Colletotrichum acutatum on apple in Norway

Abstract: Colletotrichum acutatum causes bitter rot on apple fruit. It is a pathogen with numerous hosts and adapted to various climatic conditions, and in Norway it has been found on 38 different plant species. There is an increasing awareness of C. acutatum as a postharvest pathogen on apple throughout Europe. It may occasionally be detected in the orchard, but it is the main cause of postharvest decay in Norway. This paper reviews the biology and control of C. acutatum on apple as we presently know it. Typical symptoms and signs on fruit are round, sunken lesions, with dark acervuli and orange to pink spore masses. However, these symptoms may vary. Generative buds may harbour C. acutatum on their scales and may be important as sources of inoculum in early spring. Fruit spurs and apple leaves may contain the fungus and be sources of inoculum for fruit throughout the growing season. Infections on bud scales, fruit spurs, and leaves are all asymptomatic, with the only disease symptoms occurring on maturing or harvested fruit. Fungicide applications against apple scab normally reduce the amount of postharvest development of bitter rot. Experiments with a range of different fungicides against C. acutatum on leaves did not show clear differences between treatments. In cold storage, bitter rot developed more easily in late rather than in early harvested fruit. Disease incidence increased over time in cold storage and more so after simulated shelf life. Storage at low temperature delayed disease development, and incidence after storage at 1 °C was lower than at 3 °C.