Ecology and epidemiology of Colletotrichum acutatum on symptomless strawberry leaves

Germination, appressorial production and secondary conidiation of Colletotrichum acutatum were investigated on symptomless strawberry (cv. Tristar) foliage under a range of environmental conditions, and in the presence of strawberry plant extracts. Germination started within 3 h after inoculation and melanized appressoria were formed within 9 h after inoculation on detached strawberry leaves inoculated with a conidial suspension and incubated at 26°C under continuous wetness. Secondary conidia were formed on conidial and hyphal phialides within 6 h after inoculation, resulting in up to threefold increases in conidial populations on leaf surfaces without the development of symptoms or fruiting structures. Under continuous wetness, germination was highest at 20, 25 and 30°C and lowest at 10°C, whereas appressorial development was highest at 15 and lowest at 35°C. Secondary conidiation was highest at 25 and 30°C and lowest at 10°C, and peaked within 24 h after inoculation. Germination, appressorial development and production of secondary conidia were favored by increasing wetness duration when leaves were exposed to alternating wet and dry periods of different duration. More than 4 h wetness per day were required for secondary conidiation and for significant germination and appressorial development. C. acutatum survived up to 8 weeks on symptomless leaves under dry greenhouse conditions, as indicated by production of acervuli after freezing and incubation of the leaves. Survival was strongly related to appressorial populations on the leaves. Secondary conidiation was stimulated by strawberry flower extracts, both when conidia were germinated in extracts and when extracts were applied to C. acutatum populations exposed to dryness for up to 2 weeks. Application of flower extracts resulted in up to tenfold increases in total conidial populations on leaves. This study has shown that C. acutatum can become established, survive and produce inoculum on symptomless strawberry leaves under a wide range of environmental conditions. These findings suggest that symptomless foliage may play an important role in the strawberry anthracnose disease cycle, both as a harbor for fungal survival and a source of inoculum for fruit and flower infections