Duane Greene, UMass Amherst
May 10, 2011 (Reprinted from Healthy Fruit)
All production of carbaryl in the United States was stopped earlier this year. Therefore, carbaryl used in the United States from now on will be imported. Further, carbaryl can no longer be used in many countries in Europe. These events are interpreted by many to mean that carbaryl may no longer be available for use in the very near future. Since you do have carbaryl still available now there is no panic. However, in anticipation of the loss of carbaryl, this may be a good time to evaluate alternatives in your own orchard so that you will be able to more comfortably and with some degree of confidence move to a thinning program that does not include carbaryl.
Bloom is approaching or has arrived, thus your first thinning spray of the years will soon be applied. Carbaryl has been the most popular petal fall thinner and most growers do depend heavily on this thinner at petal fall. I have stated previously that the petal fall treatments may be the most important thinner application since it sets the stage for subsequent thinner application(s) and it generally allows for less aggressive thinner applications later. In the absence of carbaryl, thinning at this timing may take on added and increased importance and urgency.
At petal fall fruit growth is extremely slow. Fertilization of ovules has or is taking place. Following fertilization there is a period of time when the fertilized ovule undergoes cell division and starts to produce the hormones which are largely responsible for the driving of fruit growth. Generally this slow growth period lasts for 6 to 7 days. Since the growth of fruit is slow the demand for photosynthate by fruit is also relatively low. It is for this reason, in part, that thinning activity of thinners applied at petal fall is generally less than at the later and more traditional 10 mm timing. Higher rates of thinners can and should be used at petal fall since these higher rates will be necessary to achieve meaningful thinning at petal fall.
Thinner Options
NAA This is a thinner that all are familiar with since it has been in common use for many years. It is less active at petal fall, so higher rates should be used. My rule-of-thumb is to apply twice the concentration of NAA at petal fall that you would consider using at the 10 mm stage. I am suggesting a base level of 10 ppm to start with. In some circumstances 15 or 20 ppm may be appropriate.
Amid-Thin Amid-Thin is no longer routinely used. In the New England Tree Fruit Management Guide it is recommended for use only as a petal fall spray on Early McIntosh, Lodi, Quinte and Yellow Transparent. It is time to resurrect this thinner, at least on a trial basis. Amid-Thin was evaluated as a thinner with NAA in the 1950s. When compared with NAA at the 10 mm timing it was less effective, so NAA became the product of choice. Amid-Thin also produces pygmy fruit on some varieties (Delicious) especially when applied at the 10 mm stage. It was evaluated as a petal fall spray during development and it was found to be quiet effective. Since thinning was generally done only once during that fruit growing era, and greater thinning was achieved with a 10 mm application, the use of Amid-Thin was relegated to use as a petal fall spray on early varieties and Macoun. Petal fall application of thinners did not come into general used until recently and Amid-Thin was long-forgotten by then. I am suggesting that you may want to consider evaluation of Amid-Thin as a petal fall spray in lieu of carbaryl. The suggested rates would be 25 to 50 ppm.
Promalin This is used in some circumstances as a thinner at bloom especially at the 2 pt/100 gal rate with a surfactant on Delicious. This is also commonly used on Gala in Chile and Washington. The thinning action is generally not strong and higher rates with the surfactant may increase thinning activity but it may also reduce return bloom.
MacCel MaxCel is generally not recommend for use at petal fall because if does not thin nearly as well or effectively at this time. Where increased fruit size is desired, a spit application may be useful, but this is infrequently done.
Ethrel While the use of this compound is generally reserved to those who are not faint-hearted, it is an effective bloom/petal fall thinner. Ethrel differs from other thinners in that it thins at bloom/petal fall and at the 20 mm stage but is less effective at the 10 mm stage.