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ReTain

The active ingredient in ReTain is aminoethoxyvinylglycine (AVG), a "look-alike" for one of the chemical precursors to ethylene. When absorbed into plant tissues, AVG binds irreversibly with a key enzyme. This prevents the ethylene precursor from binding, thus blocking the production of ethylene. Natural ripening processes are slowed, including stem loosening, fruit flesh softening, starch disappearance, and red color formation.

There are several potential harvest management benefits to slowing the fruit maturation process. Growers can spread the effective harvest window for a given variety, retaining fruit firmness and without excessive drop. Apples on unstressed healthy trees will continue to grow at the normal rate following ReTain treatment (about 1 mm per day). An additional week on the tree can add a quarter inch to fruit diameter. Fruit red color can be increased in cases where a delay in harvesting exposes the fruit to improved weather for coloring (warm sunny days and cool nights). The incidences of fruit disorders associated with ripening, such as water core and stem end cracking, can be reduced.

In order for AVG to be effective it must be applied well in advance of the climacteric rise in ethylene production that signals the onset of fruit maturity. The label recommends applying ReTain 4 weeks before anticipated harvest. This has sometimes caused confusion, as the grower is timing the spray relative to some future, unknown date. A more scientific basis for timing would be to state that ReTain should be applied 4 weeks before the natural climacteric rise in fruit ethylene, but this is still a future event with an element of uncertainty. The good news is that there is a fairly wide window when ReTain can be applied with optimal results, and a fairly easy way to determine when to apply it (Figure 1-6).

For early season varieties, such as Gala and McIntosh, start by estimating when you would normally expect to begin harvesting the variety if no ReTain or ethephon were used. Now take into consideration the season. For instance, if the bloom date and the ripening pattern of cherries, peaches, and summer apple varieties suggest that the season is about 10 days earlier than normal, the anticipated harvest date can be adjusted accordingly. Then count back 4 weeks on the calendar. Now mark the calendar from that date through the next 7 days. This is your application window for that early season variety. Watch for good spray conditions and a 6-hour drying time within that week and apply the material at the first opportunity. Congratulations! Your ReTain is on at the right time. Now mark your calendar for 7 days after the spray was applied. This is the preharvest interval, as required by the label. You cannot legally harvest before this date. Repeat the same thought process for later varieties, but keep in mind that later varieties are usually less affected by seasonal variation in maturity than stone fruit or early apple varieties. It is usually unnecessary to account for seasonal variation in fruit maturity for Empire and later varieties.

ReTain is an expensive chemical to produce, and the maximum rate of AVG allowed by the label is at the low end of the effective range. For most applications, every molecule we apply needs to be absorbed and do its duty. One exception to this rule is application to low ethylene varieties.

The active ingredient in ReTain is aminoethoxyvinylglycine (AVG), a “look-alike” for one of the chemical precursors to ethylene. When absorbed into plant tissues, AVG binds irreversibly with a key enzyme. This prevents the ethylene precursor from binding, thus blocking the production of ethylene. Natural ripening processes are slowed, including stem loosening, fruit flesh softening, starch disappearance, and red color formation.

There are several potential harvest management benefits to slowing the fruit maturation process. Growers can spread the effective harvest window for a given variety, retaining fruit firmness and without excessive drop. Apples on unstressed healthy trees will continue to grow at the normal rate following ReTain treatment (about 1 mm per day). An additional week on the tree can add a quarter inch to fruit diameter. Fruit red color can be increased in cases where a delay in harvesting exposes the fruit to improved weather for coloring (warm sunny days and cool nights). The incidences of fruit disorders associated with ripening, such as water core and stem end cracking, can be reduced.

In order for AVG to be effective it must be applied well in advance of the climacteric rise in ethylene production that signals the onset of fruit maturity. The label recommends applying ReTain 4 weeks before anticipated harvest. This has sometimes caused confusion, as the grower is timing the spray relative to some future, unknown date. A more scientific basis for timing would be to state that ReTain should be applied 4 weeks before the natural climacteric rise in fruit ethylene, but this is still a future event with an element of uncertainty. The good news is that there is a fairly wide window when ReTain can be applied with optimal results, and a fairly easy way to determine when to apply it (Figure 1-6).

For early season varieties, such as Gala and McIntosh, start by estimating when you would normally expect to begin harvesting the variety if no ReTain or ethephon were used. Now take into consideration the season. For instance, if the bloom date and the ripening pattern of cherries, peaches, and summer apple varieties suggest that the season is about 10 days earlier than normal, the anticipated harvest date can be adjusted accordingly. Then count back 4 weeks on the calendar. Now mark the calendar from that date through the next 7 days. This is your application window for that early season variety. Watch for good spray conditions and a 6-hour drying time within that week and apply the material at the first opportunity. Now mark your calendar for 7 days after the spray was applied. This is the preharvest interval, as required by the label. Repeat the same thought process for later varieties, but keep in mind that later varieties are usually less affected by seasonal variation in maturity than stone fruit or early apple varieties. It is usually unnecessary to account for seasonal variation in fruit maturity for Empire and later varieties.

ReTain is an expensive chemical to produce, and the maximum rate of AVG allowed by the label is at the low end of the effective range. For most applications, every molecule we apply needs to be absorbed and do its duty. One exception to this rule is application to low ethylene varieties.

Varieties differ in the production of ethylene and consequently differ in their response to ReTain. Low ethylene producers such as Gala are strongly influenced, while ethylene production is much harder to control for high ethylene varieties such as McIntosh. Fruit maturity of Gala on dwarfing rootstocks can be slowed with 7.5 oz (two-thirds of the full label rate) of ReTain if timing and application recommendations are followed closely. Most varieties, however, require the full labeled rate of ReTain (1 pouch or 333 grams per acre) in order to obtain satisfactory results.

Growers who are planning to use ethephon (Ethrel, Ethephon II) to color Macs should apply the full rate of ReTain at the usual time. There is some limited research that suggests ReTain can offset the deleterious effects of ethephon on fruit maturity and fruit softening.

Use a 100 percent organosilicone surfactant, such as Silwet L-77 or Sylgard 309 at 12 oz per 100 gallons. For optimum results, apply ReTain with 100 gallons of water per acre and spray each row from both sides. Split applications do not enhance the performance of ReTain. A single well-timed spray will give the best delay in maturity and best control of preharvest drop.

ReTain requires a considerable time to act, and its effects persist for a considerable period of time, so the effect of weather is little understood. It is known that trees under stress (mites, drought, etc.) are less responsive to ReTain and are poor candidates for its use. Much of what is known centers on the need to maximize coverage and absorption.

Ideally, no rain should fall for at least 6 hours after ReTain is applied; however if the coverage was good, the ReTain spray was applied with a full rate of Silwet, and the residue dried before it rained, you probably got most of the benefit of the spray. Just keep a close eye on drop and fruit maturity.

Application of ReTain under slow drying conditions is thought to be beneficial. Conversely, spraying ReTain on wet foliage can result in a loss of performance due to the material dripping off before it can be absorbed. Wait until the foliage dries. If you must apply ReTain to damp foliage, reduce the rate of Silwet to 6 fluid oz instead of 12 to reduce the sheeting action and possible runoff. Another tactic that can work on a limited scale if time is running out is to drive slowly through the block to be sprayed with just the fan on before applying the spray.