Survival and sporulation of Colletotrichum acutatum on symptomless strawberry leaves

Anthracnose caused by Colletotrichum acutatum is an important disease of strawberry worldwide. The ecology of the pathogen on symptomless plants is not well understood and may be important in disease epidemics. The aim of this study was to characterize and quantify the development of C. acutatum on strawberry foliage. Detached strawberry leaves were spray-inoculated with a conidial suspension and incubated at 26°C and 100% relative humidity. Leaf disks were sampled at 3, 6, 9, 12, 15, 18, 21, 24, 36, 48, 72, 96, 144 and 168 h after inoculation, fixed in glutaraldehyde, cleared in chloral hydrate and observed microscopically. Conidial germination, development of appressoria and the production of secondary conidia were observed and quantified over time on a unit area basis. C. acutatum germinated, formed melanized structures and produced secondary conidia on symptomless strawberry leaves. Secondary conidia were produced by microcyclic conidiation on conidial phialides and on short hyphal phialides. These conidia were produced on leaves in the absence of acervular development, suggesting that secondary sporulation on symptomless plants can be a significant source of inoculum for flower and fruit infections.