Dr. Don Elfving at Washington State University is looking for ways to stimulate branching on cherry trees. The tree pictured on the left, above, is untreated. The branched tree on the right was treated with Promalin.
Finding a way to stimulate branching of young cherry trees without resorting to pruning or scoring is a goal of Dr. Don Elfving, horticulturist with Washington State University in Wenatchee.
Pruning can stimulate branching, though not necessarily the kind of branching the grower hopes for, Elfving said. Other disadvantages are that it involves removing some of the tree growth, and generally delays and reduces cropping.
Scoring cherry tree limbs and painting with a cytokinins product such as Promalin (a mixture of gibberellic acid and benzyladenine) is another option for inducing branching, but Elving is looking for alternative methods that don’t involve cutting the tree, which would be cheaper and safer, and reduce potential for bacterial canker infecting the wounds. "We’re interested in reducing the labor costs of these kinds of practices by getting rid of the cutting, and it will reduce the hazard to workers from having knives around," he told members of the Washington Tree Fruit Research Commission, which is funding his research, along with the California sweet cherry industry.
Elfving said penetrating the epidermis of the tree (the outer papery layer of the tree) is key to making a Promalin treatment effective. In his trials, applications to uninjured bark had no effect on branching. "You have to actually interrupt the epidermis. When you do that, it works very reliably."
Removing buds does not cause enough damage for the treatment to work. However, disbudding and painting with Promalin is work and effort, and involves many decisions, such as which buds to remove and how many.
"One of the things we’ve been aiming for is something that’s simple, so the grower who wants to do this in his orchard can give simple instructions to the people doing the work and get a satisfactory, uniform response," he said.
Scoring alone can stimulate branching but is less consistent than when the cut is treated with Promalin.
The best timing for the scoring and Promalin treatment is within two weeks of green tip. Elfving’s research shows that when cuts are made on one-year-old wood, there is no advantage to making cuts any closer than 12 inches apart down the shoot, and the cuts do not need to be above buds because the material moves from the point of absorption to nearby buds.
In a 2006 trial comparing various cytokinins products with scoring, he was surprised to find that gibberellic acid alone (the control) was as effective at inducing branching as the products containing both gibberellic acid and benzyladenine. This was surprising, he reported, because GA is not thought of as a product that affects apical dominance. As a follow-up, he tested ProGibb (GA3), NovaGibb (GA4), and Provide (GA4 + 7) at various concentrations. Provide was the most effective, and GA3 (ProGibb), the product widely used in cherry production to delay fruit maturity, was the least effective. This suggests that the GA7 had a beneficial effect in terms of stimulating branching.
Elfving said some cherry growers have been reluctant to score their trees because of the risk of bacterial canker infecting the wounds. In experiments in 2006, he combined copper (the standard bacterial canker treatment) with Promalin and found that the mixture was just as effective in promoting branching as Promalin alone. He did not know if adding copper to the treatment would prevent bacterial canker, as he has never seen a bacterial canker infection resulting from scoring in the dry conditions of eastern Washington.
In experiments last season, Elfving focused on eliminating the scoring, and tested a practice used at Auvil Fruit Company in Vantage, Washington, where workers use pliers to crush the outer layer of the bark before applying Promalin. Elfving said crushing seems just as effective as scoring or notching, and the damage heals quickly without oozing.
He is now interested in finding ways to get the same kind of branching without damaging the tree, and estimates that if there was no need to injure the bark, the cost of the procedure could be cut by at least 50 percent.
Elfving said he is testing the addition of various surfactants to the cytokinins products, to see if they can help penetrate the bark without the need for cutting.