Fruit Size as an Indicator of Fruit Drop in Huanglongbing-affected ‘Valencia’

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.