Walnut Mold Confuses Growers in California

walnut mold

Moldy, off-color nuts lead to economic loss due to downgrading.
Photo by Themis Michailides

Something curious has been plaguing California walnut orchards lately. Growers became confused when their crops didn’t meet processing quality standards because of moldy, off-color nuts. Finally, when members of the California Walnut Board were hit hard with a yield reduction, they turned to University of California, Davis plant pathologist Dr. Themis Michailides for answers.

Michailides already knew about the problem. Having studied postharvest diseases for more than 30 years, he’d noticed an increase in Botryosphaeria fungi and walnut blight caused by the bacterial pathogen Xanthomonas arboricola pv. juglandis. After collecting samples and isolating various types of fungi from both nuts and hulls, he also found high rates of Alternaria and Fusarium fungi (as well as Alternaria and Aspergillus niger), which haven’t traditionally been considered pathogens to walnuts. Michailides now had the support to dive into the problem.

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“Each disease has an Achilles heel,” he says. “And that’s what we’re trying to find out and attack.”

Michailides worked with his staff Research Associate, Dan Felts, along with researchers, including Kari Arnold, Orchard and Vineyard Systems Advisor for Stanislaus County, and Elizabeth Fichtner, Orchard Systems Advisor in Tulare and Kings Counties. They started by inoculating walnuts periodically to determine when they become susceptible to the molds and found that infection rates soared as soon as hulls split, eventually invading the kernel. Walnut blight may exacerbate the problem, leading to larger lesions on the hull and greater potential for damage to the nut.

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Pre-Harvest Spray Trials

Already knowing that Botryosphaeria and Phomopsis can be effectively managed by pruning dead/diseased wood (branches and shoots) after harvest in late summer or fall, and using fungicide sprays in mid-May, mid-June, and mid-July, Michailides organized a trial with ‘Ivanhoe’ walnuts in Southern San Joaquin Valley to see if pre-harvest (pre-hull split or early hull split) sprays would be effective in combating the fungi that cause walnut mold. A second, similar trial was established in Butte County using ‘Chandler’ walnuts.

Thinking a pre-harvest spray — Merivon at 6.5 fluid ounces/ac (FRAC group 7 and 11 a.i. fluxapyroxad and pyraclostrobin) and Rhyme 7.0 fluid ounces/ac (FRAC group 3, a.i. flutriafol) — may help reduce the mold, he experimented with an application a few weeks before hull split last year. This treatment, along with the one applied at 20% to 30% hull split, seems to be working, significantly reducing walnut mold.

“We’re finding that mold requires later sprays than the bacterial pathogen causing walnut blight to prevent the fungi from infecting the hulls and eventually invading the kernel,” Michailides says.

brown apical necrosis of walnuts and walnut blight side by side

Brown apical necrosis is shown on the left, not to be confused with walnut blight, shown on the right, and caused by the bacterial pathogen, Xanthomonas arboricola pv. juglandis.
Photo by Themis Michailides

The ‘Ivanhoe’ walnuts, an early variety, have already received two applications this year and were to be harvested during the last week of August. The second trial with ‘Chandler’ is a later variety that normally harvests in early October and, hence, still awaits its first application. Michailides is looking forward to examining more samples from other varieties and from various California counties where walnuts are grown.

Additionally, next year he seeks to try three sprays, one several weeks before hull split, another one week before hull split, and the third at 10% to 20% hull split (early hull split). This will help determine the best timing and number of sprays to control the mold. He’s also considering further studies into Brown Apical Necrosis, mainly caused by the same walnut mold fungi and notable by a lesion at the blossom end. Michailides thinks it may possibly spread to the hull and then to the kernel to cause mold.

Michailides also advises growers to adjust sprinkler angles to avoid wetting tree limbs, thinking that damp walnuts and increased humidity in orchard canopies may be contributing to higher mold rates.

“California has so much agricultural land now, cultivated with irrigated plants,” he says. “You have walnut orchards so dense that canopy branches touch between one row and another, proliferating airborne mold. These fungi like high temps, and when the humidity is high, it’s just the perfect mix.”

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