Effect of Plant Growth Regulators and Fungicides on Huanglongbing-related Preharvest Fruit Drop of Citrus

The severe citrus ( Citrus sp.) disease Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus , has resulted in widespread tree decline in Florida and overall citrus production is now the lowest it has been in 50 years. More than 80% of Florida citrus trees are HLB affected, and most growers attempt to sustain production on infected trees through good asian citrus psyllid ( Diaphorina citri ) control and enhanced fertilization and irrigation management. Although production appears to benefit from these treatments, preharvest fruit drop is considerably greater than on uninfected trees. U.S. Department of Agriculture (USDA) data indicate that Florida statewide fruit drop has increased by 10%to 20%of the entire crop in the last three growing seasons, essentially doubling the historical levels. Extensive research is underway to identify solutions to 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 nonlabeled 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-dichlorophenoxyacetic acid (2,4-D), 1-naphthaleneacetic acid (NAA), S-abscisic acid (S-ABA), aminoethoxyvinylglycine (AVG), and 1-methylcyclopropene (1-MCP) were applied once or twice alone or in some combinations at standard rates to trees in various mature blocks of ‘Valencia’ and ‘Pineapple’ sweet orange ( Citrus sinensis ), ‘Star Ruby’ grapefruit ( Citrus paradisi ), or ‘Murcott’ tangor ( Citrus reticulata X C. sinensis) in central Florida in the Indian River area. Only 1 of the 10 individual trials had treatments with significantly lower drop rates than controls; and when pooled across all experiments, GAD2,4-D reduced number of fruit dropped per tree 4%, but only at P = 0.10.NAA, S-ABA,AVG, and 1-MCP had no effect and were not used the following year. Starting in2014, treatments were initiated earlier in the season with greater effort to minimize variability: GA; 2,4-D;GAD2,4-D; a natural GA, indolebutyric acid, cytokinin mix; and strobilurin fungicides were applied to 22 mature blocks of ‘Hamlin’ and ‘Valencia’ sweet orange trees. In 2014–15, only three of the 11 individual ‘Hamlin’ trials and one of the 11 ‘Valencia’ trials included a treatment with significant drop reduction compared with controls. However, when all the tests on ‘Hamlin’ were pooled, there was a significant 5% reduction in total crop drop for GAD2,4-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% of the total crop (14% drop vs. 16% drop), but fruit drop in ‘Valencia’ blocks was near the historical average in control trees. Soil conditions and tree conditions were similar across all test sites and there were no apparent relationships between product efficacy and observed tree condition or any other grove characteristics. In addition, four ‘Hamlin’ and four ‘Valencia’ blocks were treated with 1/4 rates of 2,4-DD6-benzyladenine 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% in ‘Valencia’ and 6% 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.