Dr. Duane Greene

Dr. Duane Greene

Plant growth regulators are useful tools to manipulate fruit trees and help make an orchard more productive, though they can’t work miracles, Dr. Duane Greene, horticulturist at the University of Massachusetts, told Washington growers at a recent fruit school.

It has to start with a well managed orchard, he said. “They do not undo a multitude of sins. They are not miracle workers. You can’t transform poor quality fruit on poorly managed trees into good quality fruit.”

Plant growth regulators have many uses, but in his presentation at Washington State University’s Fruit School on Apple and Pear Horticulture in November, Greene focused on use of the gibberellin synthesis inhibitor prohexadione calcium (sold under the brand names Apogee and Kudos) to control vegetative growth.

Although tree growth also can be controlled by rootstock, crop load management, tree training, pruning and root pruning, use of a plant growth regulator has several advantages.

It’s convenient, it can be started and stopped as needed, and the effect is not irreversible.

Gibberellins are a group of hormones responsible for stimulating vegetative growth, and controlling production of those hormones is a very effective way to regulate tree growth, Greene said. There are several known gibberellin biosynthesis inhibitors, but prohexadione calcium is the only one approved for use in U.S. apple orchards.

The compound is relatively benign, Greene said, and its registration was fast-tracked in the late 1990s. Besides controlling vegetative growth to enhance productivity, it is used in eastern U.S. growing regions as an alternative to streptomycin applications for controlling the shoot phase of fire blight.

Two formulations are available in the United States. Apogee has been available since the compound was registered and is the one most widely used.

However, Greene said there is still much to learn so that growers can take full advantage of it. Kudos was registered more recently. Both contain 27.5 percent active ingredient and have performed similarly in trials.

In other parts of the world, prohexadione calcium is available under the brand name Regalis, which contains only 10 percent active ingredient but also includes magnesium sulfate in the formulation.

Keys to success

For the best results, growers should include either an equal quantity of magnesium sulfate or water conditioner with the Apogee to lower the pH level. At high pH levels, Apogee becomes inactive.

A surfactant should also be included in the tank and an anti-foam agent if the surfactant is one that foams. Apogee should not be mixed with calcium, according to the product label. Greene said he believes this is because calcium products inactivate Apogee.

For maximum growth control, Greene recommends applying Apogee very early in the season — as soon as there is enough leaf area to absorb the compound. He thinks petal fall applications are too late because by then the shoots are growing rapidly.

Apogee does not stop growth immediately when sprayed on the tree, so shoots can grow significantly during the 10 to 14 days it takes for the compound to work.

“If you don’t get it on early, you’re going to lose a real opportunity,” he warned.

In Massachusetts, Greene applied Apogee in a mature block of Cortland apples to illustrate its effects to growers. The trees grew on Malling 26 rootstocks and were overly vigorous. This had been aggravated by heavy pruning each year.

Flower bud formation and fruit set were poor, and the apples were large, green, soft, affected by bitter pit, and had poor storage potential. The block had never been productive.

Greene tested two treatment timings with Apogee at a rate of 9 ounces per 100 gallons of water: one at the pink stage of bloom and the other at petal fall. The pink-stage treatment resulted in more open trees, better light penetration and improved fruit set. With the petal fall timing, much of the growth retardant effect was lost, he reported, although it did increase fruit set and opened up the trees somewhat compared to untreated trees.


In a trial on McIntosh apples, Greene assessed the effect of various rates of Apogee on fruit set and return bloom. Rates tested were 125, 250 and 375 parts per million. Fruit was thinned based on the needs of the control treatment.

Trees treated with the lowest rate set 5.3 fruit per square centimeter of limb cross-sectional area versus 9.8 for the highest rate.

However, the lowest rate had the highest return bloom (8.4 fruit per centimeter of limb cross-sectional area) versus 3.3 for the highest rate.

“As we increase the amount of Apogee we get increases in fruit set, but the reduction in return bloom is a problem,” he said, noting that growers should adopt a more aggressive thinning program in trees treated with Apogee.

Growers in the eastern U.S. tend to use lower rates of Apogee at first but apply it more frequently. This is because they sometimes have difficulty thinning the crop adequately and the theory is that using lower rates initially will reduce the increase in fruit set, though this has not been proven, Greene said.

Last year, he did an experiment with Fuji to study the effect of different rates of Apogee on fruit set. He applied 3, 6, 9 or 12 ounces per 100 gallons in late May when shoot growth was 1 to 2 inches long.

All the treatments increased the crop load in comparison with the control. The 3-ounce rate set as much fruit as the higher rates, suggesting that Apogee did not increase initial fruit set, but the reduction in vegetative growth retarded June drop.

Greene said it is not clear why there was no statistically significant rate response in the Fuji trial, as there was in the McIntosh study, and he intends to conduct another experiment to clarify this.

He also studied how long the effect of Apogee endured in the trees before the compound was metabolized and the shoots began to regrow.

Only trees treated with the 3-ounce rate regrew. The higher rates controlled growth for the whole season, but much of this growth was attributed to added weight because the terminals were pulled down horizontally by the weight of the fruit. •

– by Geraldine Warner