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Influence of Temperature and Wetness Duration on Conidia and Appressoria of Colletotrichum acutatum on Symptomless Strawberry Leaves

Strawberry leaves (cv. Tristar) inoculated with Colletotrichum acuta-tum conidia were incubated at 10, 15, 20, 25, 30, and 35°C under continuous wetness, and at 25°C under six intermittent wetness regimes. The number of conidia and appressoria was quantified on excised leaf disks. In order to assess pathogen survival, inoculated leaves were frozen and incubated to induce acervular development. Germination, secondary3 conidiation, and appressorial development were significantly (P ≤ 0.05) affected by temperature and wetness treatments. Under continuous wetness, the optimum temperature range for conidial germination was 23.0 to 27.7°C, whereas the optimum temperature for appressorial development ranged from 17.6 to 26.5°C. Secondary conidiation showed an optimum temperature range of 21.3 to 32.7°C and was most abundant between 12 and 36 h after inoculation. Conidial germination, appressorial production, and secondary conidiation were favored by increasing wetness duration and more than 4 h of wetness were required for secondary conidiation. In a greenhouse, C. acutatum survived up to 8 weeks on leaves. The number of acervuli formed on leaves after freezing and incubation was closely (r² ≥ 0.95) related to appressorial populations prior to this treatment and was greatest following periods of continuous wetness. Production of secondary conidia and appressoria of C. acutatum on symptomless strawberry leaves under a range of environmental conditions suggests that these processes also occur under field conditions and contribute to inoculum availability during the growing season.