Sunburn Damage in Walnuts

Getting sunburned is not a fun experience for most people, often resulting in painful tomato red skin in the affected area. However, humans are not the only ones capable of getting a sunburn. Walnuts, a tree nut that globally accounted for 22% of world tree nut production according to the International Nut and Dried Fruit Council can also receive damage from getting to much sunlight, though the damage looks a good bit different from a human sunburn.

So what exactly does sunburn look like on a walnut if its not a red patch of pain? According to the University of California Agriculture and Natural Resources, it typically shows as yellowish brown spots that develop into dark brown leathery lesions later in the summer. In most cases, the kernel in sun damaged nuts tend to be shriveled with darkened kernels. The difficult part, is that in some cases walnut sunburn may look similar to other walnut pests and issues, such as walnut blight, the Walnut Husk Fly, or Codling Moths.  While these dark sunken lesions are the common signal of a problem, there are some variations to tell the difference.

The dark lesions caused by walnut blight are almost always dry, while damage from Husk flies tends to remain soft, and does not sink into the walnut in the same way. Both Walnut Blight and Husk flies can affect large portions of the walnut husk, sometimes covering the entire nut. However, a sunburned husk is commonly restricted to the side of the walnut exposed to direct sunlight. Sunburned walnuts often tend to have some light tan to brown coloring leftover around the darker lesions that form as well.

For Codling moths, the damage they cause is also usually paired with a brown frass near the entry and exit holes as the larvae burrows into and out of the shell. This frass is the excrement of the larvae that is leftover during the borrowing process. Different variations of these pests are also known to affect some fruits too, being infamous for the damage they can cause to apples and pears.

When it comes to walnuts, the two most important factors to quantify the nuts quality are kernel size, and kernel color, according to Sacramento Valley Orchard Source. A kernel with a light yellow or amber color tends to be more valuable than the more bronze or black colored kernels. As such it is important to avoid heavy sun damage, as it can cause the kernels of the walnut to both shrink, and take on a darker color causing problems for both quality factors.

While sunburn can be damaging to your walnut crop, there are a few suggested methods to help mitigate some of the damage. To help keep the temperature of the nuts down, Kaolin is suggested by some researchers, such as Kathy Kelley Anderson and Dr. Bruce Lampinen from the University of California Agriculture and Nature Resources in their study on the Surround product. A later study by Kathy and Dr. Bruce along with Sam Metcalf and Claudia Negrón suggested that the Kaolin clay particle film was indeed effective in lowering leaf and nut temperatures, but not by a large margin. This amount did keep the test walnuts under the sunburn threshold however.

Another discovery from their research, was that brief interruptions in direct sunlight can also assist in preventing sunburn to walnuts. From this observation, the research team theorized that good tree training and pruning practices could help minimize sunburn conditions for the nuts.  Aside from these discoveries, the University of California’s Agriculture and Natural Resources team also suggest maintaining good irrigation and the use of fertilizer.

The RMA has updated their policy to walnuts to include walnut quality adjustment. This allows sunburned damaged walnuts to be eligible for indemnity payments through quality adjustment. (check special provisions). Additionally, the RMA now allows optional units by organic and non-organic farming practices.

Enhanced Coverage Option (ECO) is now available with walnuts as well as other specialty crops for the 2025 crop year.  Interested in learning more about crop insurance coverage for your walnuts? contact us today!

*Walnuts are only insurable currently in the state of California.