Skip to main content
ARS Home » Research » Publications at this Location » Publication #320619
Title: Effect of PGRs and fungicides on huanglongbing-related preharvest fruit drop of citrus

Author
item ALBRIGO, GENE - University Of Florida
item Stover, Eddie

Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2015
Publication Date: 12/1/2015
Citation: Albrigo, G., Stover, E.W. 2015. Effect of PGRs and fungicides on huanglongbing-related preharvest fruit drop of citrus. HortTechnology. 25:785-790.

Interpretive Summary: The severe citrus disease huanglongbing (HLB) is now widespread in Florida and overall citrus production is now the lowest it has been in 50 years. Fruit drop associated with the disease has increased, further reducing citrus production. Florida statewide citrus fruit drop has increased by 10 to 20 percent of the entire crop in the last three growing seasons. We evaluated several synthetic plant hormones (PGRs) treatments for their effectiveness in reducing fruit drop and several saved about 5 percent of the total crop. Based on results of the research, at this time we cannot recommend any of the specific PGR treatments we studied for reducing fruit drop related to HLB. Modified or additional PGR treatments could be explored.

Technical Abstract: The severe citrus disease Huanglongbing (HLB) in Florida has resulted in widespread tree decline, and overall citrus production is now the lowest it has been in 50 years. More than 80 percent of Florida citrus trees are huanglongbing-affected, and most growers attempt to sustain production on infected trees through good psyllid control and enhanced fertilization and irrigation management. While production appears to benefit from these treatments, preharvest fruit drop is considerably greater than on uninfected trees. USDA-NASS data indicates that Florida statewide fruit drop has increased by 10 to 20 percent of the entire crop in the last three growing seasons, essentially doubling historical levels. Extensive research is underway to identify solutions to huanglongbing (HLB), but it is essential to maintain production on existing trees to sustain the industry in the near term. For decades, several plant growth regulators (PGRs) have been labeled to reduce preharvest fruit drop in commercial citrus. Trials of these materials, other non-labelled PGRs, and some fungicides were conducted in two seasons to determine if fruit drop could be reduced. Randomized complete block design experiments were established using four to six replications of four- to six-tree groups as experimental units, blocked spatially. In 2013-14 sprays of gibberellic acid (GA), 2,4-D, NAA, S-ABA, AVG, and 1-MCP were applied once or twice alone or in some combinations at standard rates to various mature blocks of ‘Valencia’, ‘Pineapple’, ‘Star Ruby’, or ‘Murcott’ trees in Central Florida in the Indian River area. Only 1 of 10 individual trials had treatments with significantly lower drop rates than controls; and when pooled across all experiments, GA + 2,4-D reduced number of fruit dropped per tree 4% but only at the p = 0.10 level. Starting in 2014, treatments were initiated earlier in the season with greater effort to minimize variability: GA, 2,4-D, GA + 2,4-D, Ascend, and Headline were applied to various mature blocks of ‘Hamlin’ and ‘Valencia’ orange trees. In 2014-15 only 3 of 11 individual ‘Hamlin’ trials and 1 of 11 ‘Valencia’ trials included a treatment with significant drop reduction compared to controls. However, when all the tests on ‘Hamlin’ were pooled, there was a significant 5 percent reduction in total crop drop for GA plus 2,4-D and significant reductions with many of these PGRs alone but in only one case with fungicide treatments. When all tests on ‘Valencia’ were combined, 2,4-D reduced drop significantly but only by 2 percent of the total crop (14 percent drop versus 16 percent drop), but fruit drop in ‘Valencia’ blocks was near the historical average in control trees. There were no apparent relationships between observed tree condition nor other grove characteristics, and whether a product worked or not. Four ‘Hamlin’ and four ‘Valencia’ blocks were treated with ¼ rates of 2,4-D (Citrus Fix) plus benzyladenine (MaxCel) every 45 days during the growing season (six sprays) and three of the eight individual trials showed significant reductions in drop: when pooled, these treatments reduced drop by 3 percent in ‘Valencia’ and 6 percent in ‘Hamlin’. At this time, PGRs cannot be recommended as a consistent way to reduce fruit drop related to HLB, but further work needs to be conducted to refine the most promising treatments.