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Mature `McIntosh'/MM.111 apple (Malus domestica, Borkh.) trees were treated to evaluate the response of root pruned trees to chemical thinning and to determine if reducing the crop load increased fruit size on root pruned trees. The trees were root pruned at full bloom in 1988 and 1989, by cutting on both sides of the row 1m from the trunk and 30cm deep. Water, 600mg/liter carbaryl, 5mg/liter napthaleneacetic acid (NAA), or NAA plus carbaryl were applied when fruit diameter was approximately 10mm. Trunk cross-sectional area (TCSA) was increased by thinning treatments in 1988, but root pruning had no effect. In 1989, root pruning reduced TCSA increment by 35%. Shoot length was reduced by root pruning both years. All treatments reduced percent fruit set in 1989, however root pruned trees and trees treated with NAA had the highest fruit numbers at harvest. Preharvest fruit drop was reduced by root pruning in both 1988 and 1989. Root pruning had no influence on the response of apple trees to chemical thinning. Removing a portion of the crop with chemical thinners was partially successful in counteracting the reduction in fruit size caused by root pruning.
The risk of spreading fire blight in apples after mechanical thinning with a rotating string blossom thinner was evaluated in field and potted-tree experiments. In the field experiment, using the mechanical thinner on noninoculated trees immediately after operating the equipment on inoculated trees significantly (P < 0.01) increased fire blight incidence resulting in 90 ± 20.01 (mean ± SE) infected shoots compared with 23.5 ± 8.97 diseased shoots in similar trees that were not thinned mechanically. A similar result was obtained in greenhouse experiments whereby healthy apple plants positioned adjacent to diseased plants before the group was subjected to the mechanical thinner developed more than twice the number of infected shoots as that on similar plants that were not thinned. These results indicate that under conditions conducive to infection, the mechanical blossom thinner significantly increases the risk of spreading Erwinia amylovora. The use of the thinner should therefore be limited to orchards with no history of disease in the last 3 years and on days when predicted weather is not suitable for tree infection by E. amylovora; otherwise, a severe fire blight epidemic could develop in the orchard.
Renewal of limbs by pruning to leave a short, angled, upward-facing stub is common practice for spindle-type apple (Malus ×domestica) training systems. A short, beveled stub cut is thought to stimulate renewal growth from latent buds present underneath the base of the excised branch, and to stimulate smaller, more fruitful renewal limbs with wide crotch angles. We conducted trials over the course of 2 years that involved dormant pruning of ‘Buckeye Gala’ with renewal cuts to compare two stub lengths, 0.5 and 2 cm, and three stub orientations, upward facing, downward facing, and vertical facing, to determine the effects on renewal shoot number, position, angle, and length. We found no clear advantages with either stub length that we evaluated, and there was no improvement in renewal shoot quality with a bevel cut at any orientation. Stub length and stub angle did not influence limb renewal and may be unimportant for training orchard-pruning crews and for machine-learning and robotic pruning.
Abstract
The inhibitory effect of cropping on return bloom in fruit trees is well-known. Previous work suggests the presence of fruit inhibits vegetative meristems from initiating flowers (2). Root pruning promotes flower formation in fruit trees (4). In 1984, we began a study to investigate the interactions between cropping and root pruning on flowering and other physiological processes in field-grown apple trees. In May 1986, the trees that had been deblossomed for three consecutive years underwent a second flush of bloom. This occurrence has not been documented adequately for apple.
Hurricanes or strong winds occasionally damage apple trees in the northeastern and mid-Atlantic regions of the U.S. Following the wind event, trees may be leaning or may be lying flat on the ground with extension root damage. Commercial growers generally pull trees upright while the ground is still moist and support the tree or just the trunk with various types of posts providing support to a height of about 60 to 200 cm above ground. In some cases the trees are pulled partially upright and propped up with boards or sand bags may be placed on the upwind side of the tree. Research data are not available for comparing various methods of treating the trees following wind damage, but field observations indicate that trees perform well if trees are pulled upright within 2 or 3 weeks after the wind event. Data from rootstock research plantings from several states indicate that tree anchorage is influenced by the combination of scion cultivar and root-stock. In Virginia in 1989, Hurricane Hugo brought wind gusts of about 95 km·h–1 when the ground was completely saturated by heavy rains. Trees in several plantings designed to evaluate rootstocks or cultivars were evaluated for the extent of leaning following the storm. The percentage of leaning trees on M.26 EMLA was <10% for `McIntosh' and `Golden Delicious' and 40% for `Delicious'. Susceptibility of trees on M.7A was also influenced by scion cultivar, with 0% for `Golden Delicious' and `Empire', 2% for `Redchief Delicious', and 88% for `TripleRed Delicious'.
Abstract
Three- and 4-year-old ‘MeIrose’/M.7A apple trees were root-pruned on two sides, 50 cm from the trunk, at a depth of 35–40 cm at one of the following growth stages: dormant, full bloom, June drop, and preharvest. Root pruning at dormant or full bloom growth stages reduced trunk cross-sectional area increase and shoot length, reduced average leaf size and fruit size, and increased yield efficiency. Dormant root pruning increased the level of Ca in the fruit flesh. Root pruning at June drop and preharvest had no influence on shoot growth, but increased preharvest fruit drop. Root pruning at full bloom or later increased fruit soluble solids. Considerable root regeneration in close proximity to the cut was evident in November on trees root pruned at the dormant and full-bloom stages. Root regeneration was less on roots pruned at June drop and minimal in preharvest-treated trees. Leaf mineral nutrient levels were not influenced by root pruning treatment. Vegetative growth and fruit size were reduced less by root pruning in the 3-year-old trees than in the 4-year-old trees.
Abstract
Vigorous 15-year-old ‘Melrose’/M.26 apple (Malus domestica Borkh) trees were mechanically root-pruned annually for 4 years at full bloom on two sides of the trunk at a distance of 60 or 80 cm and to a depth of 25 or 50 cm at each distance. Compared to unpruned controls, trees that were root-pruned had reduced trunk cross-sectional area, shoot length, leaf size, pre-harvest fruit drop, fruit size, and pruning time. Although fruit yield was unaffected, yield efficiency, fruit color, and soluble solids were increased by root pruning. Canopy light penetration was increased, as was spur quality. Generally, pruning 60 cm from the trunk had a greater effect than pruning at 80 cm, while pruning depth had no influence.
A tank mix of fish oil plus liquid lime sulfur has proven to be an effective chemical thinner for apples in the bloom and postbloom periods. This combination was labeled for use as a chemical thinner in Washington State in 2003. There are several concerns with fish oil when used in this thinning mixture. Phytotoxicity is one concern. Apple growers have a reluctance to utilize this oil because of its expense and repulsive odor. Research to date has been conducted using oil from a single small source in Washington State. Shipping fish oil across the country is expensive and the consistency and purity of fish oil from other sources is unknown. Fish oil may function as a surfactant and penetrant, and it may also have a direct thinning effect. The objective of these studies was to evaluate the efficacy of several surfactants and oils in combination with lime sulfur for thinning apples. Lime sulfur has been less effective as a thinner when used alone than when used with oil in our studies. Regulaid, LI-700, and Silwet L-77 were shown to be less effective than oils for achieving thinning. Vegetable oil has been very effective in the thinning combination, while petroleum oils have been effective in some eastern U.S. trials, but less effective in the west. Tank mixing fish oil with lime sulfur has remained among the best treatments in our trials, while vegetable oil also shows promise.
The use of short-duration applications of thermal energy (thermal shock; TS) as an apple blossom thinning strategy was investigated. Effects of TS temperature and timing on stigmatic receptivity, pollen tube growth in vivo, and visible leaf injury were evaluated in multiple experiments on ‘Crimson Gala’. TS treatments were applied to blossoms and spur leaves using a variable temperature heat gun. TS temperatures ≥86 °C had a strong inhibitory effect on pollen tube growth on the stigmatic surface and in the style. TS temperatures >79 °C reduced average pollen tube length to less than the average style length. Timing of TS treatment (0 or 24 hours after pollination) was not an influential factor, indicating that effective TS temperatures reduced pollen tube growth up to 24 hours after the pollination event. The onset of thermal injury to vegetative tissues occurred at similar TS temperatures that inhibited pollen tube growth in vivo. Excessive leaf injury (>33%) was observed at 95 °C, suggesting relatively narrow differences in thermal sensitivity between reproductive and vegetative tissues. Inconsistent TS temperatures and/or responses were observed in some experiments. Ambient air temperature may have influenced heat gun output temperatures and/or plant susceptibility. While results suggest some promise, additional work is required to validate and further develop this concept.
Blossom thinning can confer significant benefits to apple growers, including increased fruit size and annual bearing. However, current blossom thinning practices can damage spur leaves and/or fruit. We evaluated the use of short duration forced heated air treatments [thermal shock (TS)] as a blossom thinning strategy for ‘York Imperial’. Using a variable-temperature heat gun, TS treatments were applied to solitary blossoms 24 hours after pollination. Effects of output temperature (five levels) and treatment duration (four levels) were evaluated using a completely randomized design with a factorial treatment structure. Short duration treatments (0.5 and 1.0 seconds) were ineffective for arresting pollen tube growth in vivo. TS temperature required to inhibit stylar pollen tube growth was inconsistent across years. In 2014, TS temperatures ≥56 °C inhibited pollen tubes from reaching the style base at 2.0 and 4.0 second durations. However, in 2015, TS temperatures ≥81 °C at 4.0 seconds prevented pollen tubes from reaching the style base. Inconsistent effects of TS across years were attributed to treatments being applied too late due to optimal conditions for pollen tube growth during the intervening 24-hour period after pollination. Excessive injury to spur leaf tissue was observed at temperatures higher than 84 °C and 70 °C (2.0 and 4.0 seconds, respectively). Pollen tube growth was reduced or arrested at temperature and duration combinations that caused minimal visible injury to spur leaves. Identifying and exploiting structural differences between apple blossoms and vegetative spur leaves may provide insight for the future development of TS or other attempts at developing selective thinning technologies.