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Citrus hybrids USDA 17-11 [Citrus grandis L. × (C. paradisi Macf. `Duncan' × Poncirus trifoliata (L.) Raf. `Gotha Road')] and 119 [(C. paradisi Macf. `Duncan' × P. trifoliata (L.) Raf. `Gotha Road') × C. sinensis (L.) Osb. `Succory'], `Hamlin' orange [C. sinensis (L.) Osb.], and satsuma mandarin (C. unshiu Marc.) were planted March 1993 and 1994. Trees were irrigated and fertilized in an identical manner. In 1993, electrolyte leakage readings were taken monthly using 17-11, 119, and satsuma leaf discs. Leaf killing point (LKP) LT₅₀ averaged from –8 to – 9C by mid-November for all selections. In 1994, leaf discs from 17-11, 119, and `Hamlin' orange were sampled weekly to determine LKP. USDA 119 had the lowest LKP and acclimated the fastest during the fall. By the end of November, there was no significant difference in LKP (–6.5C) between USDA 119 and 17-11, although both selections were significantly more freeze-tolerant than `Hamlin' orange (LKP–40C), which showed no significant decrease in LKP until the 6 weeks after the hybrid selections began acclimating. Citrus hybrids 17-11 and 119 can survive in freeze-susceptible areas that are marginal for other commercial citrus.

`Hamlin' orange trees [C. sinensis (L.) Osb.] from a commercial nursery were planted into raised beds on a site that simulated conditions typical of the flatwoods region of the citrus industry. A factorial experiment with three irrigation schedules, based on growth flushes and three nutrient application frequencies (total N, 0.136 kg/tree per year), was conducted in 1994. Trees were irrigated using 90° microsprinklers, and soil moisture content was monitored using a neutron probe. Eleven replicate trees of the nine treatments were included in a completely randomized block design. Weekly freeze tests using the electrolyte leakage method were conducted at –4, –6, and –8C. Electrolyte leakage was determined using a conductivity meter. Different irrigation scheduling based on growth flushes had no significant effect on freezing acclimation. However, increased frequency and lower amounts of fertilizer per application significantly (P = 0.05) increased freeze hardiness from 4.2 to –6.10C by the end of November. Morphological data including trunk diameter, tree height, and flushing status also were recorded. Increasing frequency of nutrient application resulted in a more rapid acclimation of young `Hamlin' orange trees.

Two USDA intergeneric, hybrid citrus scions, US 119 {[Citrus paradisi Mac. `Duncan' × Poncirus trifoliata (L.) Raf.] × C. sinensis Osb. `Succory'} and selection 17-11 {C. grandis US 145 × [Citrus paradisi Mac. `Duncan' × P. trifoliata (L.) Raf.]} on `Swingle' citrumelo (C. paradisi × P. trifoliata) rootstocks were examined for freeze hardiness traits (4 years) and general growth characteristics (2 years). Hardiness was compared with that of `Hamlin' orange [C. sinensis (L.) Osb.] and satsuma mandarin (C. unshiu Marc) from Fall 1993 to Spring 1997. As expected, US 119 and 17-11 were both hardier than `Hamlin' orange as determined by leaf disc electrolyte leakage (EL). Both showed freezing tolerance similar to that of satsuma mandarin, but 17-11 was significantly hardier than satsuma or US 119 at several times during the 4-year study. Trunk diameter and tree height were similar for US 119 and selection 17-11.

Pruning and skirting (removal of low-hanging limbs) effects on canopy temperature, relative humidity (RH), and fruit yield and quality of `Orlando' tangelo trees (Citrus paradisi Macf. × Citrus reticulata Blanco) on `Carrizo' citrange rootstock [Poncirus trifoliata (L.) Raf. × Citrus sinensis (L.) Osb.] were studied at the Univ. of Florida Fifield Farm in Gainesville, Fla., in 1996–97. In the first season, treatments consisted of skirted and non-skirted trees. In the second season, two skirting (skirted and non-skirted) and three pruning (gable-top, flat-top, and non-pruned) treatments were evaluated. Neither RH nor air temperature was affected in the lower canopy by any treatment. However, temperature in the upper canopy of flat-topped trees was higher than that in gable-topped or non-pruned trees, and reached >45 °C during spring and summer. Fruit number and yield were decreased by pruning and skirting in one season. Skirted, gable-topped trees had the lowest yields, followed by skirted, flat-topped and non-skirted, gable-topped trees. All other treatments produced yields similar to those of non-skirted, non-pruned trees. Pruning increased the percentage of large fruit and reduced the percentage of small fruit. Skirting and pruning had no effect on blemish incidence with the exception of wind scar, which was higher in skirted than in non-skirted trees in the first season. During both seasons the main causes of packout reduction were rust mite and wind scar damage. Regardless of treatment, rust mite damage was much higher in the lower than in the upper canopy because of lower average temperatures and higher RH. Pruning effects on fruit quality were similar to those reported previously, but skirting had no effect on most fruit quality factors.

Our objectives were to determine if leaf N concentration in citrus nursery trees affected subsequent growth responses to fertilization for the first 2 years after planting and how N fertilizer rate affected soil nitrate-N concentration. `Hamlin' orange [Citrus sinensis (L.) Osb.] trees on `Swingle' citrumelo rootstock [C. paradisi Macf. × P. trifoliata (L.) Raf.] were purchased from commercial nurseries and grown in the greenhouse at differing N rates. Three to five months later trees were separated into three groups (low, medium, high) based on leaf N concentration and planted in the field in Oct. 1992 (Expt. 1) or Apr. 1993 (Expt. 2). Trees were fertilized with granular material (8N–2.6P–6.6K) with N at 0 to 0.34 kg/tree yearly. Soil nitrate-N levels were also determined in Expt. 2. Preplant leaf N concentration in the nursery varied from 1.4% to 4.1% but had no effect on trunk diameter, height, shoot growth, and number or dry weight in year 1 (Expt. 1) or years 1 and 2 (Expt. 2) in the field. Similarly, N fertilizer rate had no effect on growth during year 1 in the field. However, trunk diameter increased with increasing N rate in year 2 and reached a maximum with N at 0.17 kg/tree yearly. Shoot number during the second growth flush in year 2 was much lower for nonfertilized vs. fertilized trees. Leaf N concentrations increased during the season for trees with initially low levels even for trees receiving low fertilizer rates. Soil nitrate-N levels were highest at the 0.34-kg rate, and lowest at the 0.11-kg rate. Nitrate-N levels decreased rapidly in the root zone within 2 to 3 weeks of fertilizing.

EcoLyst, a formulation of N-N-diethyl-2-(4-methylbenzyloxy) ethylamine hydrochloride containing 1 g/floz [4.5 oz/gal (33.8 g·L^-1)] a.i., is a plant growth regulator that has been reported to increase soluble solids concentration (SSC) in juice oranges by 0.6% to 1.2%. Our objectives were to determine the effectiveness of EcoLyst application for increasing SSC in Florida oranges (Citrus sinensis) and grapefruit (C. paradisi), and to identify the optimum rate and time of application. Experiments were conducted for three seasons using `Hamlin,' `Pineapple,' and `Valencia' sweet oranges; and for two seasons using `Flame,' `Marsh,' and `Ray Ruby' grapefruit, all in commercial groves. EcoLyst was applied at 6 and 12 floz/acre (0.44 and 0.88 L·ha^-1) for oranges and 16 and 32 ppm (mg·L^-1) [effectively 9 and 18 floz/acre (0.66 and 1.32 L·ha^-1) in most sprays] for grapefruit, and included Silwet L-77 adjuvant at 0.05%. Applications were made at several stages of development from prebloom to initial fruit set. In all cases, SSC was determined as juice corrected SSC, by adjusting refractometer readings based on titratable acidity. In 13 trials with sweet orange only five displayed significant increases in SSC (P ≤ 0.05) resulting from EcoLyst application. Two additional trials produced SSC increases significant at P < 0.10. Even where significant increases in SSC occurred they were typically observed in only one harvest and at one time of application and were always relatively low in magnitude (highest increase over controls was 0.38%). No rate or timing of EcoLyst application was consistently associated with best response, although eight of nine SSC increases observed in orange occurred with applications ranging from prebloom to 25% open flowers. Only one significant increase in SSC was observed in five trials with grapefruit. In these studies, increases in SSC resulting from EcoLyst application were neither sufficiently consistent nor large enough to justify a recommendation for commercial use in Florida citrus.

Our objectives were to determine the effects of leaf N concentration in citrus nursery trees on subsequent growth responses to fertilization for the first 2 years after planting and the impact of N fertilizer rate on soil NO₃-N concentration. `Hamlin' orange [Citrus sinensis (L.) Osb.] trees on `Swingle' citrumelo rootstock [C. paradisi Macf. × P. trifoliata (L.) Raf.] were purchased from commercial nurseries in Apr. 1992 (Expt. 1) and Jan. 1993 (Expt. 2) and were grown in the greenhouse at differing N rates. Five months later, trees for each experiment were separated into three groups (low, medium, and high) based on leaf N concentration and were planted in the field in Oct. 1992 (Expt. 1) or Apr. 1993 (Expt. 2). Trees were fertilized with granular material (8N-2.6P-6.6K-2Mg-0.2Mn-0.12Cu-0.27Zn-1.78Fe) with N at 0, 0.11, 0.17, 0.23, 0.28, or 0.34 kg/tree per year. Soil NO₃-N levels were determined at 0- to 15- and 16- to 30-cm depths for the 0.11-, 0.23-, and 0.34-kg rates over the first two seasons in Expt. 2. Preplant leaf N concentration in the nursery varied from 1.4% (Expt. 1) to 4.1% (Expt. 2) but had no effect on trunk diameter, height, shoot growth and number, or dry weight in year 1 (Expt. 1) or years 1 and 2 (Expt. 2) in the field. Similarly, fertilizer rate in the field had no effect on growth during year 1 in the field. However, trunk diameter increased with increasing N rate in year 2 and reached a maximum with N at 0.17 kg/tree per year but decreased at higher rates. Shoot number during the second growth flush in year 2 was much lower for nonfertilized vs. fertilized trees at all rates, which had similar shoot numbers. Nevertheless, leaf N concentrations increased during the season for trees with initially low levels, even for trees receiving low fertilizer rates. This suggests translocation of N from other organs to leaves. Soil NO₃-N levels were highest for the 0.34-kg rate and lowest at the 0.11-kg rate. Within 2 to 3 weeks of fertilizing, NO₃-N levels decreased rapidly in the root zone.

One-year-old `Hamlin' orange [Citrus sinensis (L.) Osb.] trees on sour orange rootstock (C. aurantium L.) were used to compare various fertigation frequencies and rates with application of granular materials. In Expt. 1, granular fertilizer was applied five times per year or liquid fertilizer was applied five, 10, or 30 times per year at 0.23 kg N/tree per year as an 8N-3.4P-6.6K formulation. In Expt. 2, an additional treatment of granular and liquid material was applied three times per year, but fertilizer rate and formulation were the same as in Expt. 1. Experiment 3 included the same application frequencies as Expt. 1, but with two rates of N (0.11 or 0.06 kg N/tree per year). Soil samples were taken from each treatment 1, 4, and 7 days after fertilization at depths of 0-15, 16-46, and 47-76 cm for nutrient analyses. Trunk diameter, shoot growth, and tree height were similar for all treatments 8 months after planting in Expts. 1 and 2, while trees in Expt. 3 had significantly less growth at the lower rate. Soil NH₄-N and NO₃-N concentrations for all liquid treatments within 1 week of fertilization were highest for the five times per year treatment at the 0- to 15-cm depth, but nutrient concentrations of all liquid treatments were similar at the other depths. For most dates and depths, NH₄-N and NO₃-N concentrations were similar for both fertilizer rates.

Gibberellic acid (GA₃) increases juice yield of processing oranges, but results are inconsistent. Preliminary research suggested that this variability might be related to application timing. Therefore, we conducted an experiment to determine the optimal time to apply GA₃ for increasing juice yield of `Hamlin', `Pineapple', and `Valencia' sweet oranges [Citrus sinensis (L.) Osb.]. Mature trees of each cultivar were sprayed with ≈10 L of a solution of GA<sub>3</sub> (45 g·ha<sup>-1</sup> a.i.) and organo-silicone surfactant (Silwet, 0.05%) between 2 Sept. and 9 Dec. 1998, and 25 Sept. and 9 Dec. 1999, or remained non-sprayed (control). Generally, the earliest application dates were most effective at maintaining peel puncture resistance above that of control fruit, while the latest application dates resulted in the most green peel color at harvest. Juice yield of `Hamlin' and `Valencia', but not `Pineapple', was increased by GA₃ at some application timings and harvest dates in both years. The increase in juice yield was related to time between application and harvest; juice yield of `Hamlin' was greatest ≈2 months, and `Valencia' ≈5 months after GA₃ application. Treated fruit often had lower juice Brix than non-sprayed fruit, a phenomenon that often paralleled treatment effects on peel color. When treatments did not increase juice yield but reduced juice Brix, then yield of solids was sometimes lower than for non-treated fruit. Treatments generally delayed flowering of `Pineapple' and `Valencia' but not `Hamlin'.

An experiment was designed to determine the effects of canal water and reclaimed wastewater on growth, yield, and fruit quality of mature (25-year-old) `Redblush' grapefruit (Citrus paradisi Macf.) trees on sour orange (C. aurantium L.) rootstock. The study was conducted from 1 Oct. 1990 to 18 Apr. 1994 at a site adjacent to the Indian River County municipal wastewater treatment facility located near Vero Beach, Fla. Treatments included canal water applied based on one-third or two-thirds soil water depletion and reclaimed wastewater applied using microsprinklers at 23.1 mm/week (low), 30.7 mm/week (moderate) and 38.6 mm/week (high). Trees receiving low and moderate levels of reclaimed wastewater had the largest canopies and trunk diameters and highest yields, even though the amount of fertilizer applied was less than that of canal water plots. Leaf nutrient levels were generally within acceptable ranges for N, P, K, Ca, Mg, and Na except in 1991 when levels were deficient due to excessive rainfall and leaching. Leaf B levels were similar for all reclaimed wastewater treatments but were lower for the canal water treatment in 1992 and 1993. Fruit growth rate, fruit and juice weight, total soluble solids (TSS), titratable acidity (TA), and TSS: TA ratio were similar for all treatments in 2 of 3 years. Peel thickness was similar for all treatments. Heavy metal concentration in the reclaimed wastewater was at low or nondetectable levels. Similarly, enteric viruses in the effluent were always <0.003 plaque forming units/liter. Reclaimed wastewater irrigation significantly increased weed growth compared to the canal water treatment.