New Research on HLB-Associated Fruit Drop

Ashley RobinsonFruit Drop

Tripti Vashisth revealed new research information on what causes preharvest fruit drop and possible strategies to mitigate the problem.

The Florida citrus industry, stricken by HLB, has suffered a sharp increase in preharvest fruit drop, leading to a substantial reduction in citrus production. According to Vashisth, assistant professor of horticulture at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center, excessive premature fruit drop is associated with HLB. There is no single cause for the fruit to drop, but rather a multitude of factors.

HLB-triggered events that lead up to abscission actually occur earlier in the preharvest period than many growers anticipate, Vashisth said during a recent UF/IFAS virtual OJ Break meeting.

Originally, many believed that fruit drop was caused by a lack of carbohydrates in the fruit. But after a preliminary study, Vashisth discovered that increased drop is not associated with carbohydrate starvation of the fruit at all. Instead, premature fruit drop in trees displaying HLB symptoms could be attributed to increased oxidative stress due to disease.

Trees exhibiting severe symptoms of HLB had a significantly higher rate of fruit drop compared to trees which were only mildly symptomatic. Furthermore, small fruit seemed to drop more compared to larger fruit, suggesting HLB’s negative effect on fruit growth.

“Small-sized fruit are more likely to drop,” Vashisth said, adding that practices promoting a healthy tree canopy result in less fruit drop.

According to Vashisth, altered tree water status due to HLB may also limit fruit growth during the initial stage of fruit development, immediately after flowering. This led to an increased incidence of mature fruit abscission, resulting in elevated preharvest fruit drop.

To minimize fruit drop, Vashisth said growers should focus on their irrigation and nutrient management plans. In particular, to prevent water stress, growers should develop an irrigation management plan beginning in March or April. Water stress reduces the number of fruits and fruit size — and increases the likelihood of premature fruit drop.

For optimal results, Vashisth recommended irrigating in small and frequent doses to “spoon-feed” the tree. She also recommended applying gibberellic acid during the fruit-development period.

“Use of gibberellic acid does hold potential to improve fruit set and fruit retention. We have multiple years of data to prove that,” Vashisth said. More research on gibberellic acid is underway, including timing of application.

View Vashisth’s presentation here.

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Ashley Robinson

Multimedia journalist