ReTain - Some New Insights

J. Wargo, Regional Extension Specialist,
Cornell Cooperative Extension
Reprinted from Fruit Notes, Western New York Fruit Extension Program, Vol. 00, Issue 15, July, 2000.

By now most growers are familiar with ReTain and what it will do, however there are some misconceptions about this product that users should be aware of. After three years of extensive trials with ReTain here in New York and Southern Australia, it has become clear to me that ReTain works -it delays ethylene production and fruit ripening. It also delays blush development and the onset of greasiness on susceptible varieties (Jonagold, Gala). However, this is generally a function of delayed maturity and color eventually catches up. Good coloring strains of 'Empire' and 'Red Delicious' appear to experience little, if any color delay. ReTain is an effective stop drop material for varieties like 'McIntosh', and this can have major economic impacts in blocks with a history of high amounts of drop. Fruit size can be increased by letting fruit hang on the tree for an additional week or so, but ReTain has no direct effect on fruit size enhancement.

What about fruit firmness? Many people think ReTain will give them harder apples, but this is true only if treated fruit are harvested at the same time as untreated fruit. This is not how the product is used commercially; instead, ReTain-treated fruit are usually harvested later. When harvest is delayed by a week or more, firmness of ReTain-treated fruit will only be equivalent to that of untreated fruit harvested earlier. For example, if a grower harvested untreated and ReTain-treated Empire on Oct. 2 nd , ReTain-treated fruit would invariably be firmer, but if ReTain-treated Empire were harvested a week later on Oct. 9 th , they would be at similar levels of maturity and firmness as untreated Empire harvested on Oct. 2 nd . Therefore, in most cases, ReTain fruit can safely be harvested 7 to 10 days later without fear of rapid firmness decline. This is an important consideration for growers with large acreage of one variety, where harvesting fruit within the optimal harvest window is not practical.
Better quality of ReTain-treated fruit out of storage also has been suggested, but results have been variable. In some cases ReTain-treated fruit have been harder with less breakdown after storage, but not in others. I believe this is related to stage of maturity at the time of harvest more than any other factor. If ReTain-treated fruit were harvested less mature than untreated fruit, they would most likely be harder after storage with less breakdown. Since flesh firmness decreases and fruit breakdown increases as fruit become more mature, it is understandable why ReTain would have this effect. However, if ReTain-treated fruit were harvested later at similar levels of maturity to untreated fruit, there would probably be little difference in firmness or breakdown after storage. The benefit of ReTain is that harvest can be delayed without further deterioration in fruit quality.

Furthermore, in cases where storage quality is related to orchard factors such as fruit calcium or nitrogen levels, ReTain is less likely to have an effect on
storageability. Here are some other interesting observations I found in my studies with ReTain. We know that ReTain can delay or inhibit color development mostly on hard-to-color varieties such as 'McIntosh', 'Jonagold' or 'Gala'. Based on one research trial and several field observations, it appears that ReTain delays color more when combined with other factors that can delay color development, such as 1) nitrogen fertilization, 2) vigorous trees with dense foliage, 3) heavy crop lead, and 4) hail net. Applying ReTain to trees under the above mentioned conditions is likely to delay or inhibit color more than any one factor by itself. What does this mean for fruit growers? ReTain should be applied only to trees that are in a moderate state of vigor, with normal crop loads, and well exposed to sunlight. Avoid blocks that traditionally have problems with color development for various reasons. The strategy of letting fruit hang on the tree longer for improved red color development does not appear to be a good strategy for hard-to-color varieties in Western New York. Color improvement with ReTain is usually observed in warmer climates when cooler temperatures accompanying harvest delay result in better color. ReTain-induced color delay will be less severe on dwarf trees in high density planting systems compared to semi-dwarf low density blocks. In terms of economics, ReTain should be applied to high yielding mature blocks, since yields in young or lightly cropped orchards are likely to be too low to justify cost of the product.

In conclusion, ReTain has been a savior for some growers and a disappointment for others. The most important thing to consider is what you want out of the product. If the goal is to delay maturity of some blocks for better harvest management, then ReTain may be the answer for you. In some cases, growers can use ReTain to delay maturity of one variety while they are busy completing the harvest of an earlier maturing variety. I use the example of 'Empire' and 'Jonagold' in 1999. Since the 'Empire' crop was so large, and required extra time to harvest, putting ReTain on 'Jonagold' would have allowed growers to delay harvest of 'Jonagold' without sacrificing fruit quality. Obviously, since ReTain needs to be applied 4 weeks prior to anticipated harvest date, these types of decisions need to be made well in advance. If improved fruit quality (increased firmness, reduced greasiness) is the goal, ReTain-treated fruit should be harvested at the same time or shortly after untreated fruit, but expect color reduction on some varieties. Right now is the time to start thinking about ReTain.

Predicted CA closing date for 'McIntosh' was estimated to be September 2 the for Western New York. First pick generally starts about 7 to 10 days earlier, which puts the opening of the harvest window at the middle of September. For 'McIntosh', that means ReTain should be applied near mid-August, for Empire, the beginning of September. Estimates for other varieties can be based on harvest date for 'McIntosh' to gauge earliness or lateness of the season.