Through a variety of applications, plant growth regulators (PGRs) have a huge impact on table grape production. PGRs are used to improve berry size, berry color, cluster quality, and ultimately, a grower’s yield and return on investment. In short, PGRs have literally changed the way that table grapes are grown and marketed.
PGRs for table grapes are based primarily on two active ingredients: gibberellic acid (GA3), and s-absicisic acid (S-ABA). Historically speaking, the use of ProGibb® on table grapes helped establish the true value of PGR products for commercial agriculture. More recently, the registration of S-ABA (ProTone®) for red table grapes has taken that value to new heights.
ProGibb on Grapes
Consumers probably wouldn’t even recognize today’s table grapes if they weren’t treated with the PGR gibberellic acid (GA3). Most seedless grape varieties naturally produce small berries in a very tight cluster. Untreated fruit are the size of raisins, not the plump table grapes that have become so popular with customers.
ProGibb is typically used in a three-part program. The first of these three sprays is the so-called “stretch” application. This application gets its name from its ability to stretch out the grape clusters. Because a later application greatly increases berry size, the stretch application has become critical. If the clusters aren’t loosened up, the larger berries that ProGibb produces will become so tightly bunched that air circulation can be severely limited. This can lead to a wide range of moisture-related problems. Another benefit of this spray is that light penetration is increased within the cluster, aiding sugar development.
The second application of ProGibb, which is made at bloom, will actually make some of the berries fall off, thinning clusters. That is also necessary, because it allows enough room for the remaining fruit to grow large but with enough air movement surrounding them that disease problems aren’t nearly as much of a concern. This use of ProGibb reduces thinning costs overall.
The third and final application is the original ProGibb application. Made after bloom, it is designed to “size up” the fruit.
Prestige on Grapes
Forchlorfenuron (CPPU) is a PGR that was commercialized in 2005. Known to growers by VBC’s brand names Prestige® and Caplit®, CPPU was developed primarily as a supplement for GA3. Delivering and often enhancing many of the same benefits that GA3 provides, CPPU also acts as a valuable substitute for growers of cultivars with demonstrated GA3 sensitivity. Applied at very low rates when berries are approximately 6mm in diameter, Prestige and Caplit increase berry size and yield, strengthens the skin (providing disease protection benefits), enhances berry firmness and uniformity, provides harvest flexibility, and helps to maintain fruit quality after storage and transit.
ProTone on Red Table Grapes
ProTone, one of the most impactful plant growth regulators to ever hit the market, enhances color formation in red table grape varieties (Flame Seedless, Crimson Seedless, Red Globe) and is currently approved for use in all major table grape growing countries around the world. The ProTone launch in 2010 was eagerly anticipated by growers who had seen results on experimental acreage. Today, the product is applied on the majority of all red table grape acreage for the countries in which it is registered. For more detailed information, visit www.protonepgr.com.
Getting good color on red table grapes has long been a problem for table grape growers. The same heat that helps sweeten and mature the fruit can also be a problem when it comes to color development. Color develops in the skin of the fruit through the production of anthocyanins. However, the enzymes necessary for the required chemical reactions are temperature-sensitive, so the chemical pathways slow down at very high or low temperatures. Grape growing regions can often be subject to huge temperature swings, which actually help with coloration. But when nighttime temperatures don’t get low enough, red grape varieties such as Crimson and Flame Seedless won’t “color up” as they should. Growers often refer to these periods as bad coloring years.
Using Ethephon with ProTone
A bad coloring year means a significant amount of fruit (as much as 40%) remains unpicked – a situation that can be quite costly for growers. Prior to the introduction of ProTone, ethephon was the only coloring tool growers had available. But it’s important to understand that ProTone is not intended to replace ethephon. In fact, the two work well together in an integrated program. Ethephon releases ethylene gas that enhances ripening, which, of course, turns the berries red. However, because it enhances ripening, ethephon also begins the process of softening the fruit. In softening the fruit, the storage life can be shortened considerably, causing big problems for growers seeking to tap into lucrative export markets.
ProTone works by increasing an enzyme, UDP-glucoseflavonoid 3-0-glucosyltrans ferase, or UFGT. While ethephon increases ethylene, ProTone increases UFGT and this enzyme increases anthocyanin, the red coloring agent of the berry. Growers must wait until veraison to apply. Veraison is a period in grape development referring to the onset of ripening, typically, for ProTone usage, regarded as the point when 50% of the berries are beginning to soften. ProTone can be applied earlier than ethephon — a significant benefit.
In Flame grape production in desert regions, for example, softening to harvest can be only 14 days. But since ethephon’s preharvest interval (PHI) is 14 days due to Maximum Residue Levels (MRLs) allowed in many markets, application for coloring can be tight. With ProTone, a naturally occurring PGR, MRLs are not a concern. ProTone has a (PHI) of zero days, and the re-entry Interval (REI) is just the bare minimum of 4 hours.
ProTone and Ethephon – Key Differences
Growers familiar with how ethephon works should understand that ProTone is quite different. Ethephon produces ethylene gas, which generally works faster at higher temperatures. Also, ethephon translocates through the plant, meaning application to the leaves can help color the fruit. But ProTone is not translocatable. It also does not work as fast because it increases an enzyme which then produces anthocyanin which then must work its way through the berries.
ProGibb Worldwide Performance
Virtually all table grape growers consider an application of GA3 as part of their standard program, but those same growers might not realize that most of the uses of GA3 on grapes actually emerged through the development program for ProGibb. First introduced in 1962, ProGibb has forever changed the grape industry and remains the standard by which all GA3 products are evaluated. Use started in California then spread to all major table grape producing regions. New applications continue to be developed. ProGibb can be used on a host of varieties, including:
- Thompson Seedless
- Flame Seedless
- Ruby Seedless
- Crimson Seedless
- Superior Seedless
- Black Seedless
- Autumn Royal
- Red Globe
- Waltham Cross
To re-emphasize, ProTone isn’t designed to replace ethephon, and the two products work well in combination. With ProTone, color will begin todevelop in a grape at the bottom where the last of the material evaporates. With ethephon, because it translocates throughout the plant, the color will start at the stem end. With the two materials used in a coloring program, a grower can increase marketable yield.
One final note on shipping: Fruit softening can be a problem for table grapes. Even on a refrigerated boat, firmness can prove to be a quality issue. ProTone doesn’t firm up fruit, but unlike ethephon, it doesn’t soften it. When used in combination, a grower who’s intending to ship overseas can reduce the amount of ethephon used, helping to address this challenge. Growers in the southern hemisphere are aggressively pursuing this strategy, with excellent results.
Understanding how ProTone travels through the fruit is critical to its application. Some refer to ProTone application as “painting” the fruit, meaning that if a grower only sprays one side of a cluster, the other side won’t color up . To color berries inside clusters, growers can’t just casually spritz the product on. Spray volumes of up to 1500 liters per hectare (150 gallons per acre) are recommended, which is often more than most growers are accustomed to applying. With ethephon, growers might only use 500 liters per hectare (50 gallons per acre), driving at a moderate speed. But because of the need for complete and thorough coverage, ProTone applications require a slower application and increased spray volume, with product directed at the both sides of the cluster.