Environmental requirements for infection of Colletotrichum acutatum and C. gloeosporioides sensu lato in citrus flowers and prevalence of these pathogens in Brazil

Citrus post-bloom fruit drop (PFD) is caused by Colletotrichum acutatum and C. gloeosporioides species complexes and frequently occurs in São Paulo (SP) state, Brazil. The environmental requirements for conidial germination and appressorium formation have been already determined in vitro; however, the environmental conditions for infection in citrus flowers remain unknown. C. acutatum is considered the main species complex causing PFD in citrus, but a survey of the pathogen’s prevalence has never been performed in SP state. The aims of this study were to assess the influence of different temperatures and wetness periods on infection of flowers by C. acutatum and C. gloeosporioides in potted citrus trees, and to determine the prevalence of both pathogens in different SP regions and sweet orange varieties. Environmental requirement trials were conducted in 2- to 3-year-old potted sweet orange trees. Flowers at different development stages (R2, R4 and R5) were inoculated with C. acutatum and C. gloeosporioides, and the trees were incubated at 18 or 24 °C for wetness periods of 6, 8, 12, 16 or 24 h. The percentages of inoculated flowers with PFD lesions and persistent calyces were calculated. The prevalence of both pathogens in SP regions was assessed in samples of diseased flowers collected at 16 commercial orchards of ‘Natal’, ‘Valencia’ and ‘Pera’ sweet oranges. The species complex was confirmed by isolation and PCR with species complex-specific primers. Blossom blight and persistent calyx incidences were higher in flowers inoculated with C. acutatum than in those inoculated with C. gloeosporioides. Incubation periods ranged from 2 to 5 days, and from 5 to 6 days for C. acutatum and C. gloeosporioides, respectively. In SP, irrespective of the region and sweet orange variety, C. acutatum was isolated from more than 80% of symptomatic flowers. We concluded that C. acutatum is not only more aggressive than C. gloeosporioides for causing PFD but also more prevalent in SP orchards.