University of Massachusetts tree fruit physiologist Dr. Duane Greene was one of the early researchers to work with ReTain, the growth regulator aminoethoxyvinylglycine, or AVG. It was about 1991 when he received a package of the material from Abbott Laboratories so he could study it. “The loss of Alar was the factor behind the search for a new stop-drop material,” he said. “AVG is an expensive molecule to produce, while Alar was cheap, but there was no alternative.
NAA is a much less desirable alternative stop drop on many varieties, especially McIntosh types.” Abbott Laboratories is a health-care company, Greene said, so it was not anxious to dash into the arena after Alar had been ushered out, with huge negative publicity, leaving apple consumers wary of chemistry in apple orchards.
“There was a lot of research being done behind the scenes for five years after Alar was lost,” Greene said.Today, ReTain is produced by Valent, a company spun off from Abbott in 2000 to develop and market agricultural chemicals. ReTain is a product created by a species of fungi working in fermentation vats.
Because it is expensive, costing in excess of $225 per acre for one full-rate application, researchers from all the land-grant universities in the Great Lakes and Northeast region, including Greene, have extensively studied rates and timings to get the most impact. Ethylene is a gas, produced by fruit and by other processes including fuel combustion, that causes fruit to ripen (see “Ethylene rising”).
ReTain is an ethylene biosynthesis inhibitor that blocks production of ethylene in the fruit. Since ReTain delays ripening and keeps fruit from falling off the tree, it has several uses in orchard management. It is especially effective on important northeastern varieties such as McIntosh, Macoun, and Honeycrisp for drop control, and on varieties such as Gala and Cortland, which do not drop, primarily for delaying fruit maturity, Greene said. (See “Why stop drop?”)
Rates and dates
Greene considers the growth regulator naphthaleneacetic acid (NAA, or Fruitone L) a much less desirable stop-drop option than ReTain since it advances ripening. One option is to use both materials together, although this is not always more effective. Use of NAA as a stop drop dates back more than half a century.
ReTain is sold in pouches containing 333 grams of active material, and one pouch per acre is considered a full rate on mature trees. NAA is used at a rate of 10 parts per million. Some varieties are more sensitive than others to ReTain. In Michigan, tree fruit educator Phil Schwallier recommends using a half rate on sensitive varieties such as Gala, Jonagold, and Honeycrisp.
For these three, he said, “half rate is full rate.” He has found that apple maturity is set back from 6 to 13 days by using ReTain, and the maturity delay is comparable whether it is applied 30, 21, or 14 days before harvest. Applying it only seven days before harvest is less effective, and making two applications is more effective, delaying maturity about 13 days.
Split applications at 30 and 14 days did not add significantly to maturity delay.However, on McIntosh and other summer apples, early applications 30 to 21 days before harvest were more effective. From his research, Schwallier concluded that all rates and timings shut down ethylene production, but the best results came from ReTain plus NAA anytime from 30 to 14 days before anticipated harvest.
In stressful years, earlier is better, and the rate of ReTain can be increased for better effect.The size of the crop also plays a role, Schwallier found. The heavier the crop, the greater the effect of ReTain on slowing apple maturity. Stopping drop and delaying maturity are not the same thing.
Schwallier said ReTain improves fruit quality, firmness, and shelf life, reduces greasiness and incidence of watercore, and reduces cracking as well as stopping drop. NAA stops drop but advances maturity on some varieties, such as McIntosh.
“One single application of ReTain made at the full rate will satisfactorily retard drop for about 35 days in normal years,” Greene said. “After that, drop starts to increase, and a supplemental application will be necessary to extend the drop control period.”Greene also found that earlier application is better if the goal is to delay ripening.
The earliest time of application suggested on the label is four weeks before anticipated harvest. This timing results in the maximum delay in ripening. Split applications of a half rate of ReTain applied at four and two weeks before anticipated harvest are not as effective in delaying ripening.
He recommends the full-rate strategy for pick-your-own and other blocks intended for late harvest.In an effort to reduce cost, Greene experimented with adding NAA and reducing the rate of ReTain. But it resulted in a “fickle drop control strategy.”
In the last five years, the addition of NAA to ReTain enhanced drop control in two years but not in the other three, he said.“Research over these past several years clearly shows on McIntosh types that ripening will be advanced if the ratio of NAA to ReTain is too high. Advanced ripening is unlikely if a half rate or greater of ReTain is used with 10 ppm NAA.
If more NAA or less ReTain is used, ripening is most likely to be advanced. In some situations where too much NAA is applied with ReTain, drop may be increased. Application of 20 ppm NAA or more than two applications of 10 ppm NAA may advance ripening.”
During the last few years, MCP (1-methylcyclopropene) has been adapted from its role as an in-storage treatment (SmartFresh) into an orchard spray (Harvista).“Harvista is not available in the East,” Greene said, “It is a good stop-drop material, but it works differently than ReTain.” While ReTain stops the production of ethylene inside the apple and prevents it from reaching the climateric threshold level, Harvista attaches to ethylene binding sites in fruit and prevents ethylene from ripening the apple.“Once attached, Harvista never lets go,” Greene said. “However, the apple does develop new binding sites and then drop occurs.” In New York, Cornell University horticulturists Dr. Terence Robinson and Steve Hoying think Harvista would be a great addition to the stop-drop arsenal for McIntosh. It is rapid acting and only need be applied approximately seven days before harvest and gives comparable results to the best ReTain plus NAA treatments they have tested, they report. In addition, this material preserves firmness through storage.
Ethylene is a byproduct of the oxidation of organic materials. It is found in high concentrations in smoke from brush fires, exhaust from diesel and gas vehicles, and in rotting vegetation such as rapidly decomposing grass clippings, compost, or decaying fruit.
Climacteric fruit, such as apples and pears, make their own ethylene, but even without it, they can be exposed to this gas. A hormone
to them, it causes ripening, a valuable event in the life of a fruit, but it also starts their degradation and reduces their potential storage life.
Dr. Gordon Brown wrote about this important small gas molecule in the March 2014 issue of Australian Fruitgrower magazine. The Tasmanian tree fruit physiologist was writing for Australian growers who need to store their fruit and ship it a long distance.
It is important they know where ethylene comes from and know how to stop its production, block its action, or keep it out of the fruit environment, he said.
Here are some steps he advised to assure fruit quality:
—Harvest fruit at the correct maturity.
—Rapidly market any fruit exposed to ethylene.
—Apply ReTain to inhibit ethylene production during the ripening phase.
—Keep external sources of ethylene such as exhaust fumes away from harvested fruit.
—Keep the fruit receiving area well ventilated and use ethylene absorbing or destruction systems in the cold room while loading it.
—Rapidly cool the fruit to slow down ethylene production.
—Use SmartFresh to block the ethylene binding sites in the fruit.
—Get fruit into controlled atmosphere quickly. High carbon dioxide and low oxygen inhibit ethylene production.
To read Brown’s article, visit www.apal.org.au. —R. Lehnert