May 1997

ReTain, a new harvest management tool for McIntosh

By Jim Schupp

University of Maine

ReTain is a new harvest management tool for apples, which has received a Federal label from US-EPA for 1997.

The active ingredient in ReTain is aminoethoxyvinylglycine(AVG), a natural inhibitor of ethylene synthesis. Fruit treated with ReTain produce much less ethylene than untreated fruit. This delays ripening, reduces preharvest drop, and slows the loss of fruit firmness. Fruit treated with ReTain can be picked during the normal harvest period for enhanced retention of firmness in CA storage, or harvest may be delayed, allowing the fruit to continue to grow and develop red color for an extended time.

Growers with PYO blocks can also benefit from using ReTain on a portion of the trees intended for harvest through Columbus Day weekend. The following suggestions are offered to assist growers in optimizing the use of ReTain as a harvest management tool.

Where to use it

ReTain is a natural product which is very expensive to manufacture. Because of this, ReTain should only be used in blocks with large crops of unblemished fruit.

Grower experience further suggests that ReTain will be much less effective in orchards with heavy mite infestations. Healthy, unstressed trees are needed for getting a good response to ReTain. Formal studies to determine if certain McIntosh strains respond to ReTain better or worse than others have yet to be conducted. Observations suggest that late-ripening strains, specifically stripers, may be slightly more responsive than are earlier ripening strains such as Marshall and Macspur, however all strains of McIntosh have been shown to be highly responsive to ReTain and strain effects are secondary in importance to factors such as timing of application and spray coverage.

ReTain has also been shown to reduce preharvest drop and water core in Red Delicious. There is considerable promise in using ReTain to extend the growing season for Gala by two to three weeks. This makes it possible to achieve increased fruit size while still harvesting at optimum maturity since the fruit continue to grow during this time. Another benefit is the labor savings from reducing the number of pickings in a shorter, more concentrated harvest window.

Timing

ReTain must be applied well in advance of the first visible signs of ripening in order to obtain satisfactory results.

The product label states that application should be made four weeks before anticipated harvest. Delaying the application of ReTain will not extend the effectiveness of the product later in the harvest season.

Application four weeks prior to the calculated optimum harvest date (OHD) gives the best response. For orchards in the mid-Maine region, the average OHD during the past 40 years has been September 22, suggesting that application of ReTain should be made on or about August 23 in this region.

The exact calender date of application is not critical, and growers intending to apply ReTain should time the application to coincide with optimal weather conditions for making a spray application.

Growers are cautioned not to take the product's effects on harvest date into account when timing the application. Base the timing of the application upon the calculated OHD for untreated fruit. Making a timely application is critical for getting good results.

Application

Studies in Maine in 1992 and 1996 demonstrated that ReTain must be applied with a large volume of spray water in order to get good results.

Our studies clearly show that applications to semi-dwarf trees at 125 to 150 gallons/acre (2X) gave the best results. Sprays at full dilute (250 to 300 gallons/acre) were also very effective. Applications at 100 GPA were slightly less effective but still judged to be acceptable. In both years (1992 and 1996) applications in 50 gallons/acre were much less effective than higher spray gallonage.

Since the cost of ReTain is so high, it makes no sense to skimp on water or on the extra time it takes to make the application at 2X instead of 6X. For semi-dwarf blocks, apply ReTain with 100 gal of water or more. Since coverage is so essential and since ReTain effectively doubles a grower's chemical costs for the season, it is strongly recommended that growers recalibrate their spray rigs for this application to account for the growth of the tree canopy since spring.

Another factor that is absolutely critical to obtaining satisfactory results with ReTain is the use of a high rate of a silicone-based spreader-sticker. Silwet L-77 at 0.1% (vol/vol) has enhanced the performance of ReTain compared to other non-silicone-based products. Dow Silgard is also said to be effective. These products at this concentration cause heavy foaming, which is not controlled by conventional de-foaming agents, however lower rates of spreader sticker are less effective. Since ReTain is readily dissolved in water, some growers have turned off sprayer tank agitation once the product has been thoroughly mixed to reduce foaming. Foaming is unpleasant, but much less so than unsatisfactory results.

ReTain is labeled to be applied at 333 grams (0.73 lb) per acre. Since it is expensive, growers will be tempted to experiment with reduced rates. Our studies suggest that, with optimal application conditions, 200 grams ReTain/acre (0.44 lb) can provide good control of preharvest drop for two weeks, however the full 333 gram rate is needed to provide the retention of fruit firmness in storage. In this introductory season it is recommended that growers apply most of their allocation of ReTain at the full labeled rate.

ReTain and storage

Fruits treated with ReTain produce very little ethylene, but are still responsive to ethylene from external sources, such as non-treated mature fruit.

ReTain-treated fruit that are stored with non-treated fruit typically go into storage four pounds firmer, but come out of storage only one to one and a half pounds firmer. Much of the benefit is lost. On the other hand, ReTain-treated fruit that is harvested at the proper stage of maturity and stored separately from non-treated fruit will lose firmness at a much slower rate. In order to get the maximum retention of firmness in storage, ReTain-treated fruit should be tested for firmness and maturity during picking season. Fruit destined for CA storage should test at least 14.5 lb. firmness and have a starch index reading of 6 or lower. ReTain-treated fruit should be stored separately from non-treated fruit.

Summary

ReTain is a promising new harvest management tool that delays fruit maturity. It is an expensive tool that should only be used on productive blocks with unblemished fruit and healthy, unstressed foliage. ReTain should be applied four weeks before the calculated OHD for your area. The spray should be made with 100 gallons of water or more and with 0.1% Silwet or equivalent silicone-based spray adjuvant.

ReTain shows great promise for easing some of the stress of harvest management, however fruit maturity testing will be as important as always for making good storage decisions. Finally, Retain-treated fruit should be stored separately from non-treated fruit to avoid exposure to external sources of ethylene.