It’s the time of the year when growers focus on thinning fruit trees. While apple growers have prediction models and Decision Support Systems (DSS), such as the carbohydrate or apple pollen tube growth models to guide their best decision-making skills and tools for crop load management, peach growers aren’t as lucky.

Effective chemical thinning agents for peaches – despite years of trying – just aren’t available, or are problematic, and aren’t able to significantly reduce the need for hand thinning. The reliance is on hand thinning peaches, which set many more buds than are needed to grow an annual healthy crop of high quality, commercially-sized fruit.

Guglielmo Costa, of the University of Bologna, Italy, presented a workshop on developments in peach chemical thinning for the recent International Society for Horticultural Science’s peach webinar intensive.

“Fruit thinning is mandatory in peaches,” Costa said. Thinning reduces “intrafruit competition,” with the objective of maximizing the yield of commercially-sized fruit. “Thinning is not just the size, but also the achievement of high fruit quality” and provides a balancing of leaf and crop growth for the tree, preventing an alternate bearing pattern.

In apples, there is a correlation between the first drop and the June drop, Costa said. This correlation does not exist in peaches, leaving growers without any algorithms to help guide thinning efforts.

Complications in Peaches

There are some guidelines for peach growers, even if predictive models and thinning chemicals are in short supply. Although peach thinning practices can differ slightly depending on growing region climate and cultivar, peach thinning remains a major factor in producing a commercially viable crop each season.

Flower thinning is used in some growing regions, particularly on early ripening cultivars, whose fruit development timing is different than that of later cultivars, Costa said.

The University of Florida cautions that blossom thinning can be risky if a freeze event is likely to damage remaining blooms. According to Penn State, growers need not be too concerned, as their research has shown that “any bud loss is compensated by a reduced natural drop.”

Mechanical blossom thinning can be achieved in a variety of ways: by hand, utilizing ropes or high-pressure water or mechanical string thinners. It can reduce the crop load initially, with further refinement occurring during fruit thinning.

Fruit thinning by hand remains the standard in many peach orchards. Fruit must be thinned prior to pit hardening, typically within 40 – 60 days after full bloom.

The leaf-to-fruit ratio should be approximately 40:1, Costa said. Malformed or diseased fruits should be removed, leaving healthy, well-sized fruits, spaced about eight inches apart. Fruit on the outside of the canopy is favored for proper coloration, with less interior fruit load.

Costa cautioned that nectarines and freestone peach varieties require more thinning than clingstones do.

Looking Ahead

There are significant differences between fruit growth curves in cultivars that ripen at different times. Variations in days between fruit stages of growth mean that developing algorithms for the best timing of chemical thinners has proven difficult. Several predictive models based on fruit growth have been shown to be unreliable throughout the past decades, and chemical thinning efficacy remains low.

Mechanical shakers and other methods of removing fruit can be used, and may be the best option to supplement hand thinning, Costa said. Mechanical thinning can be used in organic systems, and its success is not dependent upon environmental conditions or fruit genotype.

There are a few potential options recently studied for chemical peach thinning, he said. Metamitron is a molecule that mimics shading and reduces photosynthetic capabilities. Trials show that it is much more effective in apples than in peaches, with a greater reduction in photosynthesis in apples, resulting in thinning of fruits. Peach results have not been too successful thus far, Costa said.

Aminocylcopropane-1-carboxylic acid (ACC) is an ethylene precursor, which is naturally occurring. It is quickly metabolized into ethylene by the plant, and leaves no harvest residue. It can reduce the need for thinning, but not eliminate it in peaches.

ACC is not aggressive, and does not cause gummosis. It is not strongly temperature dependent. Used at petal fall, it’s been shown to have a greater efficacy at higher concentrations, and to be more successful for thinning peaches than abscisic acid (ABA), which is a plant growth regulator that also has thinning effects in peaches.

Climate Change

For peach growers looking to find reliable alternatives to hand thinning, climate change is one further factor complicating the search. Increasing temperatures during winter will cause a decrease in winter chilling and trees will break dormancy early. This decreased winter chill can also cause different phenological stages to occur within the same tree, complicating thinning even further with variable and prolonged bud break and flowering.

As spring is advanced and autumn is delayed, the annual cycle of tree fruit production can be shifted. Carbon balance of the trees can be affected, which in turn has an effect on fruit set. An increase in spring rainfall will decrease pollination rates, which will result in lack of fruit set. Over thinning can become a real concern.

“Climatic conditions are exceptional,” Costa said of the current changes being seen in growing regions around the world. “We know that fruit abscission is a complicated physiological process. Climate changes further increase this complexity.”

For most peach growers, hand thinning probably isn’t going to be supplanted by chemical methods in the near future. It’s a labor-intensive, difficult job to produce a bountiful, high-quality crop.