2019 ASHS Annual Conference

Huanglongbing (HLB), a citrus disease, due to its endemic presence in Florida has led to a great economic loss in citrus industry within past decade. The HLB symptoms include blotchy mottled leaves, shoot dieback, small and lopsided fruit with aborted seeds and an increase in preharvest drop of mature fruit that occurs up to 3 months before the harvest. The underlying cause of HLB-associated preharvest fruit drop is still unknown. Preliminary experiments suggest that the process of cell expansion and separation in abscission zone at calyx (AZ-C) of mature fruit on HLB-affected tree is similar to that promoted by exogenous ethylene in mature fruits of healthy citrus trees; however, the role of endogenous ethylene in HLB-affected preharvest drop and time of signal transduction pathway is not clear. Therefore, we conducted this study to understand the effect of HLB, role of endogenous ethylene, and molecular mechanism underlying preharvest fruit drop. In this experiment, ‘Hamlin’ sweet orange trees (n = 4) exhibiting mild, moderate, and severe visual symptoms of HLB were selected and preharvest fruit drop was monitored for approximately 3 months before harvest. Photosynthetically Active Radiation (PAR), as an index of canopy density was measured for each tree and was related with the disease severity level of tree. We found that total preharvest fruit drop rate was significantly related to the PAR values of the trees (R² = 0.71; p-value = 0.00061). The severely symptomatic trees (high PAR value) had an average of 28% preharvest fruit drop whereas mild trees had significantly lower preharvest fruit drop (15%). The average preharvest fruit drop and fruit detachment force (FDF), although showed similar value in beginning period of monitoring but accelerated significantly in severely symptomatic trees during the course of preharvest fruit drop monitoring. This suggests that disease severity affects the rate of preharvest fruit drop and possibly, signal transduction and HLB interaction occurs during this time period that accelerates the preharvest fruit drop. The next step is to determine the role of endogenous ethylene in HLB-associated preharvest fruit drop by analyzing relative expression of genes involved in ethylene metabolism, abscission, and senescence in AZ-C, leaf, and fruit peel during the preharvest fruit drop time period.