The nuances of stop drop applications in apples
NAA can be used as a stop drop application near harvest time to delay fruit drop, but watch moisture and temperatures.
We use NAA to thin apples in spring and growers are keenly aware of the impact of temperatures and slow drying conditions for that application. We also use NAA to delay fruit drop near harvest, so how do environmental conditions come into play at this near-harvest timing? Slow drying conditions are favorable for NAA as a stop drop material and are probably more important than temperature. Temperature is not as critical for stop drop application as it is when NAA is used for thinning in spring. However, there will be better uptake of NAA at 70 to 75°F than at lower temperatures such as 50°F.
As with most plant growth regulators, warmer temperatures result in increased activity of the material. Also, slow drying conditions are better than fast drying ones. Morning applications, when there is dew present and temperatures are rising, are ideal conditions for NAA applications. If temperatures reach 85 or 90°F (which is unlikely, but not unheard of for harvest season in Michigan), growers should use caution when applying NAA.
NAA works as a stop drop material by blocking the abscission layer (the site of where the apple stem connects to the spur). It takes about three days for NAA to kick in and its effect breaks down in about seven days after that, for a total of 10 days from the NAA application. If growers apply NAA too far ahead of the anticipated harvest date, a re-application may be warranted. A second NAA application needs to go on seven days after the initial application – don’t wait until the tenth day or it will be too late. Once the effect wears off, drop can begin quite quickly and drastically. Research has also shown that the actual process of spraying the second application can cause fruit to drop if it is done too late.
Growers should also note that the use of NAA for stop drop does sacrifice some storage life of apples, so growers need to be aware of these consequences if long-term storage of particular varieties is desired. Fruit targeted for fresh fall sales is the more appropriate use of NAA, rather than fruit for long-term CA storage.
Weather conditions aren’t as critical with Retain applications, but as a stop drop material, the timing for Retain might be too late if fruits are already producing ethylene. Retain and NAA might be best used together as a team. The reason NAA and Retain work well together is because Retain blocks the ethylene blocks that the NAA can suddenly promote when it wears off.
Related article: Doesn't the cold just frost your apples? MSU Extension