The use of ProGibb® (GA3) has become standard in the premium sweet cherry production areas of the world, including Chile, South Africa, Australia and the western United States. Such widespread adoption makes sense, as this plant growth regulator provides a host of benefits. Size of the fruit can be increased by 50%-60% by using ProGibb and the flesh is firmer – so pitting and bruising is reduced and shelf life is extended. It also maintains good stem condition in sweet cherries. Using ProGibb can delay harvest by two to three weeks allowing pickings to be staggered during the season. This delay is an excellent harvest management tool that can be used to maximize profits. The use of ProGibb has allowed sweet cherries to be shipped for long distances without losing quality. This is reflected by the significant increase in sweet cherry acreage in the U.S. and in other countries with fruit destined for export markets. Growers’ returns have increased as a result of higher demand for high-quality cherries.
The key to maximizing ProGibb’s impact is thorough, precise application timing and coverage, as ProGibb does not translocate within the tree. As most cherries mature, they change from green to yellow to red. The optimum application timing for ProGibb is between the green and yellow stages, when the fruit’s outer layer is translucent and one can almost see inside the fruit. This stage occurs about 2-4 weeks prior to harvest depending on the variety and growing region. It’s critical to keep in mind that the effects of ProGibb vary widely due to a number of factors, including the cultivar, crop load, environment and application rate.
Cultivar. Bing, the industry standard cultivar, responds quite well to ProGibb. Lambert is the most responsive cultivar, but growers should not use more than 80 gai/ha (32 gai/A). The same rate recommendation is made for Rainier, and ProGibb may inhibit red blush in some areas on this cultivar.
Brooks responds to GA3 with increased berry size, but the delay in harvest is not always an advantage with this particular variety. Tulare will respond with high rates, and self-fertile cultivars such as Lapin require a higher dose as well – and possibly, split applications.
Crop load. Large crops will require higher ProGibb rates and the resulting harvest delay will be longer. The product is more effective when maturity is uniform, and growers frequently employ multiple applications. The age of the tree is an important factor, too — older trees can carry a heavier crop load than younger trees. In addition, weak trees do not respond as well to ProGibb; instead, growers can see the best response in vigorous trees with light to moderate crops.
Environment. Cooler growing conditions will work in tandem with ProGibb to delay harvest. ProGibb should not be applied prior to a rain event.
Sour Cherry Considerations
In the U.S., more than 90% of sour cherries are grown in Michigan. A major problem is this region is sour cherry yellows virus, which makes the fruiting buds fall off, leaving bare branches, or “blind wood.”
In the 1970s, Dr. John M. Bukovac of Michigan State University found that applications of GA3 early in the spring significantly reduced the effects of the disease, and if applied early, allowed trees to recover and become very productive again. As a result, total production of sour cherry increased significantly because growers were able to harvest much more fruit from mature and old orchards. Since the adoption of this ProGibb application strategy, the total area of sour cherries has decreased slightly – and yet total production has risen due to productivity increases.
Application rate. Growers should define the rate of ProGibb they are going to use on the basis of grams of active ingredient (ai) per acre, not parts per million (ppm), taking crop load into careful account. They must pay close attention to the history of a given block, studying the pack-out reports as well – modifying their year-to-year program as needed.
Applicators should adjust water volume to fit the canopy size – 1000-2000 L/ha (100-200 gal/A) of water per acre are common. Thorough, uniform coverage is essential, but applicators should avoid runoff. They should keep water pH below 8.5 (6 to 8 is good), and should not use a surfactant or tank mix ProGibb with other agricultural products.
To optimize a crop’s response to ProGibb, growers should apply it during periods of slow drying conditions: low wind, high relative humidity and moderate temperatures such as in early morning or late afternoon or evening. Growers should water the orchard prior to application, not after, because the trees need a uniform supply of water in the soil.