Olive Cuticular Waxes Induce Conidial Germination and Appressorium Formation in Colletotrichum Species

Anthracnose, caused by several Colletotrichum species, is the most destructive disease of olive fruit. Successful infection depends on the perception of host surface signals that regulate conidial germination and appressorium formation. Although olive fruit epicuticular waxes form a hydrophobic barrier, they can also serve as chemical cues regulating fungal spore differentiation. However, their role in olive susceptibility to Colletotrichum spp. remains poorly understood. In this study, we evaluated the effects of artificial and natural olive epicuticular waxes on conidial germination and appressorium formation in C. godetiae and C. nymphaeae, the two dominant species in Mediterranean olive-growing regions. Surface hydrophobicity strongly favoured conidial differentiation. The addition of olive epicuticular waxes restored conidial germination and appressorium formation when added to hydrophilic surfaces. Increasing wax concentration progressively enhanced both processes. Waxes extracted from olive fruits and leaves stimulated both developmental processes in both species, although waxes from ripe fruits induced a stronger response than those from unripe fruits. In contrast, waxes from the non-host avocado completely inhibited conidial germination. Although olive epicuticular waxes are generally regarded as physical barriers protecting plants against abiotic and biotic stresses, our results suggest that, in the olive-Colletotrichum pathosystem, they may also provide host-derived chemical cues that stimulate the early stages of infection.