By Tripti Vashisth, Lisa Tang and Sukhdeep Singh
In citrus, huanglongbing (HLB) causes an increase in the mature fruit drop rate up to three months prior to commercial harvest. This preharvest fruit drop results in a great reduction in yield and overall grove productivity.
Since the plant growth regulators that improve fruit retention on healthy trees do not have consistent effects for HLB-affected trees, their use is not recommended for controlling HLB-associated fruit drop in Florida. Currently, how HLB escalates preharvest fruit drop in citrus remains unclear. Therefore, a study with trees exhibiting three HLB symptom levels (mild, moderate and severe) of early-season Hamlin (Figure 1) and late-season Valencia sweet oranges was carried out to gain more understanding of mature fruit drop and to determine the potential causes of fruit drop in the presence of HLB.
Starting from mid-October through harvest in late-December, Hamlin trees with severe HLB symptoms had a greater drop rate of fruit compared to moderate or mild HLB trees (Figure 1). Total fruit drop rate was correlated with HLB severity and canopy density.
The severity of HLB for trees in this study was based on visual symptoms and canopy density, which is represented using photosynthetically active radiation (PAR) that indicates the amount of sunlight passing through the canopy to the orchard floor (i.e., a high PAR value suggests a thin canopy). Results demonstrate that trees with a thin canopy, mainly due to HLB-caused twig dieback, have a high rate of preharvest fruit drop.
Fruit drop over time was also related to HLB severity for Valencia trees (Figure 2). In late January (approximately 4 months before anticipated harvest time), the drop rate of mature fruit for trees at different HLB symptom levels was similar. Interestingly, starting in mid-February, severe trees had more fruit drop than less symptomatic trees, and the increase in fruit drop became more noticeable as time progressed.
Consistent with the previous reports on HLB and fruit size, Hamlin and Valencia fruit of severe trees were smaller than those of mild or moderate trees (Figure 3). Attached fruit (those that did not drop and were still attached to trees at the time of harvest) were overall larger than dropped fruit (those that dropped naturally without external force) for both cultivars.
To determine the characteristics of fruit that tend to drop, a set of attached fruit with 6 to 8 inches of branches was clipped from trees. Fruit was further separated into tight fruit and loose fruit based on the value of fruit detachment force (FDF), which determines how much force is required for individual fruit to be pulled or detached from the tree/branch. Whereas tight fruit [FDF > 6 kilogram-force (kgf)] were not physiologically ready to drop, loose fruit (FDF < 6 kgf) represented the fruit with a high tendency to drop at the time of collection. For both Hamlin and Valencia, there is a significant and positive correlation between fruit size and FDF, indicating that small fruit require less force to be detached from trees (Figure 4). In other words, small fruit are more prone to drop from trees compared to large fruit.
The growth of fruit can be attributed to cell division and enlargement, which increases the number and size of cells, respectively, during the first two months following the beginning of fruit set (stage I of fruit development) and about four to six months thereafter (stage II). Given that severe trees produced small fruit and that small fruit had a high tendency to drop, it is likely that HLB restrains fruit growth during early developmental stages, which leads to a high inclination for fruit drop at maturity.
For citrus, fruit size increases rapidly as a result of water accumulation in cells at stage II of fruit development. Therefore, adequate water supply during this period is critical to ensure uninterrupted fruit development and growth for better fruit retention.
Interestingly, in another preliminary experiment, lower midday leaf water potential was observed for severe Valencia trees in comparison with mild trees in March (Figure 5). Although the value of water potential for severe trees was not in the range of water-deficit stress, and fruit were mature at the time of measurement, results imply that the internal water requirement or usage is different for trees with different HLB severity. Therefore, it is recommended to avoid any stress conditions, including drought, in order to prevent exacerbating preharvest fruit drop in the presence of HLB.
THE ROLE OF CARBOHYDRATES
It has been hypothesized that HLB results in the disruption of carbohydrate flow in the phloem, which leads to a carbohydrate shortage in fruit. This could increase preharvest fruit drop in a manner similar to the cause of fruitlet abscission during drop that occurs in summer months after fruit set.
Nevertheless, the results of recent analysis demonstrate that carbohydrate levels in fruit are independent of HLB severity or the tendency of fruit drop. For Valencia, there was no difference in the concentration of sucrose, fructose or glucose in juice of fruit from mild, moderate and severe trees. Carbohydrate levels were not different between attached and dropped fruit either.
Moreover, between attached and dropped fruit, no difference in the callose (a carbohydrate component of plant cell walls) accumulation in the peduncle phloem was observed, suggesting that phloem plugging is not related to HLB-associated fruit drop.
TREE HEALTH MANAGEMENT
In summary, research results demonstrate a clear relationship between HLB severity, fruit size and preharvest fruit drop. Fruit of severely symptomatic trees are smaller and have a higher tendency to drop prior to harvest compared to those of trees displaying less symptoms. This strongly suggests that maintaining tree health is a key to control preharvest fruit drop in HLB-affected trees.
Since there is no cure for HLB, an adequate nutritional program in combination with an optimal irrigation regime is important to sustain normal tree physiology, especially during early fruit development and growth phases. Prevention of any stress condition — drought, in particular — to citrus trees also benefits fruit retention until harvest.
Tripti Vashisth is an assistant professor, Lisa Tang is a postdoctoral associate, and Sukhdeep Singh is a graduate student — all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.