Cooling off with clouds

To manage heat stress in fruit crops, fogging could be an efficient way forward.
An image of a field of crops with a fog net over them. On top of the image sit the words "Cooling off with clouds."
(Photo: Lav Khot)

Washington is the primary producer of fresh market apples in the U.S. with farm gate value of about $3 billion in 2021. However, growing fresh market produce is becoming a challenge with a range of weather- and market-driven issues. Growers in the region continue to face enormous challenges to deliver high-quality produce due to crop losses attributed to sunburn, fruit splitting, frost damage, pests, diseases, and harvest and storage disorders. Sunburn and associated heat stress are particularly detrimental to apple production. Excess heat and sunlight can cause considerable yield loss and propagate disorders during storage, reducing fruit size and returns for growers. The last two seasons with hotter summers and entire weeks with temperatures above critical thresholds for fruit sunburn has triggered the search for innovative ways to manage or mitigate heat stress in apples.

Fogging and fognet-based heat stress mitigation in commercial apple orchard near Prosser, Washington (Photos: Lav Khot)

Growers utilize active heat stress mitigation methods such as overhead sprinklers for evaporative cooling, protectant spraying (e.g., kaolin, calcium carbonate particle films or antioxidant wax materials), or protective shade covers. Several issues have been reported with these above methods including excessive use of water and energy, reduced fruit quality (color development, size and maturity), food safety risks, and higher cost.

In addition, there are no real-time heat stress monitoring systems that link in-orchard data on crop physiology and microclimate with the mitigation technology. This knowledge gap has placed growers in the position of reducing yields due to heat stress-related losses and excess use of resources.

To address both the monitoring and effective management challenges, our team (Carolina Torres, PhD, and Troy Peters, PhD, at Washington State University are additional investigators) has been investigating and evaluating sensor-driven, real-time heat stress monitoring and management strategies in two premium apple cultivars, i.e., Honeycrisp and WA 38.

Both fogging and conventional evaporative cooling seem to adequately mitigate heat stress. Fogging tends to perform better during the peak heat hours. (Photo: Lav Khot)

This article summarizes findings associated with the evaluation of current and emerging heat stress mitigation methods. Since 2021, we have been evaluating conventional evaporative cooling (applied at 55.5 gallons per minute per acre), fogging (applied at 26.2 gpm/a), netting (shade net at 12% shade) and a combination of netting and fogging in a commercial Honeycrisp orchard near Prosser, Washington. Table 1 summarizes the pertinent operational details of water-based heat stress mitigation techniques. Our team has been working with an industry collaborator to provide hardware to establish fogging and fognet treatments.

In all the treatments, we have been monitoring microclimate (air temperature, relative humidity, wind speed and direction, solar radiation [1 min.]) as well as localized fruit surface temperature throughout the season. Fruit quality at harvest and postharvest is being evaluated as well.

Data from the 2021 and 2022 seasons suggest that fogging and netting combined had better protection against heat damage compared to the other treatments. While the netting provided good protection against direct sunburn, adding foggers underneath the net reduced the excessive heat accumulation typically observed under netting alone. The heat accumulation was quantified by air and fruit surface temperature data. Overall, excessive heat accumulation delayed fruit color development and reduced fruit growth. Contrary to netting, water-based cooling methods seem to promote fruit growth and the lower temperature favored color development.

Combining the understanding of heat stress mitigation approaches to mediate crop losses in a variety of crops and cultivars can help streamline available resources and technologies for climate change adaptation, i.e., climate smart production management in coming years.

Regarding water-based mitigation, both fogging and conventional evaporative cooling seem to adequately mitigate heat stress. Incidentally, fogging seems to perform better during the peak heat hours where the cyclic nature of conventional evaporative cooling can fail to regulate fruit surface temperature below the literature-recommended sunburn threshold, which is 113 degrees Fahrenheit. Fixed-cycle frequency-based actuation of conventional evaporative cooling can be automated to realize a variable on/off cycle frequency on hotter days with inputs from localized weather. We are exploring such automation scenarios.

Table 1. Operational details of water-based heat stress mitigation techniques

Heat stress is becoming a dominant issue in grapes, blueberries, cherries and many other temperate fruit crops grown in the U.S. and around the globe (e.g., Chile, China, European Union). Combining the understanding of heat stress mitigation approaches to mediate crop losses in a variety of crops and cultivars can help streamline available resources and technologies for climate change adaptation, i.e., climate smart production management in coming years.

This study was funded in parts by the U.S. Department of Agriculture and National Institute of Food and Agriculture/National Science Foundation Cyber-Physical Systems program, Washington Tree Fruit Research Commission, and WNP0745. We would like to acknowledge industry and grower collaborators for their in-kind support.
Lav Khot, PhD, is an associate professor of precision agriculture at Washington State University and interim director of WSU AgWeatherNet. He works in the agricultural automation engineering research emphasis area of the department of biological systems engineering.
Basavaraj Amogi is a doctoral student advised by Lav Khot, PhD. His research focuses on heat stress management in apple orchards.
Bernardita Sallato is a tree fruit extension specialist at Washington State University. Her program provides leadership in applied research, extension and outreach for the Pacific Northwest tree fruit industry.

Additional resources

Evans, R. G., Kroeger, M. W., & Mahan, M. O. (1995). Evaporative cooling of apples by overtree sprinkling. Applied Engineering in Agriculture, 11(1), 93-99.
Rakesh Ranjan, Basavaraj Amogi, Lav Khot, R. Troy Peters (July 2021). Real-Time In-Orchard Apple Heat Stress Monitoring System. WSU Tree Fruit Research and Extension,
Serra, S., Borghi, S., Mupambi, G., Camargo-Alvarez, H., Layne, D., Schmidt, T., Kalcsits, L.& Musacchi, S. (2020). Photoselective protective netting improves “Honeycrisp” fruit quality. Plants, 9(12), 1708.
Tory Schmidt (June 22, 2018). Apple Sunburn 101. WSU Tree Fruit Research and Extension,



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