The growth rate of apple bitter rot lesion, caused by Colletotrichum spp., is affected by temperature, fungal species, and cultivar.

The growth and symptom development of two fungal species, Colletotrichum siamense, and C. fioriniae were examined using in vitro and in planta assays. In a plate assay, C. siamense grew fastest at 30 °C and at 25 °C, but C. fioriniae grew slower at 30 °C than at 25 °C. With apple inoculation assay, the mean lesion diameters at 30 °C were significantly larger (P ≤ 0.05) than that at 20 °C on a cultivar Ida Red for both fungal species. On the other hand, the mean lesion diameter on a cultivar Golden Delicious was significantly larger at 20 °C than at 30 °C for both fungal species. Therefore, the rate of lesion development was influenced not only by fungal species and incubation temperature but also by apple cultivar. In this study, cultivar ‘Ida red’ was found to be more susceptible to C. siamense under warm environmental condition. Although both species were able to grow at 10 °C in the plate assay, no disease symptoms were developed at 10 °C with the inoculation assay. Although the numbers of cultivar and fungal species are small, to our knowledge, this is the first report to describe potential interactive effect among Colletotrichum species, temperature, and apple cultivar on the rate of bitter rot symptom development.