceptible (‘Galega’) fruits.