Physiology-Based Peach Thinning Considerations

Over the past 40 years, consumer preference for large fruit has increased, so growers are planting varieties with the genetic potential to produce larger fruit. In the 1930s, researchers reported that about 30 to 45 leaves were required to obtain maximum fruit size, and in the early 1960s, the importance of early thinning was demonstrated. Over the past 30 years, as our understanding of peach tree physiology and fruit development has improved, we have modified thinning practices to maximize fruit size and quality. This article is intended to summarize the research results for growers to consider while developing a fruit thinning strategy for the future.

Final fruit size is a function of the number of cells in a fruit and the size of those cells, but the number of cells is the most important. So, to enhance fruit size, there should be an emphasis on increasing cell numbers. Even by early autumn, varieties that produce large fruit already have more cells in the flower buds than varieties that produce small fruit. There has been little research on factors influencing fruit cell division before bloom. In the 1980s, when I was at Rutgers University, we studied the influence of fall ethephon applications on fruit bud development to delay bloom the following spring. We found that within two weeks of application, fruit buds on treated trees had fewer cells, resulting in smaller fruit at harvest the following summer. Although I am not aware of supporting data, it seems that late-season stress on the tree, caused by nutrient deficiencies, shade, drought stress, or reduced leaf function due to insects or diseases could result in fewer cells in the flower buds, so maintaining healthy trees after harvest is essential.  

In the 1930s, H.B. Tukey reported that cherries grew in three stages. During the first stage (bloom to about 50 days after bloom), fruit growth is primarily due to cell division. During the second stage, the fruit increases little in size, but dry weight increases as the pit develops. The duration of the second stage varies from a few days to several weeks, depending on the variety. During the third stage (final swell), which lasts about six weeks, fruits grow rapidly due to cell expansion. Adequate water is needed to maximize growth during the final swell. Other researchers showed that this double sigmoidal growth curve was common to all stone fruit and grapes.

Although potential fruit size is related to the number of cells per fruit, there has been little research on factors affecting cell numbers. About 12 years ago, the peach genome was published, and since then, geneticists have been studying the involvement of different genes in stone fruit growth a development. In 2013 a group at Michigan State University published a paper in Molecular Breeding (2013, vol. 32:311-326) and reported that 23 cell number regulating (CNR) gene families were identified in peach, with at least one CNR gene on each of the eight chromosomes. They also found that the genetic effect on fruit size was mainly due to differences in the flesh rather than pit size. Two of the CNR family members likely also control fruit size in sweet and sour cherry.

The Fw2.2/CNR gene family members are negative cell regulators, and the level of expression is negatively correlated with cell division in the early stages of fruit development. The same gene family is also responsible for fruit size in tomato. The genes involved in fruit size are cytochrome P450 genes and control many metabolic pathways in plants and animals. In plants, they are involved in auxin synthesis and are upregulated by various biotic and abiotic stresses. I hypothesize that carbohydrate stress, induced by heavy early-season crop loads, increases the expression of these genes and suppresses cell division. Therefore, crop load must be reduced before carbohydrates become limiting to cell division.

Apple trees produce nearly 30% of their leaf surface before bloom as spur leaves. Thus, early-season fruit and shoot growth are supported by reserve carbohydrates in the woody tissues, plus the carbohydrates produced by the photosynthetic activity of the spur leaves. However, peach trees produce little leaf area until well after bloom, so early-season fruit growth depends entirely on stored carbohydrates (starch in the woody tissues is converted to sucrose that is transported to areas of active growth). In a series of experiments in California, Dr. Ted DeJong and his group showed that peach trees have too little carbohydrate reserves to support early season growth of fruit and vegetative organs, so fruit growth is compromised. This carbohydrate stress is most severe during warm weather that stimulates vegetative growth.

They also found that fruit mature early when temperatures are above average during the 30 days after bloom, resulting in smaller fruit. Water stress during the first stage of fruit growth can also negatively impact fruit growth, which occurs about 30 days after bloom.

Most growers know that fruit size at harvest is most significant when the crop load is adjusted early. Thinning at bloom time stimulates cell division and fruit growth. A light frost that kills up to about 80% of the blossoms is usually beneficial.

Because hand removal of flowers or fruit is expensive, researchers have tested chemicals that kill flower parts to prevent fertilization and fruit set. Unfortunately, results have been inconsistent because efficacy depends on weather conditions and the bloom stage. Another problem is that peach fruits that have not been fertilized will continue to grow for about 40 days before they cease growing. So, follow-up hand thinning must be delayed until fruit set can be assessed. Physically removing blossoms by hand with fingers or brushes or with rope drags to remove about half the blossoms is effective but usually requires some follow-up hand thinning.

The number of flower buds per tree can be adjusted with pruning. There are usually several hundred fruiting shoots on a tree, and the number of flower buds per shoot depends on the variety and the previous season's crop load and growing conditions. We found that pruning more severely to remove excess shoots significantly reduced the time for hand thinning, increased fruit size, and advanced fruit maturity. The largest fruit are produced on shoots 12 to 24" long because some axillary buds on these shoots produce leafy shoots that support the growth of fruits developing on those shoots. Therefore about four fruits can be retained per shoot if excess shoots were removed while pruning.

The position of a fruit on a shoot does not influence fruit size, so the most prominent 3 or 4 fruit on each shoot should be retained while thinning. If possible, include fruit at nodes with leafy shoots because photosynthates from those shoots will be preferentially translocated to the fruit at the same node to support fruit growth. Also, keep in mind that fewer fruits should be retained on shoots shorter than 12" because they produce fewer leafy axillary shoots to support fruit growth.