Managing the Effects of Spray Water Quality on the Efficacy of NAA Stop-Drop Sprays—Things We Used to Know(?), but Forgot

Way back in 1943, scientists in Washington State tested NAA at 10 PPM, with and without various additives, for apple fruit drop control. They observed that adding ½ pound of lime (calcium carbonate) reduced the effectiveness of NAA. Adding zinc sulfate reversed the negative effect of adding lime, and even improved the effectiveness of NAA when no supplemental lime was added (they didn’t measure hardness of their spray water). The loss of efficacy could have been caused by increasing water hardness (Ca ions in solution), by increasing the alkalinity of the spray water (CO3 in solution), or from both, since adding calcium carbonate increases both calcium concentration (hardness) and pH. The reversal of the lime effect when zinc sulfate was added seems to indicate that the cause of the loss of efficacy was due to hardness, since the sulfate would have combined with the calcium to form an insoluble precipitate, that is, the zinc sulfate presumably acted as a water conditioner.

Most growers have observed that young apple leaves “flag” when NAA is applied as a chemical thinner in spring. This bending of the leaf stem is caused by a burst of ethylene from the plant, and the degree of flagging is proportional to the amount of NAA that is absorbed by the leaves. In 1960, Westwood and Batjer showed that hard water (333 PPM) reduced flagging from 6 PPM NAA by 57%, compared to NAA applied in distilled water. Softening the hard water eliminated this inhibition, again indicating that the cause was due to hardness. They also evaluated the effects of adding a non-ionic surfactant to NAA sprays in both distilled water and hard water. While the surfactant improved the response to NAA in distilled water by 30%, it increased the response to NAA in hard water by120%. Thus using a surfactant partially overcame the effects of hard water on NAA absorption.

So for a time, we knew that hard water reduces the effectiveness of NAA, and that conditioning the hard water restores this effectiveness. Based upon studies of leaf flagging in young potted apple trees growing in a greenhouse, we knew that using a non-ionic surfactant also went a long way to overcome the negative effects of hard water.

In 1972, Duane Greene and John Bukovac showed that uptake of NAA into pear leaf disks was much more rapid when the pH of the spray mix was 3.2 than at pH 5.2. So the pH of the spray mix also was shown to be important to leaf penetration of NAA.

Next, we jump forward to 2000, when German scientists reported their findings on the effects of water quality, various adjuvants, temperature and humidity on penetration of NAA across excised pear leaf cuticle. They reported that while hard water inhibited NAA penetration, high pH inhibited it even more. A non-ionic surfactant increased penetration a little bit, but addition of an “accelerator adjuvant” greatly increased penetration of NAA. Based on their findings, and considering the effects of temperature, sunlight and humidity on NAA uptake and on NAA breakdown, these scientists concluded that water quality (hardness and especially pH) is crucial to the effectiveness of NAA as a plant growth regulator and that use of a penetrating surfactant was beneficial to getting adequate uptake of NAA into the leaf. It would seem to be an open and shut case, would it not?

Why then call these conclusions into question? Consider the following:

•    Only one of these studies actually looked at drop control on whole apple trees in the orchard. The loss of drop control caused by lime amounted to only 4% more than that of the NAA spray with no lime added. Compared to the 54% drop on their untreated checks, this suggests that 10 PPM NAA still worked pretty good, even with lime in the tank.
•    The experiment that addressed this question in 1960 was on young potted trees in the greenhouse, and leaf angle was measured. Leaf flagging doesn’t seem to be well-linked to the effectiveness of an NAA spray as a thinner or as a stop-drop. It seems that the temperatures and sunlight for the 3 to 4 days after the spray have more to do with the effectiveness.
•    The other studies I listed here evaluated uptake of NAA into leaf disks or across excised leaf cuticles. Studies on the impact of water quality on NAA efficacy for drop control (or chemical thinning) on whole trees in the orchard are lacking.

Is uptake of NAA really the limiting factor to its success as a stop-drop or thinner? Just about everything we know about the impact of water quality on the effectiveness of NAA as a stop-drop is based upon studies of leaf penetration. There is more to the story not covered by these studies. First, while it is true that in order for NAA to work it must be absorbed by the leaf in a sufficient amount, often plant growth regulators are applied at higher concentration than that which is actually needed to influence the desired result. Next the NAA must be translocated to the site of action (the stem of the apple) in sufficient concentration to act, and then the fruit must be at a receptive stage of development to respond to it. Some of the NAA will be metabolized by the plant, and become neutralized. All these factors, each one critical to the success of the growth regulator, are influenced by the plant’s metabolism. That in turn is greatly influenced by sunlight and temperature, before, during, and especially after the growth regulator is applied.

Nevertheless, it seems there is strong evidence that water quality can affect the penetration of NAA into leaves, and perhaps some of us overlooked this in recent years. Perhaps this oversight reduced the effectiveness of our NAA stop-drop sprays. It is also possible that we “forgot” these factors because other factors, such as NAA concentration, surfactants, timing, and weather were just as important or more so. NAA is a tool we’ve had for a couple of generations, and we’re still not sure exactly how to use it to best advantage. So what to do?

Penetration of the leaf is the first step to an effective plant growth regulator application, and there is a large body of evidence that at least suggests that it could be a limiting step in getting good results. Fast drying conditions (low humidity) and high sunlight often go hand-in-hand—conditions that could shorten the time for absorption and speed the breakdown of the NAA residue. Penetration of NAA into the leaf is also much slower when the air temperature is 50° F as compared to when it is 70° F. Until we know more about whether this reduces efficacy as a stop-drop, the following guidelines should be followed:

•    Use a concentration of NAA that will do the job. I suggest using a minimum of 10 PPM (4 oz. of product per 100 gallons), applied in an adequate volume of water to get thorough wetting of the leaves.
•    Use a surfactant, and consider one that is listed as a penetrant as well as a spreader-sticker.
•    If your spray water source provides hard water, add a conditioner before adding the stop-drop. One pound of ammonium sulfate per 100 gallons of finished spray mix, or one of the propriety conditioning products may be used for this purpose.
•    Use an acidifying buffer at a rate to reduce the pH of your spray water to 4.0.
•    Don’t apply NAA with calcium fertilizers.
•    Refer to the Plant Growth Regulator section of the Pennsylvania Tree Fruit Production Guide for more information on monitoring drop and other guidelines.