Thinning is required to optimize fruit size and quality in apple orchards. Thinning also helps keep trees from biennial production and maintains a balance between vegetative growth and fruiting. Apples can be thinned chemically, by hand, or by a combination of both.
The primary advantage of hand-thinning is that poor quality or damaged fruit can be removed. Hand-thinning is traditionally done after the "June" drop (late May) when the fruit are between the size of a cherry and a walnut, but this is too late to greatly affect biennial bearing. When hand-thinning, thin apples so that fruit clusters are about 4 to 6 inches apart with one fruit per cluster for fresh-market varieties, especially red ones, or two to three fruit per cluster for processing apples, green varieties, or light crops. Before hand-thinning, take into account the total number of fruit on the tree so that the crop is not over-thinned; this is especially important following chemical thinning.
Successful use of chemical thinners on apples is determined by many factors that influence the response of trees to applied chemicals. These include (but are not limited to):
Weather before application
Cool, wet conditions may precondition leaves for greater absorption of thinning agents, leading to increased activity.
Weather during/after application
Cool, wet, or humid weather prolongs drying, giving greater activity. High temperatures following application, particularly if they follow cool periods, can cause mild tree stress and tend to increase thinning activity of some chemicals.
Stress from any source (low nitrogen, lack of water, root damage, heavy crop in previous year, shading within canopy, etc.) may increase thinning response to applied chemicals.
Young, excessively vigorous trees are easier to thin than older, moderately vigorous trees.
Natural postbloom drop
Over-thinning may occur when natural postbloom fruit drop is heavy, particularly on Gala and Granny Smith.
The rates and timings recommended here are those that have given the most acceptable and consistent results in field trials and grower experience to date; they are provided as suggested guidelines for growers who wish to begin accumulating experience in using chemical thinning as a management tool.
Rates of chemical thinning agents should be adjusted within the recommended ranges based on bloom density and conditions during pollination and fruit set. When bloom is moderate or environmental conditions are favorable for thinning, use lower rates within the ranges specified. Use higher rates where bloom is heavy or environmental conditions are less favorable for thinning.
Careful timing of spray application is important if predictable and acceptable results are to be expected. Proper timing, choice of chemical and rate varies dramatically according to variety. Most varieties are thinned at petal fall or shortly after fruit set. Because chemicals do not thin uniformly throughout the tree, supplement chemical thinning with hand-thinning to break up large clusters.
Rates given pertain to applications using sufficient water to thoroughly wet trees (150–250 gallons/acre) without run-off. The optimum spray volume depends on tree size, planting density, row spacing, and stage of growth. Field experience with low-volume applications is lacking, and they are not recommended. Reports from other areas indicate that low-volume applications yield less consistent results than dilute applications. To help prevent over-thinning in the lower portion of tree canopies, adjust the spray pattern to reduce the volume of solution applied to this area.
1) In mixed variety blocks, take precautions to minimize drift of applied chemical thinners to non-target varieties. Spray drift from applications made to hard-to-thin varieties such as Fuji may over-thin Gala and Granny Smith.
2) When used at the rates suggested, carbaryl is toxic to honeybees if applied to bees during daytime while they are active in the orchard. Remove colonies from the orchard before spraying, and/or spray at night to prevent direct application to foraging bees. Remove flowering weeds or cover crop in the orchard before spraying.
The best and most consistent results to date have been achieved using a single application of Sevin XLR Plus (carbaryl) at 1.5 to 4 pints (0.75–2 lb a.i.) per acre, applied when the largest fruit are 3/8 to 9/16 inch (10 –15 mm) in diameter.
The best thinning of the Gala variety has been achieved with a single petal fall application of Sevin XLR Plus at 3 to 4 pints (1.5–2 lb a.i) per acre plus the growth regulator Amid-Thin W (1-napthaleneacetamide) at 25 ppm.
Follow-up hand-thinning is usually required following this treatment.
Of the varieties discussed here, Fuji is the most difficult to thin with chemicals. The best and most consistent experimental results have been obtained with the following combination treatment:
Sevin XLR Plus at 6 pints (3 lb a.i.) per acre plus the growth regulator Amid-Thin W at 50 ppm at petal fall, followed by another application of Sevin XLR Plus alone at 6 pints (3 lb a.i.) per acre when largest fruit are 3/8 to 9/16 inch (10–15 mm) in diameter.
This combination has not provided complete thinning of Fuji, and follow-up hand-thinning is usually necessary.
Experience with chemical thinning of this variety (in controlled experiments and commercial practice) is very limited, so the following treatment is suggested as a tentative guideline only. Pink Lady appears to be relatively easy to thin, increasing the risk of over-thinning under conditions favorable to thinning.
The best and most consistent experimental results to date have been obtained with an early petal fall application of Sevin XLR Plus at 2 to 4 pints (1–2 lb a.i.) per acre.
There are several alternative treatments that alone or in combination are effective for thinning Golden Delicious.
Tank mix combinations of Fruitone N (sodium salt of 1-naphthalenaecitic acid) at 5 to 10 ppm and Sevin XLR Plus at 4 to 6 pints (2-3 lb a.i.) per acre applied when the largest fruit are 3/8 inch (10mm) in diameter have provided more effective and consistent thinning of Golden Delicious than applications of either material alone.