By Tripti Vashisth and Christopher Vincent
This article summarizes current knowledge about HLB-associated preharvest fruit drop. Unfortunately, exactly how HLB induces preharvest fruit drop is still unknown. Nonetheless, this article links what we do know about HLB-associated fruit drop to fruit drop in HLB-free trees and how strategies, such as managing tree water status and use of gibberellic acid, can be useful in reducing drop.
HLB-associated fruit drop can be up to 60% of the total yield depending on the disease severity, cultivar and environmental factors. The severity of HLB or tree canopy density directly affects preharvest fruit drop. The greater the HLB symptoms, the higher the drop. In addition, fruit size is affected by HLB symptom severity. Trees with more HLB symptoms produce smaller fruit than trees with mild HLB symptoms. These scientific observations are evident from historical U.S. Department of Agriculture citrus crop data. As HLB became endemic in Florida, an obvious reduction in fruit size and increase in preharvest fruit drop were reported.
It was once suspected that the limited supply of carbohydrates from phloem blockage was the cause of HLB-associated preharvest fruit drop. Recently, three different experiments on Valencia and Hamlin concluded that the fruit likely to drop or that do drop have the same or higher amounts of sugars as the fruit that will likely remain attached to the tree. Therefore, fruit drop cannot be solely attributed to carbohydrate shortage.
FRUIT WEIGHT AND SIZE
In a study focused on characterizing attributes of dropped fruit versus retained fruit, only fruit weight and size were linked to a fruit’s likeliness to drop out of many physical and compositional attributes such as seed number, leaf number, chlorophyll and soluble sugars. Fruit with a smaller diameter or less weight are more likely to drop than larger or heavier fruit in both Hamlin and Valencia, irrespective of overall tree HLB symptoms.
In citrus, fruit size increases rapidly because of water accumulation in cells at stage two of fruit development. Interestingly, differences in the size of developing fruit on mild or severe HLB trees can be observed as early as May (stage two of fruit development).
Figure 1 shows the fruit size measurements in trees with mild and severe HLB symptoms throughout the season. A difference in fruit size between mild and severe HLB trees appears in May, but the actual impact of these subtle differences is not realized until late in the season.
February through June are usually dry months with limited rainfall in Florida. This is when stage one and early stage two of fruit development take place. HLB-symptomatic fruit have been found to have higher accumulation abscisic acid (ABA) than healthy and non-symptomatic fruit. ABA is a plant hormone that is correlated with drought stress, where low water supply and high transpirational demands result in ABA accumulation.
In studies focused on HLB-associated fruit drop, ABA- and ethylene-related genes were upregulated in dropped fruit compared to retained fruit. In citrus, ABA indirectly increases the fruit abscission through promotion of ethylene. In fruit crops including citrus, during low-water availability conditions, ABA accumulation occurs in leaves. In a preliminary study, Valencia fruit that are ready to drop (mature as well as immature ‘June drop’ fruit) had higher ABA content in the peel than the fruit that were retained tightly on the tree.
HLB-affected trees undergo significant root dieback. Feeder root mass can decrease up to 80%, limiting water and nutrient uptake. Lower midday leaf water potential was observed for severe HLB trees in comparison with mild HLB trees of Valencia sweet orange in March. This suggests that water deficiency increases with more severe HLB symptoms, which may be exacerbated during low rainfall periods in Florida.
Remarkably, even in well-watered, healthy citrus, the lowest leaf water potential (highest water deficit) is found around noon. In parallel, mature Valencia fruit are easiest to detach (most likely to drop) and most susceptible to abscission agents around noon. This suggests that water availability to the fruit is linked with fruit drop.
Potential strategies to address fruit drop can be devised based on this background knowledge.
One strategy is to maintain a constant water supply through irrigation. Applying large volumes of water at once is not beneficial for HLB-affected trees as significant portions of water leach out of the root system before the tree can take up the water. Instead, trees should be provided with small amounts of frequent irrigation. The goal is to give trees enough water, so fruit growth and drop is not influenced.
A second strategy is to reduce transpiration. In HLB-free trees in Italy, reflective materials like kaolin and calcium carbonate improved water relations and photosynthesis. This resulted in at least 80 pounds per tree more fruit and reduction in preharvest fruit drop compared to untreated trees.
Plant growth regulators are a third strategy. Foliar applications of gibberellic acid (20 grams active ingredient per acre) on Valencia from September to January have shown promising results in HLB-affected sweet oranges. Generally, gibberellic acid (GA) is antagonistic to ABA and ethylene in plant tissues. GA increases fruit size, delays senescence and reduces fruit drop in healthy citrus.
Figure 2 shows the four-year yield (pounds per tree) from GA-treated and control trees. Overall, with use of GA, the four-year average yield was significantly higher than untreated control trees. GA resulted in 228 pounds per tree compared to 176 pounds per tree in the control.
The GA positively influenced tree productivity in two ways, first by reducing the number of flowers and fruit set, which reduced the competition; and second, by increasing the vegetative growth in HLB-affected trees as compared to the control.
In this trial, control trees experienced a significant decrease (-8.19%) in canopy density. No decrease in canopy density was found in GA-treated trees.
To summarize, HLB-associated fruit drop is related to tree canopy density and limited water availability to the fruit. Practices to improve canopy density have the potential to reduce fruit drop. Use of frequent irrigation scheduling and GA may improve canopy growth and potentially improve yield. However, targeted research should test these hypotheses under field and HLB conditions in Florida.
Tripti Vashisth and Christopher Vincent are assistant professors at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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