Epidemiological Relevance of Colletotrichum Species Isolated from Glomerella Leaf Spot Causing Symptoms in Apple Fruit

Abstract

Colletotrichum isolates from apple leaves with symptoms of Glomerella leaf spot (GLS) can cause fruit rot and several small lesion spots, here called Colletotrichum fruit spot (CFS). This work investigated the epidemiological relevance of Colletotrichum species obtained from leaves with GLS in causing diseases in immature apple fruit by comparing different fruit sizes (phenological stages) for symptom development. In the first experiment, five Colletotrichum species were inoculated in ‘Gala’ (Ø = 5.5 cm) and ‘Eva’ (Ø = 4.8 cm) fruit in the field (2016/17 season). Subsequently, C. chrysophilum and C. nymphaeae were inoculated in fruit of different sizes (Ø = 2.4 to 6.3 cm) in the field (2017/18 and 2021/22 seasons) and in the laboratory according to the phenological stages of growing fruit. At harvest of the immature inoculated fruit in the field, only CFS symptoms were observed in both cultivars. For Gala, the CFS incidence reached 50% regardless of season, pathogen species, and fruit size. For Eva, CFS symptoms were observed after inoculation with C. melonis in the 2016/17 season and in smaller fruit inoculated with C. chrysophilum and C. nymphaeae in 2021/22. During postharvest, bitter rot symptoms developed, but did not seem to come from CFS symptoms. It can be concluded that the Gala cultivar has a high susceptibility to CFS caused by the two Colletotrichum species of the greatest epidemiological importance for GLS in Brazil in all fruit sizes tested.