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  • Author or Editor: Ed Stover x
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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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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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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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Analysis of apple (Malus×domestica Borkh.) and citrus thinning experiments indicates that the relationships between cropload, fruit size, and total yield can be used to assess optimal cropload for highest crop value. Mean fruit size increased and total yield declined as the cropload (number of fruit/cm2 trunk cross-sectional area) was reduced through the use of chemical thinners. Because crop value is influenced by fruit size and total yield, intermediate croploads gave the highest economic returns in all experiments evaluated. For `Empire' apple, croploads greater than those expected to provide good return bloom often produced the highest crop value for a single year. In citrus, optimal crop values resulted from a broad range of intermediate croploads. A method is described to analyze optimum cropload from thinning experiments.

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Grapefruit are susceptible to melanose from initial set until fruit diam. is 6-7 cm, which can span 3 months. Common Indian River melanose-control practice has been application of Cu fungicides at petal fall, with reapplication every three wks. through the infection period. Research data were previously used to develop a computer model to estimate Cu levels on fruit and indicate when reapplication is needed to prevent potential infection. The purpose of this study was to compare melanose control using spray timings suggested by the computer model vs. standard 3 week intervals vs. non-sprayed checks and was conducted over 3 years in mature grapefruit groves near Ft. Pierce, Fla. All applications were made using airblast at 1180 L· ha-1. Melanose and melanose-like Cu injury could not be distinguished and were combined in a melanose/Cu marking (MCM) score for each fruit. Separate fruit samples from the interior and exterior of tree canopies were randomly selected from each tree. In no year was there a significant difference in interior fruit MCM from computer model vs. calendar spray timings when treated with standard rates of Cu fungicide. However, rainfall never occurred when calendar-sprayed fruit were projected to be at low Cu levels. In 2 of 3 yrs. exterior fruit in the non-sprayed checks had less MCM than those from trees treated with Cu, indicating that Cu injury predominated over melanose on exterior fruit. In these fruit, MCM increased linearly with maximum fruit Cu concentration, which was lower on trees managed using the computer model. The computer model appears to be a sound approach to managing melanose, but economic benefit over calendar-based spray timing may only become apparent when practiced over numerous groves and seasons.

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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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Pesticide spray practices for citrus (Citrus spp.) in the Indian River region of Florida were surveyed in 2001 as the first step in identifying opportunities for improving efficiency and reducing potential environmental impact. The survey covered 73% of grapefruit (C. paradisi) acreage in Indian River, St. Lucie, Martin and Palm Beach counties, comprising 70% of all Indian River commercial grapefruit. Large differences in spray practices were revealed. The focus of this survey was grapefruit spraying, since grapefruit represent 59% of fresh citrus shipped from the Indian River region, and are sprayed more intensively than citrus fruit grown for processing. In commercial groves, almost all foliar sprays to grapefruit are applied using air-assisted sprayers pulled through the groves by tractors. Use of engine-driven and power-takeoff-driven sprayers were reported with equal frequency and accounted for 89% of spray machines used. Lowvolume Curtec sprayers comprised the remainder. Spray volume for grape-fruit varied: 7.6% of acreage was sprayed at 25 to 35 gal/acre (230 to 330 L·ha-1) for all sprays; 4.2% was sprayed at 100 to 170 gal/acre (940 to 1600 L·ha-1) for all sprays; 15.3% was sprayed at 200 to 380 gal/acre (1900 to 3600 L·ha-1) for all sprays; 28.2% was sprayed at 450 to 750 gal/acre (4200 to 7000 L·ha-1) for all sprays; and 44.5% of grapefruit acreage was sprayed in a progressive manner from lower to higher volume as the season progresses. Many mid and high spray volume growers reported unacceptable results when they lowered spray volume. Although correlation was moderate (r = 0.35 to 0.45), regressions indicated that both total foliar pesticide spray material costs, and annual fungicidal copper (Cu) use increased with spray volume used for postbloom fungicides. Mean Cu use per acre was in the middle of the recommended range. All growers reported adjusting nozzling for tree height within a grove, and since Indian River groves are bedded, growers adjusted sprayer output differently for trees on bed tops versus furrows on 85% of acreage. Sprayers were shut off for missing trees on 83% of acreage, but this was done only for two or more adjacent trees on almost half of this area. Sensor-actuated sprayers were used to minimize off-target application on 14.7% of grapefruit acreage, but for an additional 21% of acreage, growers reported trying and abandoning this technology. While 88% of grove acreage was sprayed during the day, 75% of acreage sprayed using less than 100 gal/acre was sprayed at night. Growers reported no defined protocol for ceasing spray operations based on environmental conditions.

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