The Huanglongbing-associated Preharvest Fruit Drop Signal Arises Several Weeks Before the Physical Separation of Sweet Orange Fruit

Abstract

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.