ReTain and NAA as a standard practice for apples
Dissecting the use of harvest management tools for apple growers.
Harvest management is a primary reason growers use ReTain on apples. However, there are many other benefits that ReTain can offer apple growers. The ReTain effect on improving storage quality is a major desire for shippers and processors. ReTain stops the production of ethylene in the apple fruit and thus delays maturity and lengthens shelf life. Michigan State University Extension’s best recommendation is to use ReTain at full rate split into two sprays at 30 and 14 days before harvest. Add NAA at 20 ppm with the 14-day before harvest application. This will give you maximum stop drop and good harvest management. Remember, some sensitive varieties like Gala and Jonagold should only receive a seasonal rate of half the full rate.
ReTain will delay apple maturity, thus allowing harvest management of large blocks of one variety. ReTain is also dose-dependent, that is, a full rate will give maximum delay of maturity, say, 14 days. Half rate will only give half of the delay of maturity, say, seven days. This allows growers to better plan harvest. For example, one-third of your Galas could be treated with a half rate of ReTain and one-third of your Galas could be treated with one-quarter rate and thus the grower could harvest untreated Galas one week, the one-quarter rate treated Galas the next week, and the half rate treated Galas the third week. All the Galas would be picked at prime maturity over a three-week period.
ReTain is dose-dependent and time-dependent. The standard recommendation of ReTain use is to apply full rate at 30 DBH (days before harvest). However, a grower can apply the ReTain 14 DBH or even in split applications. There can be some complications from delaying ReTain applications closer to harvest, but it also allows for more grower flexibility of ReTain use. Later applications of ReTain tend to have less impact on red color development and allow for combinations with NAA. Typically, NAA is applied about 14 DBH of any variety. Waiting to apply ReTain 14 DBH allows growers to make one application of the combination ReTain + NAA instead of two trips (ReTain at 30 DBH followed by NAA at 14 DBH).
Other major benefits of ReTain include its stop drop impact on apples. ReTain stops ethylene production, which stops the ethylene ripening effect and, thus, dropping fruit – a ripening effect. NAA does this as well, but by a different effect, that is delaying the formation of the fruit abscission zone. Unfortunately, NAA also turns on ethylene which, in turn, will ripen fruit and after NAA wears off, fruit drop will be enhanced. ReTain, however, will eliminate the NAA-induced ethylene. So in using the combination of ReTain + NAA you get the best of both worlds, improved stop drop from both materials and ReTain’s control of the NAA induced ethylene ripening.
ReTain also reduces watercore, greasiness, cracking and improves fruit size. Shippers and processors like the improved storage quality and longer shelf life of ReTain-treated fruit, especially the greater fruit firmness from ReTain. The combination of ReTain + NAA is an improvement over using either material alone. We still have to work out what is the lower rate of ReTain that will work with NAA and more work on the timing of each material.
Overall ReTain + NAA has performed well in the past four years of trials on all varieties.