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Increased preharvest fruit drop rates and reduced fruit sizes are common symptoms associated with Huanglongbing (HLB) in sweet orange. Small fruit may be more likely to drop during preharvest fruit drop. The objective of the current study was to determine whether fruit size could be used as an indicator of the preharvest fruit drop likelihood. Nearly 1400 fruit were surveyed over the following three timepoints across two years: 11 Mar 2022, 15 Apr 2022, and 22 Mar 2023. Each fruit was evaluated to determine the equator and peduncle diameter, fruit detachment force (FDF), type of detachment (mechanically broken or abscission), and lopsidedness. The total soluble solids content was determined for a subset of fruit. The FDF was consistently lower in abscising fruit than in mechanically broken fruit, suggesting that the physiological process of abscission had begun in these fruit and that they were more likely to drop. The fruit diameter was significantly smaller in abscising fruit than in mechanically broken fruit on 11 Mar 2022 and 22 Mar 2023, but not on 15 Apr 2022. Similarly, the fruit diameter and FDF were also significantly and positively correlated, but this relationship was weaker at time points late in the season (closer to harvest). These findings suggest that small fruit are indeed more likely to drop early in the preharvest period. The abscising fruit had total soluble solids contents similar to those of the mechanically broken fruit. Therefore, we hypothesized that the smaller fruit ceased growing and are able to respond to abscission signals earlier than the larger fruit. Therefore, any strategies to mitigate fruit drop, such as the use of plant growth regulators, should be applied early in the season when the fruit are still actively growing. When growth ceases, the fruit are vulnerable to drop.

The spread of Huanglongbing (HLB), a bacterial disease presumed to be caused by Candidatus Liberibacter asiaticus, throughout the state of Florida has coincided with a steady decline in total citrus (Citrus sp.) production. This decline is partially attributable to the high rates of preharvest fruit drop seen in HLB-affected trees. Although mature fruit drop is a natural phenomenon, the drop rates continue to increase as HLB symptom severity worsens. Unfortunately, how HLB causes this increase in fruit drop remains unknown. The current study aimed to determine the fruit characteristics associated with mature fruit drop in sweet orange (Citrus ×sinensis) and to provide an understanding of the possible role of endogenous ethylene, carbohydrates, and water deficit in HLB-associated preharvest fruit drop. Therefore, preharvest fruit drop rates of ‘Hamlin’ and ‘Valencia’ trees exhibiting mild, moderate, or severe HLB symptoms were monitored during the preharvest period (October–December for ‘Hamlin’ and January–May for ‘Valencia’). In addition, a subset of 20 fruit was collected to measure the fruit detachment force (FDF), which is the amount of force necessary to detach the fruit from the stem. After performing FDF measurements, eight additional physical and biochemical variables of tight and loose fruit (categorized by FDF) were measured. The total fruit drop rate during the preharvest period was higher for trees with severe visual HLB symptoms than for mild trees. Similarly, this increase in drop rates was negatively correlated with the canopy density. The fruit from severe trees (with high preharvest drop) showed increases in gene activity related to ethylene and abscisic acid earlier in the preharvest drop season, but not late in the season. No consistent carbohydrate pattern in tight and loose fruit was observed. Fruit likely to drop (those with lower FDF) were also consistently smaller than the fruit likely to be maintained on the tree (those with higher FDF). Therefore, it is proposed that the suppression of fruit growth early in the developmental period (possibly caused by water deficit) determines the fate (to drop or not) of the fruit before they have reached physiological maturity. Thus, strategies to mitigate preharvest fruit drop should be applied earlier in the season, and possibly during early stages of fruit development. By the time actual fruit drop becomes evident, the fruit drop-related signals have already been triggered, and treatments may not effectively reduce drop.