Raymond Mireles, UCCE Fruit and Almond Advisor Tulare and Fresno Counties

Many growers in the Central Valley were challenged with consecutive days of high temperatures in 2024, making it difficult to meet orchard demands throughout the season. The early warm spring temperatures initially promoted good fruit set, but then negatively impacted fruit size and quality for some peach growers. Understanding the key factors that influence fruit development and growth may help growers adapt to future seasons with high spring temperatures and employ techniques to improve fruit size and quality.

After receiving sufficient chill during the winter months, the orchards in the Central Valley begin to break dormancy and start blooming. Once full bloom and fertilization are achieved, rapid growth and development of the fruit follow. The growth of stone fruits follows a pattern known as the double-sigmoid curve, which can be divided into three main stages or phases. After full bloom, the flowers enter Stage I (S1), where initial fruit size is determined by cell division and cell expansion. In Stage II (S2), known as the pit hardening stage, fruit growth slows down and stone lignification occurs. Stage III (S3) resembles S1 in terms of fruit sizing, but in this stage, cell expansion becomes the predominant process, leading to fruit maturity and ripening. The timing and duration of each of these phases depend on environmental conditions as well as the genetic makeup of the cultivar. 

Fruit development, or maturation, is influenced by temperature, while fruit growth, specifically size, relies on the accumulation of available carbohydrates. When warm spring temperatures occur, fruit develop quicker, resulting in a shorter time between bloom and harvest. However, this rapid development means that each fruit has less time to accumulate carbohydrates, leading to smaller sizes. It is widely accepted that the first 30 days after bloom (S1) are critical in determining the duration of the fruit development period for stone fruits. For peaches, high temperatures during this initial 30-day period can result in smaller fruit sizes and reduced yields.

To mitigate the effects of high spring temperatures on fruit size and potential yield decreases, UCCE recommends early thinning. Thinning offers several advantages, including improved fruit weight, quality, synchronized maturity and ripening, as well as optimal flower bud formation. By thinning flowers, competition between vegetative growth and fruit development is reduced, allowing for larger final fruit sizes. It is important to note that fruit thinning has a smaller impact on fruit size than flower thinning. While earlier thinning has the greatest influence on fruit size, considerations must be made regarding its economic feasibility, desired yield, and the risks associated with late spring frosts.

Research conducted by the University of California calculated and quantified the Growing Degree Hours during the first 30 days after full bloom (GDH30) for multiple peach cultivars. The higher the accumulated GDH30, the faster the fruit will develop. The Harvest Prediction Model, available on the UC Fruit & Nut Research Information Center website, serves as a tool to predict whether earlier thinning is necessary. 

High early spring temperatures significantly influence final fruit size, but elevated temperatures throughout the growing season also impact fruit size, quality, and tree health. It is crucial to maintain proper water levels in trees to prevent water stress. Adequate water helps accumulate fresh fruit weight and size as harvest approaches. When trees experience water stress, this can lead to issues such as leaf stomatal closure, which restricts transpiration and gas exchange. This limitation affects photosynthesis, reducing the availability of carbohydrates essential for fruit development and growth.

It’s important to note that trees with heavier crop loads are more susceptible to the effects of water stress, as their photosynthetic productivity declines and carbohydrate supply becomes restricted. During the hotter months, particularly when fruit is in the S3 stage of development, cell expansion relies on cell turgor pressure, which is the result of accumulated water pushing against the cell walls. On hot, dry summer afternoons, leaf water potential can drop significantly, leading to water loss from fruit tissues to adjacent leaves, resulting in noticeable fruit shrinkage. Water stress in peaches and cherries during the S3 stage and post-harvest can increase the occurrence of fruit doubles, deep sutures, and decreased fruit set in the following season.

Proper irrigation is essential to meet the needs of the orchard, maintain healthy tree vigor, and promote good yields for the following year. The aim of irrigation management is to provide the right amount of water at the right time. During the S1 and S3 phases, it is critical for water and carbohydrates to be available for the developing fruit. In the S2 phase, water conservation practices can be implemented; however, it is important to consider the differences between early-season and late-season varieties, as S2 occurs rapidly for early varieties compared to late ones. Postharvest water savings can also be practiced, but care should be taken during bud differentiation to avoid issues like fruit doubling.

Balancing irrigation is crucial, as both under-irrigation and over-irrigation can adversely affect tree health and fruit size. Monitoring tools such as soil moisture sensors, pressure bombs, and plant-based water sensors can be used together to assess the amount of water available to the plants and their stress levels. Additionally, staying updated with weather conditions and crop evapotranspiration (ET) estimates will help monitor heat waves and changes in water demand. A thorough understanding of your irrigation system, including its distribution uniformity and efficiency, is key to ensuring that your orchard receives the proper amount of water. For further information and guidance, growers, farm managers, PCAs, CCAs, and other industry professionals are encouraged to contact their local UCCE farm advisor. Best of luck and stay cool!