Appressorium Formation and Tomato Fruit Infection by Colletotrichum coccodes

Tomato fruit grown for commercial processing are harvested when the majority of the fruit are at the full, red-ripe stage of development. At this physiological stage, marketable yields often are reduced significantly by Colletotrichum coccodes. Appressorium formation and the infection of tomato fruit by C. coccodes were investigated in controlled-environment experiments. Conidia of C. coccodes were subjected to five temperature treatments (10 to 34°C with 6°C increments), and eight incubation periods (3 to 24 h with 3-h increments). The highest proportion of conidia that formed appressoria occurred at 16 and 22°C. Appressoria were formed within as few as 6 h of incubation at 16, 22, and 28°C. In contrast, incubation periods of at least 15 and 18 h were required for appressoria to form at 34 and 10°C, respectively. Appressorium formation was significantly reduced by 0.1 to 0.2 ppm of the fungicide chlorothalonil, and no appressoria formed at concentrations >0.4 ppm. When tomato fruit were inoculated with C. coccodes at three inoculum densities (2 × 10⁵, 6 × 10⁵, and 10 × 10⁵ conidia/ml) and incubated in dew chambers for 8, 16, and 24 h at 5°C increments from 15 to 35°C, there was no significant interaction among inoculum density, dew period, and temperature. In general, across all inoculum densities and dew periods, anthracnose severity levels were greater for each 5°C increase in temperature from 15°C until its maximum level was observed at 30°C. However, when the fruit were exposed to 35°C, disease development was minimal. At temperatures from 15 to 30°C anthracnose severity increased proportionally as dew-period duration and inoculum density increased.