Fruit thinning is a management practice that reduces the number of fruits per tree in the current season, resulting in increased fruit size of the remaining fruits and increased return bloom and yield in the next season. Although it can be difficult to achieve, fruit thinning is the single most important management strategy in determining the annual profitability of apple orchards.
Management of crop load is a balancing act between reducing crop load sufficiently to achieve optimum fruit size and adequate return bloom and not reducing yield excessively.
Inadequate thinning can produce the following results:
Overthinning can result in the following consequences:
Reduction of crop load can be accomplished through hand thinning or chemically induced thinning, although chemical thinning typically is the preferred method.
Hand thinning usually is not a viable option for managing crop load. It is expensive, ranging from $1,000 to $2,000 per acre, and the supply of available labor on most fruit farms is too limited to hand thin the crop. Moreover, when growers do perform hand thinning, they usually do so six to eight weeks after bloom, after a significant reduction in fruit size already has occurred.
For the past 50 years, chemical thinning using plant-growth-regulating chemicals applied shortly after bloom has been the preferred method growers use to achieve fruit thinning. Various chemicals can cause fruit thinning. The most common chemical thinners are synthetic versions of natural plant hormones in the auxin and cytokinin categories. Despite 50 years of use, chemical thinning remains an unpredictable part of apple production as its results vary significantly from year to year and within years due to weather. Chemical thinning is not an exact science, and each grower must weigh and evaluate the many factors that affect chemical thinning response when deciding on a thinning program.
The following factors are major sources of variability in chemical thinning response:
Variability in spray uptake relates to the chemical thinner concentration, the environment at the time of application (temperature and humidity), application method and coverage, drying conditions, and leaf epicuticular wax. However, temperature and humidity largely compensate for one another in affecting drying time and uptake. Many of these factors are directly related to the balance of carbohydrate supply from photosynthesis in relation to the demand for carbohydrates from all the competing organs of the tree (crop, shoots, roots, and woody structure). After fertilization, young fruit require currently produced carbohydrates for continuous development, and the extent of this demand appears to be associated with the stage of fruit development and level of light. After petal fall, rapid fruit growth results in an ever-increasingly large carbohydrate demand that may not be met by current photosynthesis. After bloom, the total carbohydrate demand of growing shoots and developing young fruitlets increases more rapidly than does photosynthesis, leading to carbohydrate deficits during periods of cloudy weather or excessive heat. Chemical thinners are reputed to work by providing a transient stress on the tree when fruits are most susceptible to a carbohydrate deficit. Chemical thinners appear to have the capability to create a carbohydrate stress by reducing photosynthesis, increasing respiration, or impeding carbohydrate movement to the fruit. Many have observed that the greatest fruit abscission caused by thinners is associated with periods (three to five days) of reduced carbohydrate availability immediately following thinner application. These weather conditions are generally a combination of warm temperatures and low light.
Depending on the chemical used, chemical thinning can be done at various times, beginning with full bloom and ending when fruits have reached 20 mm in diameter. The most common timings are at full bloom, at petal fall, and when fruits are 10-12 mm in diameter. Bloom thinning can be done with caustic thinning chemicals that burn flower pistils or with hormone-type thinners such as naphthaleneacetic acid (NAA). Postbloom thinning at petal fall or when fruits are 10-12 mm in diameter is usually with NAA, an auxin-type thinner, or with benzyladenine (BA), a cytokinin-type thinner. A carbamate insecticide (carbaryl) also is used to cause fruit thinning. Combinations of NAA and carbaryl or BA and carbaryl are the most commonly used sprays. A myriad of possible combinations of chemicals, timings, rates, and varieties provide many possible thinning program options for growers. Most growers apply two or three applications to hard-to-thin cultivars, such as Empire, Gala, Jonamac, Macoun, Spur Delicious, Golden Delicious, and Romes to achieve the optimum crop load. With easy-to-thin cultivars, growers typically use a single application when fruits are 10-12 mm in diameter.
Terence Robinson, Cornell University