Citrus greening or huanglonbing (HLB) is thought to be caused by Candidatus Liberibacter asiaticus (CLas) and is devastating the citrus industry worldwide. One symptom of HLB disease is excessive pre-harvest fruit drop. Recently, higher incidence of Lasiodiplodia theobromae (Diplodia) was found in HLB-symptomatic orange calyx abscission zones(AZ-C) than in non-symptomatic fruit, and the infection was positively correlated with the reduction in fruit detachment force (FDF), suggesting that Diplodia infection may be involved in the HLB-related pre-harvest fruit drop. To verify the hypothesis, we conducted two experiments. Experiment I was conducted by shaking HLB-affected 'Hamlin' and 'Valencia' orange trees during the harvest season (twice for 'Hamlin' and once for 'Valencia'). The fruit that dropped from trees upon shaking were collected (D), and the fruit retained on trees after shaking were harvested (R). Fruit ethylene production was measured, and the levels of Diplodia and CLas in AZ-C of D and R fruit were analyzed. The results revealed significantly higher levels of Diplodia in D compared with R fruit; and ethylene was produced from more than half of the D fruit but none of the R fruit. Ethylene production was positively correlated with Diplodia level in D fruit. In experiment 2, a preliminary trial on the effect of fungicide (Quadris Top) application on incidence of Diplodia infection and fruit drop was investigated. The experiment was conducted in a commercial grove with essentially 100% of the trees being HLB-symptomatic, and included five citrus cultivars ('Early Gold' orange, `Midsweet' orange, 'Murcott' tangor, 'Navel' orange and 'Ray Ruby' grapefruit). Diplodia levels were lower and FDF significantly higher in fungicide-treated compared to non-treated 'Early Gold', 'Midsweef and 'Murcott' fruit, and consequently, the fruit drop was reduced by 45%, 30% and 46% that of non-sprayed controls, respectively. For 'Navel' or 'Ray Ruby' fruit, there was no significant change between sprayed and non-sprayed controls in the level of Diplodia, FDF or fruit-drop. The results consistently showed a positive correlation between Diplodia infection and fruit drop in HLB-affected fruit, indicating the possible involvement of the fungus in HLB-related excessive fruit drop. This suggests that control of Diplodia fungal infection in the field may reduce HLB-associated pre-harvest fruit drop. Published by Elsevier B.V.