The Use of Plant Growth Regulators for Branching of Nursery Trees in NY State
The quality of nursery trees has a large impact on early production and profitability of high density systems. Today, nurserymen are not only asked to produce trees of good caliper but also highly branched trees for the Tall Spindle system with short, well positioned lateral branches with wide crotch angles. This has required nurseries to improve their management to induce lateral branching.
Lateral branching or “feathering” of nursery trees is controlled by apic al dominance within the stem of the tree wherein the terminal bud of a nursery apple tree exerts control over the development of the lateral buds. The young leaves in the apical bud produces a hormone called auxin that diffuses downward and inhibits the development of lateral buds. Therefore, in order to produce a well-branched, highly marketable tree, apical dominance must be interrupted. Traditionally nurserymen have accomplished this through the removal of small undeveloped leaves, called “leaf pinching” or “plucking”, resulting in a a transient reduction of auxin content in the stem content in then stem allowing lateral buds to develop into lateral shoots. With some cultivars, years, and climates a ‘markeatable’ feathered nursery apple tree can be produced with this technique. Unfortunately, it is inconsistent in the NY climate and most of the nursery apple cultivars require more than one “leaf pinching” to maintain low auxin content due to rapid stem elongation. This labor-intensive practice (a well-skilled nursery worker can “leaf pinch” a maximum of five to six thousand trees per day) substantially increases the costs in an apple nursery and must be conducted in a short period of time.
The use of plant growth regulator chemicals has been seen as a more consistent and less expensive method to induce lateral branching than manual “leaf pinching”. Currently, there are three PGRs that can potentially be used in nursery apple production. Promalin (mixture of a cytokinin and 2 gibberellins) and Maxcel (a cytokinin) have been sporadically used in nursery apple production in the US usually as a single spray. In the last 4 years we have done 2 new studies with multiple applications (3) of Maxcel to improve branching in NY nurseries. Our results show that 3 applications of 500 ppm have given very good branching with most cultivars. However this use has not yet been added to the Maxcel label. Interestingly, the use of Maxcel has been adopted by Italian nurserymen and is a key step for the successful production of well feathered “knip-boom” apple trees. Depending on cultivar, Italian nurserymen apply from 1 to 4 Maxcel treatments with spray intervals of 5-7 days depending on temperatures after application (Dr. Walter Guerra, Italy, personal communication). Since 2009, a new branching agent, cyclanilide (Tiberon formulated by Bayer Environmental Science, N.C. USA), has been available in the US for use in outdoor nurseries of apple, sweet cherry, pear, and plum in Florida, Idaho, Oregon, Michigan, and Washington States. It is not currently registered for tree fruit nursery use in New York State, Europe, or elsewhere.