The adoption of high-density apple orchards during the last several decades has resulted in a significant improvement in precocity, cumulative yield, and fruit quality. However, as planting density is increased, the additional benefit in yield is smaller and smaller with each additional tree (Robinson, 2008; Robinson et al., 2007). Therefore, high early yields are needed to pay back the initial investment. With the use of highly feathered trees, significant yield can be achieved in the 2nd and 3rd years after planting, which is an essential asset to help pay for increased tree numbers and establishment costs (Robinson and Stiles, 1995). As the benefits of highly feathered trees were discovered, it became necessary to develop nursery management techniques to stimulate lateral branch development. Therefore, besides large trunk caliper, nurseries are asked to provide trees with a relatively high number of short lateral branches (feathers) with wide crotch angles (Van den Berg, 2003; Weis, 2004; Wertheim and Webster, 2003). In the 1970s, leaf removal and pinching were commonly used to induce branching on nursery trees (Wertheim, 1978). In the United States, the number of feathers on grown nursery trees has improved significantly in the last decade. Before 2009, most nurseries used a single spray of Promalin® (benzyladenine + GA4+7; Valent BioSciences Corp., Libertyville, IL) combined with leaf removal to obtain trees with three-to-five feathers. In the spring of 2009, a new branching chemical, Tiberon™ SC (cyclanilide, Bayer Environmental Sciences, Research Triangle Park, NC), was registered and was used commercially in the United States, improving the quality of apple nursery trees (Elfving and Visser, 2005, 2006b). In 2012, Tiberon™ SC was withdrawn from the market. European nurseries have been using 6-benzyladenine in the form of MaxCel® (Valent BioSciences Corp.)/Exilis (Fine Agrochemicals, Worcester, UK) or 10% BA Paturyl (Reanal, Budapest, Hungary) to induce branching (Basak et al., 1992). MaxCel®, a cytokinin plant growth regulator that is already labeled for several uses on apples, was registered for chemical branching of nursery apple trees in the United States in 2013.
Most of the published branching studies were done many years ago on varieties that are no longer grown (Basak et al., 1992; Cody et al., 1985; Forshey, 1982; Jaumien et al., 1993; Miller and Eldridge, 1986). Other studies reported notching (Greene and Autio, 1994) or a combination of notching plus hormone sprays (Greene and Miller, 1988; Mcartney and Obermiller, 2015) as a technique to increase branching. In 2010, we evaluated the use of Tiberon™ SC on ‘Macoun’ trees in New York and found Tiberon™ SC to significantly reduce tree height and caliper, resulting in poor tree architecture under Eastern climatic conditions (Miranda Sazo and Robinson, 2011). To further study the use of MaxCel® and Promalin® in comparison with Tiberon™ SC, we have conducted 10 experiments at several nurseries in NY, WA, DE, Ontario (Canada), and Chile over the last 4 years.
Nursery-produced apple trees of many varieties bloom prolifically when subsequently planted in commercial orchards. Once trees are moved from the nursery to the orchard, new spring planted trees bloom later than established orchards when temperatures are warmer. These trees are at higher risk of blossom fire blight (Erwinia amylovora Burrill) if left unprotected or with fewer streptomycin sprays (Vanneste, 2000). Hence, an apple tree that doesn’t flower in the 1st year after planting may be a good strategy for fire blight control. Although many studies have reported the use of hormone sprays to reduce flower induction and improve biennial bearing (Greene, 2000; Jonkers, 1979; Mcartney and Li, 1998; Unrath and Whitworth, 1991), there is a dearth of studies that focus on flower inhibition for nursery trees. A second objective of our study was to determine if gibberellin sprays in the nursery could inhibit flower bud formation thus reducing flowering the following season in the orchard to minimize the risk of fire blight.
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