Good Fruit Grower

August 2014

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I t took eight years after the Alar debacle of 1989 for apple growers to obtain a plant growth regulator that would control preharvest fruit drop as effectively as Alar did. But since 1997, commercial growers—especially in the East—have increasingly been using ReTain as part of their preharvest management program. 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 Lab- oratories 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 anx- ious 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 chemi- cals. 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 eth- ylene 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 con- trol, 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 ripen- ing. 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 effec- tive, delaying maturity about 13 days. 16 AUGUST 2014 Good Fruit Grower Harvest E thylene is a byproduct of the oxidation of organic materials. It is found in high concentra- tions in smoke from brush fires, exhaust from diesel and gas vehi- cles, and in rotting vegetation such as rapidly decomposing grass clip- pings, compost, or decaying fruit. Climateric 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 Austra- lian 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 eth- ylene production during the ripen- ing phase. —Keep external sources of eth- ylene 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 atmo- sphere quickly. High carbon diox- ide and low oxygen inhibit ethylene production. To read Brown's article, visit —R. Lehnert ETHYLENE rising How to RETAIN the crop Growers can't afford to let expensive apples fall off the trees. by Richard Lehnert "The loss of Alar was the factor behind the search for a new stop- drop material." —Duane Greene

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