Modern high-density apple orchard systems require the establishment of trees with many temporary fruiting scaffolds arising uniformly along the main axis or axes. This process is facilitated by planting a highly feathered tree as a result of growth regulator applications in the nursery to stimulate sylleptic shoot formation (Elfving and Visser, 2005, 2006; Miranda Sazo and Robinson, 2011, 2012). After planting, the leader can develop sections of blind wood, where axillary buds remain dormant rather than producing shoots capable of developing into fruiting scaffolds. Buds on sections of blind wood are paradormant according to the definition provided by Lang et al. (1987); that is, factors outside the meristem itself but still within the plant are responsible for the resting state. Blind wood can also develop on lateral branches (feathers) in cultivars such as Jazz (Palmer et al., 2005), Cox (Hipps, 1992), and Jonagold (Wertheim and de Groene, 1991). Blind wood along the leader limits full canopy development, whereas blind wood on lateral shoots results in lack of cropping in the center of the tree; both can greatly reduce the productivity and profitability of modern high-density apple orchard systems.
Lateral branching can be stimulated on young fruit trees using cultural techniques or application of various plant growth regulators. Removing a narrow strip of bark from above a bud, referred to as notching, is a technique for increasing lateral branching of young apple trees (Niu et al., 1998; Oullette et al., 1996; Zimian et al., 1998). Notching of buds on 1-year-old wood greatly increased budbreak and the proportion of buds that developed into shoots (Zimian et al., 1998). Greene and Autio (1994) reported that notching of vegetative buds on apple trees with a spur-type growth habit increased the chance of the bud growing into a lateral shoot. Earlier research had shown that notching of vegetative buds on 2-year-old ‘Starkrimson Delicious’ apple trees increased the percentage budbreak and proportion of shoots that grew >5 cm, and a whole-tree spray of 0.4 g·L−1 6-BA caused a larger percentage of notched buds to develop into lateral shoots compared with notching alone (Greene and Miller, 1988). Palmer et al. (2005) reported that localized application of thidiazuron was far more effective than 6-BA at inducing growth of dormant buds on lateral shoots of ‘Jazz’ apple trees.
Notching has not been widely adopted by commercial apple growers in the southeastern United States for several reasons including the high and skillful labor input needed to perform this practice, the inconsistency of results, with a low percentage of notched buds developing into new shoots/scaffolds or only a weak growth response, and concern that notching may create sites for infection by pathogenic bacteria such as fire blight (Erwinia amylovora). Palmer et al. (2005) noted that notching can increase the risk of canker caused by Nectria gallegena. Inconsistent responses to notching have also been reported in sweet cherry (Prunus avium) trees (Elfving and Visser, 2007). Concerns of increased risk of bacterial canker (Pseudomonas syringae) infection were raised when scoring or nicking cuts were suggested as a technique for stimulating lateral branch development in young sweet cherry trees (Elfving and Visser, 2009; Elfving et al., 2011). Elfving and Visser (2007) reported that combining notching with a proprietary formulation of 6-BA plus gibberellin A4 + A7 (GA4+7) greatly improved the branching response in sweet cherry compared with notching alone. To reduce the labor required to stimulate branching and to reduce the risk of pathogen infection, the effects of combining plant growth regulator treatments with penetrating surfactants was investigated in sweet cherry trees (Elfving and Visser, 2009). Subsequent studies demonstrated that combining a penetrant with gibberellic acid (GA3) resulted in successful branch induction without the need to apply GA3 to scoring or nicking cuts in the bark (Elfving et al., 2011).
The objectives of the present research were to evaluate the potential for notching and 6-BA treatments to stimulate shoot development from paradormant buds on blind wood on the leader of young apple trees. Additional research was conducted to determine the optimum timing for notching plus 6-BA treatments, the minimum effective concentration of 6-BA for inducing shoot development from notched paradormant buds, and the potential for a penetrant to substitute for notching when using 6-BA.
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