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Ed Stover

The relationship between intensity of flowering and various aspects of cropping are reviewed for fruit species. Relatively light flowering can limit yield in most fruit species. This commonly occurs in young trees that have not achieved full production and in “off” years for varieties that display alternate bearing. When trees mature, many species will carry fruit numbers that exceed commercially desired levels, resulting in excessively small fruit and accentuating alternate bearing. The economic disadvantages of excess cropload have resulted in considerable research on fruit thinning and widespread commercial application of this practice. Heavy flowering intensity in some crop species results in economic disadvantages beyond the problems of excessive cropload and resultant small fruit size. Many species flower profusely and have initial fruit set that greatly exceeds tree capacity, resulting in abscission of numerous flowers and fruitlets. Abscised organs can represent a substantial amount of carbohydrates and nutrients, compromising availability of these materials at critical periods in flower and fruit development. The potential implications of this process are best exemplified in `Navel' orange [Citrus sinensis (L.) Osbeck], where an increase in flowering beyond intermediate intensity results in a reduction in both initial fruit set and final fruit yield at harvest. In several species, there is evidence that fruit size may be reduced by excessive flowering, even when cropload is quickly adjusted to an acceptable level. These data suggest that further research on the advantages of controlling flowering intensity is warranted.

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Ed Stover and Greg McCollum

Incidence and severity of Huanglongbing (HLB) disease were assessed in Apr. 2010 among eight citrus cultivars representing diverse scion types growing in commercial groves in Florida's Indian River region, an area with a high incidence of HLB. In each grove, 20 trees of each cultivar were rated for visual HLB symptoms and leaves were collected for quantitative polymerase chain reaction quantification of Candidatus Liberibacter asiaticus (CLas), the presumptive causal agent of HLB. There was a strong correlation between HLB rating and CLas titer (titer represented by Ct, r 2 = 0.37 and 0.40, for whole tree and leaf sample, respectively, both with P < 0.0001) across all cultivars and groves. Although incidence and severity of HLB varied considerably among the groves, scion-specific differences were apparent, even when analyses excluded potentially confounding grove effects. ‘Temple’ tangor showed the most consistently low incidence of HLB symptoms and CLas titer; in contrast, ‘Murcott’ tangor and ‘Minneola’ tangelo had the highest incidence of HLB symptoms and highest CLas titer. These results suggest useful resistance to HLB with reduced symptoms and reduced CLas titer may be found in conventional scion cultivars and further work is needed to assess this potential and its commercial value.

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Ed Stover and William Castle

Sour orange (Citrus aurantium) has been the dominant citrus rootstock in the Indian River region of Florida since the initial plantings in the 1880s. Use of this rootstock in new plantings has been rare since 1990 because of heightened concern about decline strains of citrus tristeza virus (CTV), to which this rootstock is highly susceptible. Because the proportion of trees remaining on sour orange rootstock and the rate of decline among them are important in predicting the economic consequences for the Indian River citrus industry, two surveys of rootstock usage were conducted for citrus in this growing region. In the first survey, growers were asked about rootstock usage and problems observed across all types of citrus, and responses represented 35% of acreage. In the second survey, growers were restricted to rootstock usage and grower observations on decline for grapefruit (C. paradisi), and responses represented 53% of acreage. Even though 44% of all current Indian River grove area has been planted since 1987, when use of sour orange for new plantings largely ceased, 48% of all citrus and 55% of all grapefruit grove area are currently on sour orange rootstock. The percentage of grapefruit trees on sour orange rootstock that showed significantly health decline in 2000 was 8% based on grower reports. The other root-stocks representing substantial commercial grove area have known problems and limitations that are likely to prevent any of them from gaining the prominence once held by sour orange. Swingle citrumelo (C. paradisi × Poncirus trifoliata) at about 25% of grove area, Cleopatra mandarin (Citrus reticulata) at about 8%, and Smooth Flat Seville (Citrus hybrid) at about 3% all represented similar acreage for grapefruit and across all cultivars, while Carrizo citrange (C. sinensis × P. trifoliata) use was reported for 4% of grapefruit and 13% overall. Evaluation and development of new rootstocks is vitally important for the Indian River area, especially for soils with significant clay and calcium content.

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Ed Stover and Eric W. Mercure

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Kim E. Hummer and Ed Stover

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Ed Stover, Scott Ciliento, and Monty Myers

In spring 1999, a commercial NAA (1-naphthaleneacetic acid) preparation for trunk sprout inhibition was compared with a corrugated plastic trunk wrap, aluminum foil wrap, bimonthly hand removal of sprouts, use of NAA preparation plus bimonthly hand removal when sprouts appeared, and a nontreated control. Three recently planted groves on three different rootstocks [`Midsweet' orange (Citrus sinensis)] on Swingle citrumelo (Citrus paradisi × Poncirus trifoliata), `Valencia' orange on Volkamer lemon (Volk, Citrus limon), and `Minneola' tangelo (Citrus paradisi × C. reticulata) on Smooth Flat Seville (SFS, Citrus hybrid) received each of the treatments in a randomized complete block experimental design with trees blocked by initial height and circumference. Every 2 months, sprouts were counted on each tree and removed from the hand removal treatments. After 1 year, all sprouts were removed and counted and height and circumference of trees was determined. Across all experiments, 82% to 100% of nontreated trees produced trunk sprouts and all sprout control methods significantly reduced sprouts per tree. NAA treatments were never significantly less effective at sprout suppression than the wraps at the P = 0.05 level, although in two experiments, wraps were more effective than NAA at P = 0.10. Time of sprout appearance varied between the three experimental blocks. Plastic and foil trunk wraps enhanced development of trunk circumference compared with nontreated controls in `Midsweet'/Swingle and `Valencia'/Volk. Greater trunk circumference resulted from use of wraps versus NAA in all three experiments, which appeared unrelated to differential sprout suppression. In these experiments, it appears that either wraps enhanced tree development beyond the suppression of sprouts or NAA influence on tree metabolism somewhat reduced trunk growth. The economics of the sprout suppression methods are also discussed.

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Leo G. Albrigo and Ed W. Stover

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 ×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, GA + 2,4-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 in 2014, treatments were initiated earlier in the season with greater effort to minimize variability: GA; 2,4-D; GA + 2,4-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 GA + 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% 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-D + 6-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.

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Chris Wilson, Ed Stover, and Brian Boman

Off-target deposition of pesticidal spray material is both an economic loss to the grower and a potential environmental problem in southern Florida. This study evaluated the reduction in non-target deposition of copper resulting from different approaches to spraying row-ends in typical Indian River citrus (Citrus) production systems. Using copper as a model pesticide, applications were made in a commercial citrus grove in June and July 2001. Non-target deposition on the water surface within an adjacent drainage canal, as well as on surrounding ground surfaces, was measured using Teflon spray targets. Specific row-end spraying scenarios included: 1) leaving both banks of nozzles on while turning; 2) turning the outside-facing nozzles off (leaving tree-facing nozzles on); 3) turning both banks of nozzles off at the tree trunk; and 4) turning all nozzles off at the end of the foliage of the last tree within the row. Deposition directly onto surface water contained within drainage canals was reduced significantly when nozzles were turned off at the last tree within a row, or when the outside-facing nozzles-only were turned off through the turn. Likewise, deposition was reduced on ground surfaces adjacent to the sprayer under the same scenarios. No differences were observed on ground surfaces on the opposite side of the canal. Significant reductions in direct application of agrichemicals to surface waters within Indian River citrus production groves can be achieved by turning nozzles off when turning from one tree row into the next.