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  • Author or Editor: Gerard Krewer x
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Rabbiteye blueberry (Vaccinium ashei R.) flowers often suffer slight freeze damage that prevents fertilization and fruit development. To determine if gibberellic acid (GA3) might be useful in rescuing freeze-damaged flowers the following treatments were applied before anthesis to two cultivars at different locations: 1) undamaged control, 2) approximately two-thirds of the corolla and most of the style removed, 3) approximately half of the style removed, and 4) ovules lanced with an insect pin by driving it through the equator of the undeveloped berry until the point came out the other side. Half the bushes were not sprayed, and half were sprayed with GA3 (312 ppm, v/v) the night following treatment. `Climax' at Chula, Ga., had good fruit set for treatment 1 with and without GA3 (70% to 85%). Good fruit set also occurred for treatment 2, 3, and 4 where GA3 was applied (47% to 54%), but poor fruit set without GA3 (4% to 16%). `Tifblue' at Chula had significantly better fruit set for treatment 1 with GA3 (54% vs. 27%). Excellent fruit set occurred for treatment 2, 3, and 4 where GA3 was applied (81% to 96%), and poor fruit set without GA3 (6% to 7%). `Tifblue' fruit set by GA3 sized better than `Climax' fruit set by GA3. The experiments provide corroborative evidence that flowers that have suffered freeze damage to the stigma, style, corolla, and perhaps ovules can be set with GA3.

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Recent research in Georgia indicated gibberellic acid (GA3) could possibly be used to induce fruit set of freeze damaged rabbiteye blueberry (Vaccinium ashei) blooms. This research was conducted to determine the subfreezing temperature limit at which GA3 could be expected to be of use in salvaging a crop with freeze-damaged flowers. Rabbiteye blueberries with flower buds at stages 5 to 6 of development (fully elongated corollas and open blooms) were exposed to temperatures of 0, –1, –3, and –4.5°C in growth chambers to simulate overnight freezing events. After cold exposure, plants were placed in a greenhouse with a hive of bumblebees. Half of the plants were treated with GA3 and half were not. The number of flowers and subsequent fruit were recorded in order to calculate fruit set. Temperatures of –1°C and below caused fruit set resulting from pollination by bees to decline compared to control plants; whereas, flowers treated with GA3 had fruit set comparable to control plants down to –3°C. Plants exposed to –3°C had 50% to 80% fruit set when treated with GA3 compared to 5% to 19% fruit set for untreated plants. Temperatures of –4.5°C caused severe flower damage, and fruit set by pollination or GA3 was very poor (<2%). These results indicate that GA3 should be useful in salvaging a blueberry crop exposed to temperatures of – 1 to –3.5°C during bloom.

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Southeastern peach and pecan orchards weathered hurricanes in the 1980s and 1990s that left long-term effects on tree health and productivity. Pecan trees were affected the most, due to being blown down from strong winds and wet soils or suffering considerable damage to branches and immature nuts resulting in massive nut drops. Premature nut drop triggered or enhanced alternate bearing problems. Cultivar differences were evident in the ability of trees to withstand wind damage, with open-canopy trees being most resistant, but essentially all trees were damaged when they exceeded ≈17m in height. Hurricanes in older, alternate-bearing orchards sometimes broke enough limbs to induce sufficient vegetative regrowth to reestablish an equilibrium between sink (nuts) and source (foliage), thus enhancing yields in subsequent years. Peach trees which were less than 4.5 m tall and already harvested usually did not blow over unless the soil was very wet. However, peach trees were often twisted about the tree axis from the change in wind directions as the hurricane passed over. Afterwards, many trees leaned more than 30 °, especially trees less than 6 to 7 years of age. Root damage was significant and increased when trees were manually repositioned as additional root breakage occured from which these trees often later died. Trees not repositioned but instead retrained to vertical by pruning lived longer. Ambrosia beetles also attacked wind-stressed trees and caused a long-term decline. Slow moving hurricanes significantly damaged peach trees by waterlogging the soil, which killed roots and helped primary pathogens such as Phytophthora sp. to attack the tree crown. This was followed by secondary pathogens like Oxyporous sp., which attacked the internal woody cylinder. Initial trunk damage appeared localized; however, trees continued to die over a number of years. Experience showed that whole orchard removal on severe waterlogged sites was the best economical response.

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Experiments were conducted during 1999 at the Univ. of Georgia Research Farm near Alapaha with the rabbiteye blueberry (Vaccinium ashei Reade) cultivar Brightwell to determine how various harvesting and handling tactics influenced firmness. The research was facilitated by availability of a mechanical harvester and a commercial packing line. Firmness was determined with a FirmTech II firmness tester on fruit samples before and after cold storage. Fruit harvesting methods included machine harvesting in bulk, hand harvesting in bulk, and hand harvesting directly into clam shell containers. Assessment of precooling effects were made by comparing firmness of fruit that were placed immediately over ice after harvest to fruit that remained at ambient temperatures for 24 hours after harvest. Additional measurements were made to discern the effects of grading and sorting on fruit firmness. The data overall indicated that `Brightwell' fruit firmness was “acceptable” regardless of the harvesting and handling methods experienced. However, there were considerable firmness losses caused by the various procedures. The greatest loss in fruit firmness (20% to 25%) was caused by machine harvesting. This was followed by a 15% to 18% loss of firmness due to grading and sorting. Immediate cooling of fruit after harvest resulted in only a 8% to 12% increase in firmness as compared to keeping fruit at ambient temperature for 24 hours. These findings should be useful to growers and packers in targeting segments of their operations that can be manipulated to improve berry firmness and quality for fresh market sales.

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In a series of experiments, gibberellic acid (GA3) was applied to rabbiteye blueberries (Vaccinium ashei Reade) under field and greenhouse conditions to determine if fruit set could be improved following physical or freeze injury to flowers. In field experiments, physically damaged flowers (i.e., corollas and styles removed, styles only removed, or ovaries lanced) of `Climax' and `Tifblue' treated with GA3 (4% ProGib at 250 mg·liter–1) set substantially more fruit than nontreated, damaged flowers. Under green-house conditions, GA3 applied postfreeze to `Tifblue' and `Brightwell' resulted in increased fruit set compared to unsprayed control plants of the same cultivars. Freeze-damaged plants had substantially reduced fruit set overall but to a much lesser extent for GA3-treated plants than for those not treated with GA3. Individual fruit weight was reduced by GA3 applications, as was berry seediness. Results from these greenhouse and field trials suggest that GA3 can be used to salvage a blueberry crop following a moderate freeze during bloom.

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Since 2004, growers and scientists have observed a disorder described as “yellow twig” or “yellow stem” affecting a major selection of southern highbush blueberry, FL 86-19, in the south Georgia blueberry production region. The initial symptom observed was leaf marginal chlorosis and subsequent necrosis, which eventually progressed throughout the whole leaf resulting in early leaf fall. Thin, yellow twigs or yellow stems became evident on some cultivars. The described symptoms on blueberry were similar to those exhibited on grapes with Pierce's disease and on plum with leaf scald disease. This prompted the enzyme-linked immunosorbent assay (ELISA) tests and isolations of Xylella fastidiosa, which is the causal agent of the previously mentioned grape and plum diseases. Two leaf and two root tissue samples were collected from a diseased FL 86-19 plant for isolation and ELISA testing on 2 Mar. 2006. ELISA results indicated all four tissues tested positive for the bacterial pathogen, X. fastidiosa, whereas only the two root tissues provided positive isolations. One leaf and one root tissue sample were later collected from each of five additional diseased plants for isolation and ELISA testing. Both isolation and ELISA testing methods obtained positive results. Cultures were multiplied to inoculate seedlings of three cultivars: ‘Southern Belle’ (eight plants), ‘Premier’ (six), and ‘Powderblue’ (six) on 23 May 2006 and one selection, FL 86-19 (eight), on 31 May 2006. Two FL 86-19 plants started to show symptoms of marginal necrosis 54 days postinoculation, whereas one plant each of ‘Southern Belle’ and ‘Powderblue’ started to show symptoms of marginal necrosis 63 days postinoculation and ‘Premier’ stayed symptomless. All eight culture-inoculated FL 86-19 plants (100%) showed symptoms 72 days postinoculation, but no symptoms were observed on the control plants. One hundred twenty-six days postinoculation, two ‘Powderblue’ and four ‘Southern Belle’ plants showed mild symptoms, whereas all ‘Premier’ plants were asymptomatic. Positive reisolations of the bacteria from the inoculated symptomatic plants, not from asymptomatic plants, fulfilled Koch's postulates, which confirmed X. fastidiosa was the causal bacterium of the new blueberry disorder, the bacterial leaf scorch of blueberry.

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Abstract

There was an inverse linear relationship between fruit set of ‘Bicentennial’ and ‘Redhaven’ peach trees [Prunus persica (L.) Batsch] and treatments of 0 − 4000 ppm of (2-chloroethyl)methylbis(phenylmethoxy) silane (CGA 15281) in 1980. Increases in fruit weight occurred at the highest rate of CGA 15281 when compared to lower rates with ‘Bicentennial’, but no significant increase was obtained with ‘Redhaven’. Terminal fruit were larger than basal and middle node position fruits in ‘Bicentennial’. In 1981, all applications of CGA 15281 (0 − 3000 ppm) to ‘Candor’ and ‘Jefferson’ thinned blossoms and resulted in an increase in fruit size, when compared to the hand-thinned control. Treatments of 2250, 2500, and 2750 ppm to ‘Candor’ resulted in adequate thinning and increased yields. Applications at 1500 and 1750 ppm resulted in overthinning and reduced yields in ‘Jefferson’.

Open Access

Growing southern highbush blueberries in milled pine bark beds ≈15 cm deep has become a popular fruit production system in Georgia and Florida. One of the primary limiting economic factors in this system is the cost of the growing media, which can exceed $10,000 U.S. per ha. In an effort to discover low-cost substitutes for milled pine bark, available waste or low-cost organic materials were screened for there suitability as growing media for southern highbush blueberries. Cotton gin waste, pecan shells, hardwood “flume” dirt, milled composted urban yard waste, composted urban tree trimmings, pine telephone pole peelings, and pine fence post peelings were evaluated. Only pine derived materials had a suitable pH (<5.3). Fresh pine telephone pole peelings (≈25% bark to 75% elongated fibers of cambial wood) and pine fence post peelings (≈75% bark to 25% elongated fibers of cambial wood) were evaluated for several seasons in containers and field trials. The growth index of blueberries in these materials was slightly less or equal to milled pine bark. Surprisingly, nitrogen deficiency was slight or not a problem. The results indicate that pine pole and post peelings may offer an excellent, low-cost substitute for milled pine bark for blueberry production.

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