Effects of temperature on sporulation and latent period of Colletotrichum spp. infecting strawberry fruit

Effects of temperature on sporulation of Colletotrichum acutatum, C. gloeosporioides, and C. fragariae, causes of anthracnose of strawberry, were determined in controlled-environment studies. Detached immature fruit were inoculated with a conidial suspension and incubated up to 36 days at constant temperatures of 5, 10, 15, 20, 25, 30, and 35 degrees C. Latent period (time to first sporulation) depended on temperature and ranged from 2 to 3 days at 25 degrees C to 6 to 17 days at 5 degrees C. C. acutatum had a shorter latent period than the other species at 5 and 10 degrees C; at higher temperatures, latent periods of the species were very similar. During the first 4 days of sporulation, there was an optimum-type relationship between the logarithm of conidia per fruit [log(Y)] and temperature, with maximum observed sporulation (generally 10(6) to 10(7) conidia per fruit) from 15 to 30 degrees C. Sporulation increased over time at temperatures of 15 degrees C and above. The greatest difference among the species was at 5 and 10 degrees C, where tested C. acutatum isolates produced from 10 to 100 more conidia per fruit than the other species. Polynomial regression equations were used successfully to represent log(Y) as a function of temperature and incubation time. The rate of increase in sporulation over time was a function of temperature, with a predicted optimum of 22 to 26 degrees C. Equations were validated by predicting sporulation of the three species infecting fruit attached to plants growing in controlled-environment chambers. Although the predictions tended to be slightly larger than observed, mean error [100(observed - predicted)/ observed] was only -0.7% (95% confidence interval: -2.4 to 1.0%).