Huanglongbing Increases Diplodia Stem End Rot in Citrus sinensis

Huanglongbing (HLB), one of the most devastating diseases of citrus is caused by the alpha-Proteobacteria Candidatus Liberibacter. Diplodia natalensis Pole-Evans is a fungal pathogen which has been known to cause a postharvest stem-end rot of citrus, the pathogen infects citrus fruit under the calyx, and the fruit decay typically occurs following harvest and is exacerbated by exposure to ethylene. In this study, we report that high incidence of Diplodia infection in HLB-symptomatic fruit. The incidence of SER in 300 'Hamlin' and 300 'Valencia' orange fruit, with or without ethylene treatment was determined. Two weeks following exposure to ethylene (10 ppm, four days), the incidence of SER in HLB-symptomatic fruit was as high as 66.7% ('Hamlin') and 58.7% ('Valencia'); whereas for asymptomatic fruit, less than 10% of the fruit were affected by SER. Confirmation of Diplodia in calyx abscission zone was by qPCR validation of the isolates and morphology of conidia.