University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) citrus scientists Tripti Vashisth and Fernando Alferez are trying to reduce the amount of fruit drop from Hamlin and Valencia trees prior to harvest. If fruit drops, growers can’t harvest it, and that leads to losses for farmers and less fruit at the market for consumers.
Normally, about 10 to 15 percent of citrus fruit drops from a healthy tree, said Alferez. With citrus greening now prevalent throughout Florida’s citrus growing regions, scientists and growers have seen preharvest fruit drop increase in the last decade, he said. In the 2012–13 season, researchers and citrus farmers noticed about 25 percent of their crop lost because of greening-associated preharvest fruit drop. Now, depending on the citrus variety, growers may see preharvest drop up to 50 percent.
The two UF/IFAS scientists are studying many fruit-drop issues. For example, they’re looking into the effect of removing fruitlets — fruit that’s less than 2 centimeters in diameter — from so-called “off-blooms.”
“Typically, healthy citrus trees bloom early in spring, and they may have another wave of flowers later in the spring, but it is atypical for the tree to bloom in late summer,” said Alferez. Those would be an example of “off-bloom” fruit.
Preliminary experiments from the two UF/IFAS researchers show that removing fruitlets from off-blooms when Hamlin oranges are mature results in more mature fruit hanging onto citrus trees — as opposed to dropping — Alferez said. This may increase yield because the mature fruit drops less.
Valencia oranges are more complex, as several blooms occur during the last part of fruit maturation, he said.
“Also, we have found that fruit drop is associated with fruit development,” said Alferez, an assistant professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee. “Small fruit are more likely to drop.”
Another preliminary finding is that insufficient carbohydrates do not cause fruit drop. Therefore, the researchers recommend growers use good irrigation and nutrition practices during the fruit-development period. Vashisth and Alferez also say if growers apply gibberellic acid during the fruit-development period, it can help reduce fruit drop.
“Our preliminary findings are important because they show that the fruit drop related to HLB is not a simple process,” said Vashisth, an assistant professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred. “There is no single cause for the fruit to drop. There have to be more causes, and that is what we are trying to understand now.”
In healthy trees, competition for resources between mature fruit and fruitlets causes some ready-to-harvest fruit to drop.
Citrus greening disease — known as huanglongbing (HLB) — may contribute to fruit dropping from trees.
“In general, trees displaying fewer HLB symptoms or that are relatively healthy tend to have less preharvest fruit drop and can hold on to fruit for a longer time,” said Vashisth.
Usually, off-blooms are not abundant; they’re only on a few branches. However, off-blooms may affect tree performance if the tree is sick with HLB.
“In Hamlin and Valencia oranges, we have observed several off-blooms resulting in new fruit as the current crop is already developing,” Vashisth said. “This is normal in healthy trees growing in tropical or subtropical regions, such as Florida. However, in an HLB-affected tree, this may compromise the tree’s ability to keep fruit. It may also reduce yield, because the new fruitlets and mature citrus compete for carbohydrates.”
This research was made possible by state legislative funding for the UF/IFAS Citrus Initiative and UF/IFAS new faculty funding.