By Tripti Vashisth and Lisa Tang
Fruit drop is a natural, physiological phenomenon that occurs in all citrus varieties. However, pre-harvest fruit drop often causes grower distress and concern as it can result in significant yield reduction as well as loss of tree resources into non-harvested fruit.
THREE PERIODS OF FRUIT DROP
In general, citrus trees bloom profusely in spring, but only a small number of flowers become fruit that stay on the tree and reach maturity until harvest. In sweet orange (Citrus sinensis), for example, less than 1 percent of flowers turn into harvestable mature fruit. The actual percentage of flowers to harvestable fruit will vary from year to year, with heavy-blooming years setting less flower to fruit than in years with lighter bloom. The low percentage of flowers turning to fruit can be attributed to fruit drop during different stages of growth and development.
The physiological process of fruit drop is called abscission. Naturally, citrus fruitlets and fruit generally detach from trees during the following three periods: post-bloom, June and pre-harvest.
Shortly after bloom, a number of flowers and fruitlets of citrus abscise due to poor pollination or in response to nutrient shortages and/or inadequate environmental conditions. The infection of the fungus Colletotrichum acutatum also causes the drop of small fruitlets that set immediately after bloom. Eighty to 90 percent of all flowers drop their fruit in this period.
Approximately one to two months after bloom, young developing fruit abscise from trees with excessive fruit set in June (often happens in May in Florida), consisting of about 10 percent of the total drop. The drop during this stage mainly results from the competition among young fruit for energy (carbohydrates) for growth and development. Although June drop of citrus fruit is typically considered a natural event during the course of fruit development, water-deficit in addition to warm temperatures in summer can exacerbate fruit drop (Figure 1).
Another wave of fruit drop in citrus occurs starting at three to four months prior to harvest. Unlike earlier drops, the pre-harvest fruit drop occurs in mature fruit. In addition, these mature fruit abscise distinctly at the calyx abscission zone (“button”), instead of the peduncle (stem) abscission zone (Figure 2).
For citrus trees grown in Florida, increased drop of mature fruit prior to harvest is one of the many symptoms of huanglongbing (HLB) disease, caused by Candidatus Liberibacter asiaticus. This HLB-associated pre-harvest fruit drop, in addition to tree decline, results in a reduction in yield (Figure 3). For example, for the late-season Valencia, pre-harvest fruit drop rose from 14 percent in the 2009–2010 production season to 30 percent during the 2016–2017 season (Table 1). To date, how HLB escalates pre-harvest drop in citrus remains unclear.
THE ROLE OF ENDOGENOUS PLANT HORMONES
The balance of two endogenous (growing from within) plant hormones, auxin and ethylene, plays an important role in signaling the process of abscission. A higher ratio of ethylene to auxin induces the enzymes responsible for the separation of the main body and fruit from the tree. Use of ethephon (ethylene) in abscission studies effectively loosens the fruit in healthy citrus.
Furthermore, the application of 2,4-dichlorophenoxyacetic acid, an auxin, delays the abscission of mature fruit in healthy citrus trees. Nevertheless, the effects of ethylene biosynthesis and action inhibitors and auxins on preventing pre-harvest fruit drop are inconsistent in HLB-affected citrus trees. Based on recent studies, it is not recommended to apply plant growth regulators (PGRs) in order to reduce pre-harvest drop.
A better understanding of mature fruit abscission associated with HLB is necessary to develop strategies to mitigate increased pre-harvest drop for HLB-affected citrus in Florida. Thus, a study on HLB-affected Valencia trees was set up in an experimental grove in Lake Alfred, Florida.
This study compared the effects of different HLB severities on fruit abscission, based on HLB symptom levels. Low, moderate and severe symptom levels were included in the study (Figure 4). For each tree in the study, mature fruit drop was monitored during the period of January through harvest at the end of May (Figure 5). Trees at the severe symptom level had a significantly higher fruit drop rate compared to low and moderate symptom trees.
A subset of mature fruit that was still attached to the trees was collected and further separated into “loose” and “tight” fruit based on the value of fruit detachment force (FDF), which determines how much force is required for a fruit to be pulled or detached from the tree. Loose fruit represented those fruit that would potentially drop shortly, whereas tight fruit were physiologically not ready to abscise yet (Figure 6). For severe symptom trees, 60 percent of the fruit were loose. At the low symptom level, only 13 percent of fruit were loose.
For HLB-affected Valencia, fruit harvested from severe symptom trees was significantly smaller and lower in weight as compared to low and moderate symptom trees. Overall, the loose fruit were smaller in size (diameter) and weight than tight fruit (Figure 7). Across all three HLB symptom levels, the FDF value was positively correlated with the fruit size and weight, respectively (Figure 8). This suggests that smaller fruit require less force to be removed from the trees compared to the larger ones. Thus, it is possible that small fruit have a higher tendency to drop. With the arrival of HLB, a decrease in fruit size has been observed throughout the state in addition to increased pre-harvest fruit drop. This suggests that these two observations are correlated.
Between the two types of fruit, there were no consistent patterns of ethylene-related gene expression in May, but the possibility of early ethylene activity prior to the triggering of citrus fruit abscission (before May) cannot be excluded. Interestingly, for trees at each of the HLB symptom levels, the carbohydrate concentration in juice of loose fruit was either greater than or equal to those of tight fruit.
Overall, fruit juice from severe symptom trees had the highest concentrated sugars as compared to the juice harvested from low and moderate symptom trees (Figure 9). This preliminary result, that loose fruit did not have lower carbohydrate content than tight fruit, suggests that the carbohydrate stress, which triggers June drop, is not likely to be the dominant cause of pre-harvest fruit drop in HLB-affected trees. The generally accepted hypothesis on the carbohydrate stress as the primary cause of pre-harvest fruit drop in HLB-affected trees needs further scrutiny.
To conclude, HLB-associated pre-harvest fruit drop is a complex phenomenon. Use of PGRs to reduce HLB-associated pre-harvest fruit drop is not currently being recommended. The current focus in horticultural grove management should be on reducing any type of stress on the tree while intensively managing irrigation and nutrition. Trees displaying less HLB symptoms or less decline tend to have less pre-harvest fruit drop and can hold on to fruit for a longer time.
Tripti Vashisth is an assistant professor, and Lisa Tang is a postdoctoral research associate — both at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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