Weather determines the thinning window
Chemicals thinners can be applied over a wide range of times, and depending on the year, this may be from bloom until fruit reach a diameter of 25 mm.
The weather largely determines the length of the thinning window of opportunity. If the weather following bloom remains relatively cool and sunny, fruit growth may be slow and developing fruit are placed under little stress, so thinning can be carried out over a longer period of time. Conversely, if the weather is warm to hot and sunlight limited, fruit growth is rapid, fruit are stressed by competition from other fruit and growing shoots and the time period when fruit can be thinned is shortened.
Growth and development of fruit during the thinning period is dynamic and continuous. There are specific times during this developmental process when fruitlets may be especially vulnerable to chemicals with specific modes of action, while at other times all thinners may be effective. Each time period during the developmental process provides thinning opportunities, and strategies may be devised to thin at these times.
The thinning strategy that has evolved in the eastern United States in recent years is to have a thinning program that applies thinners two or more times over the thinning period. This is good, since it spreads the thinning out over time and developmental stages and increases the possibility that thinner application will coincide with favorable thinning weather and conditions. It also tends to be safer, since less aggressive thinning treatments are generally used, and thus the chance of over-thinning is reduced.
The thinning stages selected and presented here should be considered ranges rather than distinct periods. These may change slightly, depending on the year.
Bloom. Thinning at bloom offers several advantages over thinning at other times. Early flower/fruit removal usually results in the largest fruit at harvest and the greatest return bloom the following year. Efficacy of caustic blossom thinners is less influenced by the weather, since it may not be necessary to have specific physiological conditions exist within the fruit to be effective.
Growers are reluctant to embrace blossom thinners, especially caustic ones, for several reasons. The potential for undesirable phytotoxic effects is generally quite high in the humid regions of the east. The possibility of frost and freezes still exists and the occurrence of rain and the development of backdoor-cool cold fronts are prevalent on the East Coast, and they can influence pollination and fruit set. The pros and cons must be weighed, but application of thinners at bloom is a viable time to make an application.
Petal fall to 5 mm. Specific apple varieties bloom at different times and they are usually categorized as early, midseason or late flowering. Consequently, petal fall occurs at different times. From a practical standpoint, petal fall for an orchard should be considered the time when bees are removed. For many growers in the east, this is considered the first thinning opportunity, and application at this time is a critical component in the multiple thinner approach that is now embraced by most growers.
Carbaryl is the most popular thinner used at this time, primarily because it is relatively safe to use, and over-thinning with it is remote at best. NAD is a useful thinner at this time. NAA, or a combination spray of NAA plus carbaryl, is frequently recommended on more difficult to thin varieties where aggressive thinning is appropriate. BA is not recommended for use at this time, because it has been shown to be weak or ineffective when applied. We recommend that the petal thinning spray should go on soon after removal of bees from the orchard, rather than waiting for specific and perhaps more favorable weather conditions. You will always have another thinning opportunity, so the sooner it is applied and can start to work, the better.
7 to 14 mm. This is the growth stage when apples are most susceptible to thinners. All thinners are effective at this developmental stage, although NAD is less frequently used at this timing. Weather plays a critical role in thinner response. For the first six to eight days after petal fall, growth of the fruit is slow – thus the need for carbohydrate to fuel fruit growth is low. However, when fruit size reaches 6 to 8 mm, a logarithmic phase of growth starts, which in turn fuels a demand for the limited carbohydrates being produced by the spur leaves.
There is competition for these resources among developing fruit on a spur and the growth of the bourse shoot. Application of thinners accentuates the stress. If this application is followed by three to four days of warm/hot weather with reduced light, carbohydrate resources for the developing fruit will be reduced, which results in a signal that is sent by the weakest fruit to initiates the abscission process. In the absence of this weather-created carbohydrate stress, thinning will be much less.
It is much more important to time your thinning spray application to take advantage of this weather-imposed stress, rather than completely relying on fruit size as a clue to make the application. A good rule of thumb is once fruit reach the 6 mm size range, look for the first three-day period of predicted favorable thinning weather, then make your application just prior to the start of this period.
15 to 19 mm. This is a much more difficult time to thin, since spur leaf area has expanded and some fruit have initiated the abscission process – thus the carbohydrate deficit is usually not as great. If fruit have not been stressed to this point, adequate thinning is often possible. Carbaryl and BA can be useful at this time, NAD is not used and the use of NAA carries the risk of negatively impacting fruit size. This is especially true if the weather is hot and elevated rates of NAA are applied. Ethephon may be used at this time, but it may be preferable to try alternatives before resorting to this thinner.
20 to 25 mm. When fruit reach this stage, experience has shown that ethephon or ethephon plus carbaryl are the only sprays that have a chance of reducing crop load, and this is by no means a certainty. Alternatives include hand thinning and accepting reduced return bloom for the next year. Ethephon at 1 pint/100 gal. (300 ppm), plus 0.5 to 1 lb./100 gal. carbaryl may be a good compromise between getting a thinning response while avoiding excessive thinning. An added benefit of applying ethephon at this time is the potential to achieve increased return bloom.
By Duane W. Greene, University of Massachusetts