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NAA
Shortly after synthetic auxins (NAA, 2,4-D, 2,4,5-TP) were discovered, it was learned that these materials had activity as stop drops. Auxins interfere directly with the enzymes that create the abscission zone. Today, the one remaining auxin that is registered for this use is NAA (Fruitone N, Fruitone L).
Unlike ReTain, fruit maturity of apples treated with NAA is not delayed; in some cases, it may be accelerated. Since the optimal application time for NAA is just before the onset of drop, NAA offers a “rescue” treatment, should the threat of preharvest drop be increased due to unforeseen circumstances. Examples of such situations include unavoidable delays in harvest due to bad weather or labor issues, slow red color development, and overlapping harvest schedules of varieties with similar maturity windows, such as that of Empire and Delicious.
A single spray of NAA can provide about 7 days of drop control. Since it is less expensive than ReTain, it may be more cost effective to use NAA when only a few days of drop control are needed to conduct an orderly harvest. For example, when using ethephon (Ethrel, Ethephon II) to promote fruit coloring, growers should also use NAA to prevent excessive fruit drop resulting from accelerated fruit maturation. When NAA is used to control drop on ethephon-treated trees, the two may be tank-mixed if the fruit are to be harvested within 7 days. If the fruit are to be left on the tree longer than 7 days after the ethephon, then NAA should be applied 2 to 3 days after the ethephon.
Other than when applying NAA with ethephon, timing an NAA stop-drop spray is a little like a game of chicken, requiring both steely nerves and a good understanding of your opponent (Figure 1-7). The label says to apply NAA when the first sound fruit begin to drop. Based on research with McIntosh, a variety that is very prone to drop, a single spray of 10 to 20 ppm NAA can control drop for about 7 days from the date of application, but it takes 2 or 4 days to “kick in.” If NAA is applied too early, then effective drop control may wear off when it is needed most. On the other hand, if NAA is applied just a few days too late, a significant portion of the crop may be on the ground before it takes effect. Predictive degree-day models and the pattern of starch disappearance, as gauged by the starch index test, can provide a general indication of whether the potential for drop is earlier or later than normal, but more direct monitoring is required for the actual timing of the sprays.
Varieties that are susceptible to preharvest drop should be monitored to determine when fruit drop is beginning. Limb tapping is one method that can be used to determine the onset of drop as fruit near maturity. Bump several scaffold limbs of 3 or 4 inches in diameter throughout the block on a daily basis. Use the palm of your hand with a short firm stroke, striking the limb at its mid-point (just like golf, this skill improves with practice and experience). If zero to one apple per limb drops on average, it is too soon to apply NAA. If the average is about two, check again later the same day or the next morning. When several apples drop in response to limb bumping, it is time to harvest within 2 days or apply NAA.
Rates of 10 to 20 ppm NAA are usually needed to be an effective stop-drop. To obtain the maximum drop control, use a split application of 10 ppm in the first spray, followed by a second spray of 10 ppm five days after the first. Split applications can provide some drop control for about 12 days from the date of the first application.
NAA must be taken up by the spur leaves in order to be effective and does not translocate very far within the tree, so it must be applied with good coverage and plenty of water. Concentrating beyond 4X (less than 75 gallons of water per acre for 300 gallon TRV trees) may diminish the effectiveness. The use of alternate row spraying is discouraged. Use of a nonionic or organosilicone surfactant is recommended to enhance uptake.
Calcium in the spray water is detrimental to NAA efficacy. This includes both tank-mixed calcium for bitter pit control as well as calcium present because of hard water. If your water source provides hard water, use of a water conditioner is advisable.
Weather conditions following the application also impact efficacy. Rewetting within 2 or 3 days of the spray application and spraying under slow drying conditions (high humidity) will both increase the uptake of NAA. Temperatures in the mid-70s produce a better response than cooler temperatures, while excessively hot weather immediately following an NAA spray may result in accelerated ripening.
When used as a stop-drop, NAA may advance ripening, especially at the maximum label rate of 20 ppm. The primary impact of this advance in maturity is reduced storage potential of the fruit, particularly in the loss of firmness. This effect is not consistent from year to year and may be attributed to high temperatures, as mentioned previously.
Some research showed that the deleterious effects of NAA sprays on fruit maturity and fruit softening were minimized in Delicious by making repeated applications of 5 ppm NAA at four weekly intervals prior to harvest. This “preloading” technique has recently been included as an application option on the Fruitone labels. This research was repeated on Delicious in 2004, and NAA preloading accelerated ethylene and fruit softening. Also, using NAA preloading on McIntosh resulted in more advanced ripening, accelerated drop, and fruit softening, not less! NAA preloading for McIntosh and other early season, high-ethylene varieties is not recommended. Growers should use caution when trying preloading on high ethylene-producing varieties until more is known about how different varieties will respond.
The question then arises whether NAA-treated fruit have potential for CA storage or treatment with SmartFresh (1-MCP). Perhaps the simplest way to answer the question with regard to CA is to remember the adage “garbage in, garbage out.” If the fruit were left on the tree to the bitter end of the drop control, are measurably softer than previously harvested fruit, and have elevated starch index values, then they should be marketed in the short term. On the other hand, if the fruit were harvested within a week after treatment and have appropriate firmness and starch values for CA storage for the variety, there is little reason to expect performance different than similar fruit that received no NAA.
The question of whether NAA stop-drop sprays have advanced fruit maturity may be most critical when using SmartFresh on McIntosh, where the maturity of the fruit is an overwhelming influence on whether the fruit will respond to 1-MCP. Quoting Dr. Chris Watkins in the Proceedings of the 2003 Cornell Apple Storage Workshop, “We do not have any data yet, but we assume that induced ethylene production that results from use of NAA will deleteriously affect fruit responses to 1-MCP. If you use stickers [NAA stop-drop], your storage operator should be informed.”
Finally, a comment about use of NAA on trees previously treated with ReTain. The use of both stop drops at the respective correct times results in drop control that is superior to that obtained by using either one alone. Fruit treated in this manner, then left for an extended time on the tree, often have limited storage potential (see above). This combination, however, can be an effective way of getting the ultimate in drop control. This drop control comes at a high price and should therefore only be used on high value fruit with little or no storage period, such as for a few rows of trees held for late picking in pick-your-own blocks.
Table 1-17 summarizes the attributes of ReTain and NAA. Both stop drops are useful compounds, although quite different with respect to activity and timing.
