Written by: Brent Arnoldussen, PhD Candidate, WSU Horticulture; Matthew Whitting, Professor, WSU Horticulture. Feb. 5, 2020
Cold damage to the reproductive buds and flowers is a perennial concern to tree fruit producers worldwide. Every year low temperatures threaten productivity and periodically, sudden cold snaps between bud break and bloom can cause near-total crop failure. The potential for springtime cold damage is predicted to increase as the result of a changing climate. Currently, fruit growers use various methods to protect against cold damage, including heaters, wind machines, and irrigation, often in combination. These techniques have remained relatively unchanged for decades and each has limitations. Our team of engineers and horticulturists at WSU has been investigating an alternative method of freeze protection for fruit crops: sprayable cellulose nanocrystal (CNC) dispersions.
Cellulose nanocrystals represent an advancement in nanotechnology known as bio-nanomaterials. These are plant-based by-products such as those from the forest industry, which are treated with a series of green chemistry buffers and enzymes to break down the structural carbohydrates in the material into inert nanosized constituents of cellulose. This process makes it a renewable and sustainable product. The material’s nanostructure gives it unique physical and chemical properties. Most importantly, it has a very low thermal conductivity, meaning it holds in heat. Our team at WSU has developed a novel method for suspending these materials into a sprayable aqueous dispersion.
We have demonstrated that using electrostatic application technology, these dispersions form a thin (ca. 25µm-40µm; Fig. 2) and durable insulating film around the surface of the buds. When coated, the bud is slower to lose heat into the environment than untreated buds, improving their resistance to cold temperatures. Since 2018 we have been investigating the protective effects of CNC coatings in apple, grape, and sweet cherry. Our initial small plot work revealed significant improvement in cold tolerance, with treated flower buds having up to 6 F improved tolerance. More recently we have compared cold damage of trees treated with CNC using commercially-available electrostatic sprayers (On Target Spray Systems) to damage in untreated trees. We again documented significant improvements in cold tolerance – treatment of about 3% CNC in a ‘Benton’ orchard reduced the temperature of first damage (i.e., flower death) by about 8°F (Fig. 3). The protection can last at least 7 days post-application, but since the material is water-soluble, precipitation will reduce/eliminate any protection.
In addition to the protection given to dormant buds, CNC coatings can also protect buds during anthesis when floral tissue is most sensitive. Dispersions can be applied as late as the popcorn flower stage and can add as much as 5.4°F protection to the developing floral tissues. However, this protection appears to be short-lived, lasting about 48 hours at early stages of flowering (e.g., popcorn), and CNC treatments appear to be ineffective once the flowers are fully open.
This emerging data on CNC’s effectiveness for improving bud hardiness suggest it may be a promising new advancement as a freeze prevention tool.
The authors received financial support for this research from USDA-NIFA and the Washington Tree Fruit Research Commission
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