Mar 28, 2022

Viewing physiology of fruit thinning, available online tools

As most of you know thinning is not an exact science. There are many factors that can make a thinning chemical work effectively and others that make it less effective.

Alan Lakso, Emeritus at Cornell University developed a carbohydrate balance model for apples which is now available as a tool on the NEWA (The Network for Environment and Weather Applications, Cornell University) platform.

During times of high supply and low demand (sunny and cool), the balance is positive, and it is difficult to thin chemically. When the balance is slightly negative, chemical thinning becomes easier. When the balance drops to a critical limit, the trees are under significant carbohydrate stress, and chemical thinning will be strong. Below a critical limit, the natural stress may be so severe that some fruits will be shed even when thinners aren’t used because the response to chemical thinners is predicted to be very strong.

In general, tree physiology and environmental conditions are the two major factors affecting the response of most chemical thinners. The physiological factors in the tree that affect thinning response include variety, tree age, tree health, crop load, and severity of pruning. A weak and a very old tree is relatively easier to thin than a healthy and young tree.

Similarly, a heavily cropped tree is relatively easier to thin than a lightly cropped tree. However, you should know that even though the chemical may have knocked a large number of fruit off the tree, you may still have more fruit left due to poor chemical coverage, making it a better management practice to monitor the number of the fruits that are left on the tree, not those on the ground.

Another factor that contributes greatly to the effectiveness of a thinner is the variety. For example, ‘Fuji’ fruits are harder to thin than ‘Gala’ fruits. The type of tree is also important; for example, ‘Spur’ varieties are harder to thin than non-spur varieties. Some studies also suggest that pruning may have an influence on the effectiveness of thinning.

Application timing is also important. Some thinners work best when applied at bloom time. But this has generally been a time when growers prefer not to apply thinners because of the uncertainty of late spring frost. However, early thinning plays an important role in return bloom, especially in biennial bearing cultivars like Honeycrisp and Fuji.

The apple pollen tube growth model was developed at Virginia Tech to help apple growers more precisely time thinning sprays during bloom and is also available as a tool on the NEWA platform for Fuji, Gala, Golden Delicious, Granny Smith, Honeycrisp, Red Delicious, and Pink Lady. For more information on how this model works, see Penn State’s March 26, 2019, article on Apple Crop Load Management: Blossom Thinning Apples with Lime Sulfur.

Penetration of the chemical thinner is greater through the lower side of the leaf than the upper side, and these differences increase as the leaves age because older leaves have more wax deposited on the upper surface than the lower surface. Interestingly, very little NAA enters the plant leaf through the stomates and instead most enters through active transport channels across the plasmalemma. In other words, the leaves spend energy to get NAA inside so it can cause the fruit to abscise.

A tree that has heavy bloom will require a significant amount of energy to take up the thinner, and so it is advisable to supplement the tree with some nitrogen at bloom to offset the amount of energy spent on producing the flower and getting rid of the fruit during thinning. Environmental factors also have a strong influence on the effectiveness of thinning chemicals.

Ed Stover and Duane Green published a very nice review in HortTechnology on the effect of environment on the performance of thinners. In this article they pointed out several pre-, during, and post-application factors that impact the effectiveness of growth regulators on fruit thinning. During the early stages of growth, leaves deposit waxes on the surface. The amount, structure and composition of these waxes influence leaf wetness and penetration of thinning chemicals. Deposition of these waxes is affected by the environment.

Low light, high humidity, frost damage and low temperatures were listed as factors that can increase the response of thinning chemicals, while high temperature and dry conditions reduce the effectiveness of the thinning chemicals. Many of these factors work synergistically, so if the weather is warm and humid the thinning chemical will work more effectively than if the weather is cold and dry.

The greatest uptake occurs right before the chemical has completely dried (because of the increase in the concentration of the chemical). However, uptake decreases drastically when the chemical has completely dried. Therefore, conditions that allow for longer drying periods will increase the effectiveness of thinning.

Field studies have also shown that if the chemical droplet dries before it is washed by rain, its activity is not lost. However, if the droplet is still wet and a rain occurs, then the chemical will be less effective because it washes off before it gets into the leaf. These are only a few of the factors that impact the effectiveness of the thinning chemicals, and that is why it is difficult to predict if a thinner will work or not.

– Adapted and edited by Elizabeth Wahle, University of Illinois, from an article originally submitted by Mosbah Kushad