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Why walnut hulls can turn black and mushyWhy walnut hulls can turn black and mushy

Greg Northcutt

October 2, 2014

3 Min Read
California's walnut crop in 2018 is projected at 690,000 tons, according to the National Agricultural Statistics Service.

If you’re seeing black, mushy walnuts on your trees, the walnut husk fly or vinegar flies may be the reason. Then, again, this damage could reflect environmental stresses or physiology of the tree, itself.

In the September, 2014, issue of Sacramento Valley Walnut News, Rick Buchner, University of California Cooperative Extension farm advisor for Tehama County, and Emily Symmes, UCCE integrated pest management advisor for the Sacramento Valley, describe the various causes.

Walnut husk fly (WHF).

In the Sacramento Valley, WHF egg laying can occur as early as late June or the first week of July. That’s why they recommended putting up traps by early June. It’s the maggots feeding inside the husks that cause the problem. Because the outer skin of the husk usually remains intact, you’ll have to look inside the husks to find them. These maggots are whitish after they hatch and turn yellow as they mature. Mature WHF maggots are distinctly yellow with black mouthparts and measure about 3/16-of-an-inch long. After feeding in the husk for three to five weeks, these maggots drop to the ground and burrow several inches into the soil to pupate and emerge in following years. They produce one generation annually. More information on WHF is available at: http://www.ipm.ucdavis.edu/PMG/r881301211.html

Vinegar flies.

Nuts on the ground or blackened husks may be secondarily infested with the larvae of vinegar flies (Drosophila spp.). As Buchner and Symmes point out, damage caused by vinegar flies could be mistaken for that resulting from WHF. “Vinegar-fly maggots are smaller and remain white in color compared to WHF maggots,” they write. “For this reason, the mature yellow maggot stage is the best indicator to distinguish WHF and vinegar-fly infestation.

Reduced carbohydrate levels.

Hulls may also become black and mushy due to lack of adequate carbohydrates when there aren’t enough photosynthates to mature the nuts properly. Because a carbohydrate deficit occurs later in the season, the kernels are well-formed and often appear normal. Although affected nuts can occur in many different situations, usually they are in the lower inside shaded areas of the tree canopy.

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This condition appears to be more prevalent in heavy-bearing varieties like Chandler, Howard and Tulare and in heavy crop years. That may be due to the increased nutritional demands placed on the tree by the heavier crop loads. Also, reduced light penetration into the lower inside spurs for these and other varieties contributes to hull breakdown.

Environmental stresses.

Any stresses experienced by the tree could impact kernel and husk condition, Buckner and Symmes note. “Moisture stress can be problematic and irrigation management that allows trees to experience large changes in stem water potential – becoming either too wet or too dry – may influence husk and kernel integrity,” they report.


Some of the black husks could be related to Botryosphaeria (Bot) and/or Phomopsis infections. Initially, these diseases cause the hull to turn black and soft. However, by late summer or early fall, the hull dries to a black or brown color. Currently, Themis Michailides, University of California plant pathologist at the Kearney Agricultural Center, Parlier, and cooperating farm advisors and growers are conducting research to understand the blight phase of these two diseases.

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