Apple Thinning and Promoting Return Bloom

It will soon be time to start hand thinning and applying return bloom sprays to apple orchards.
Apple Thinning and Promoting Return Bloom - News

Updated: August 8, 2017

Apple Thinning and Promoting Return Bloom

Chemical thinning in our region has been a challenge in 2017, to say the least. The growing season started very early. Bloom was about three weeks early with adequate pollination activity, leading to a strong initial set. Temperatures then shifted to cooler than normal for a protracted period of time. Prolonged periods of cloud cover led to long stretches of carbon deficit, but temperatures were too cold for chemical thinners to be very effective. With little stress to hold them back, fruit set strong and grew well during the long cool spring. Many growers were forced to make thinning applications under less than optimal conditions, while others like me waited for good conditions that never came. Rescue thinning with ethephon was prevalent this season, and many are still waiting to see how well it worked. The conditions of heavy initial set and sub-optimal thinning weather suggest that many of us will have heavily cropped trees in 2017.

Hand Thinning

It will soon be time to start hand thinning early maturing/high value apple varieties, such as Gala and Honeycrisp. Best outcomes from hand thinning are achieved when thinning is done early. Later thinning has diminished effect on fruit size and return bloom. For optimal results, thinning crews should be trained to break clusters and to remove small or damaged fruits first.

Research suggests that over-thinning is a common occurrence when apple fruits are thinned by hand. Calibration of thinning crews to thin to an appropriate level can be done with the Equilifruit disk (Figure 1). For each size of limb, there is an optimal number of fruit that can be carried. How many fruits are left is determined by selecting the notch on the Equilifruit disc that fits the base of the limb, then reading the fruit value listed on that notch. Workers quickly develop an eye for how many fruit to leave, which increases the accuracy of their thinning and prevents over-thinning. The disc can also be used to spot check and re-calibrate the thinning, as needed.

Figure 1. Equilifruit disk in use. Photo: J. Schupp, Penn State

How to use the Equilifruit disc for hand thinning fruits

  • To assure that each limb is properly measured and adjusted, start at the lowest limb on the tree and systematically work to the top of the tree until completed.
  • Approach a limb, place disc approximately 3 cm away from the trunk of the tree. Find the notch in the disc that fits snugly around the limb. Refer to the F-value that corresponds to the selected notch. This is the number of fruit that should be left on the limb after hand thinning.
  • Count / estimate the number of fruit that are present on the selected limb. If the number of fruit is greater than the F-value, then hand-thinning is necessary.
  • It is important to be selective when hand-thinning. Remove the following fruit in order of importance:
  1. injured/damaged fruit
  2. small fruit, and
  3. break up clusters of fruit

Note: Other hand-thinning recommendations emphasize the spatial distribution of fruit throughout the canopy; however, this system focuses on adjusting the number of fruit per unit limb size.

Other Considerations

Thinning the leader

Either select an arbitrary point near the terminus of the central leader (ex: the top wire) or take note of the point at which the there are many small limbs that cannot be measured with the smallest notch of the disc. At this point, use the disc on the leader and thin accordingly.

Small branches

If limbs are smaller than 0.5 cm2 (the smallest notch on the disc), then discretion is used to designate the number of fruit that will remain on the limb, either 1 or 2 fruit.

Delta values

The delta value (Δ) is a means of adjusting the F-value. Subtracting or adding delta from the F value can be utilized to intensify or reduce the hand thinning treatment and reduce or increase cropload in a consistent fashion. While delta values may be useful to further reduce cropload on younger trees, the standard F value gave the best results for mature trees in our trials.

Return Bloom Sprays

Return bloom (RB) sprays will be valuable on trees with heavy fruit crops this year, but note that excessively-heavy trees of biennial varieties are unlikely to generate adequate return bloom in 2018, even with RB sprays. Start RB sprays when fruits are 30 to 35 mm (about 1.25 inches) in diameter. Ethephon is the strongest RB promoter. Ethephon for this purpose is used at a low rate, 8 oz per 100, to minimize unintended effects such as premature ripening. Low rates of NAA (2 oz per 100) are also effective for stimulating RB. If there are concerns about reducing crop load by additional thinning, some growers prefer to start with an initial spray of NAA, then switch to ethephon for the subsequent sprays. Some growers tank mix ethephon and NAA, but there is no evidence to show that the addition of NAA promotes more flowers than use of ethephon alone. Likewise, the addition of foliar nitrogen to RB sprays is thought by some to promote additional flowering, but we have not been able to document this in research trials.

Addition of a nonionic surfactant can enhance the performance of the RB agent, however use caution not to increase the risk of leaf or fruit damage from increased uptake of other chemicals, especially when tank mixing RB materials with cover sprays. When in doubt, leave the surfactant out. RB promoters can go on with cover sprays, and can go on as ARM. Four RB applications are best and 3 is the minimum number of sprays for biennial varieties. The scheduling of these sprays should be weekly during May and June, and all RB sprays should be applied by early July. Later RB sprays, especially ethephon, can accelerate ripening in early maturing varieties such as Honeycrisp.


Tree fruit production Orchard management systems Crop load management of tree fruit Fruit tree pruning and training

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