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Strawberry Plant Extracts Stimulate Secondary Conidiation by Colletotrichum acutatum on Symptomless Leaves

Conidial suspensions of Colletotrichum acutatum were prepared in 1:27, 1:45, and 1:81 (wt/vol) dilutions of an extract of strawberry (cv. Tristar) flowers or leaves in water. Strawberry leaves and plastic coverslips were sprayed with the conidial suspensions, incubated at 25°C and continuous wetness for 48 h, and the number of conidia and appressoria were counted. In another experiment, leaves and coverslips were sprayed with a conidial suspension in water, incubated for 72 h to establish C. acutatum populations, and placed in a growth chamber under dry conditions for up to 6 weeks. At each sampling time, leaves and coverslips were sprayed with flower extracts, leaf extracts, or water, incubated for 48 h at 25°C and continuous wetness, and the number of conidia and appressoria were counted. Flower extracts significantly (P ≤ 0.05) increased the number of conidia on leaves and coverslips compared with water, both when conidia were applied with the extracts and when the extracts were applied to C. acutatum populations exposed to dryness for up to 2 weeks. Application of flower extracts resulted in up to 10- and 16-fold increases in conidia on leaves and coverslips, respectively. Number of conidia increased more when exposed to flower extracts than to leaf extracts. Production of appressoria was not significantly affected by flower or leaf extracts. Our results suggest that inoculum levels of C. acutatum on foliage may increase during flowering of strawberry plants.