Luke Milliron, UCCE Orchards Advisor, Butte, Glenn & Tehama
Dr. Themis Michailides, UC Davis Plant Pathologist at the Kearney Ag Research and Extension Center
What is walnut mold?
Growers and processors have reported elevated mold levels in harvested walnuts to farm advisors, which has resulted in pathology sample submissions to Dr. Themis Michailides, UC Davis Plant Pathologist (Photo 1). Although Botryosphaeria and Phomopsis (BOT) can cause walnut mold, most walnut mold develops from Fusarium and Alternaria species. Furthermore, walnut mold spray timing is later than BOT prevention spray applications and therefore your fungicide program for BOT will not control mold. Walnut mold does not begin to develop until the hull completes maturity and begins to split, long after most BOT-controlling sprays have been applied. Before these alerts and ensuing research, not much was known about managing mold in walnut.
Increased mold reports have led to the California Walnut Board funding the lab of Dr. Michailides to investigate the management of walnut mold. Although all possible predisposing factors have not yet been investigated, conditions that compromise the integrity of the hull, such as walnut blight, sunburn or insect-damage can serve as an entry point for mold fungi.
Secondary blight predisposes nuts to mold:
Secondary walnut blight (Xanthomonas arboricola pv. juglandis) infections that do not penetrate to the kernel nor result in nut drop, can create an entry for not only moth pests, but for the mold causing Fusarium and Alternaria species. These infections are a specific type of walnut mold called brown apical necrosis (BAN), named because the black blighted lesions at the apical (aka: stylar – floral remnant) end of the fruit, that turn brown following fungal colonization (Photo 2). As the fungal infection expands under the hull, the hull is decayed and the infection spreads to the kernel, most likely through the apical end. It stands to reason that improved walnut blight management, particularly of secondary infections will lead to less BAN, however this has not been studied. More on walnut blight best management practices here.
Predisposing factors and cultural controls for mold:
Practices that help maintain hull integrity are part of the pre-hull split management of walnut mold. One predisposing factor is sunburn, with mold commonly isolated from the sunburnt side of developing walnuts. Higher incidence of mold has also been found in insect infected nuts. Thus, controlling sunburn and insect damage will also help keep down mold infections. Finally, a critical management practice is timely shake and pick up. Bill Olson (UCCE Butte Emeritus) did early work showing that mold and other quality problems increase the longer walnuts remain both on the tree and especially on the ground. Picking up the same day as shaking is a critical best practice for overall quality and grower returns, particularly for non-Chandler blocks.
Some varieties are more susceptible to mold than others, and therefore more diligent attention to mold management may be required. In earlier research, higher incidence of mold was discovered in walnut varieties with larger openings at the stem end and larger sized nuts.
Fungicide management for mold:
Investigating fungicide efficacy1 for walnut mold is just in its infancy, with initial studies first conducted in 2019. In a trial in Chandler located in Butte County, the fungicide Rhyme (flutriafol) was tested because of its short preharvest interval (PHI). A single spray at either 30 or 60% hull split, reduced mold incidence by over 70%. More efficacy testing of various chemical controls for walnut mold are planned and these results will be reported here.
1Mention of pesticides and spray timings do not constitute a pesticide recommendation; it is merely the sharing of research results. Always follow the pesticide label and consult with your PCA.