Managing Walnut Mold

Kari Arnold Ph.D., UCCE Area Orchard and Vineyard Systems Advisor, Stanislaus County
Themis Michailides Ph.D., UC ANR Plant Pathologist and Lecturer, Kearney Agricultural Research & Extension Center

Fig. 1 (Left) 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. Figure provided by Themis Michailides.

Something has been plaguing walnut orchards for many years without a known cause. Often referred to as Brown Apical Necrosis, or BAN (Fig. 1), growers and PCAs have scratched their heads for years when walnut grades come back dinged due to moldy, off color nuts. Dr. Themis Michailides, a UC Davis Plant Pathologist, recently decided to take a stab at this issue and can now offer a solution. The findings are:

After collecting samples and isolating various types of fungi from both nuts and hulls, the Michailides Lab at the Kearney Agricultural Research and Extension Center in Parlier, Calif. consistently found Alternaria, Aspergillus niger, Fusarium, Botryosphaeria, and Phomopsis present, please see Fig. 2. Botryosphaeria and Phomopsis; we know from previous work can be managed by pruning dead/diseased wood/spurs, pruning after harvest in the fall, reducing sprinkler angles to avoid wetting tree limbs and depending on severity, fungicide sprays applied in mid-May, mid-June, and mid-July (further product information can be found at http://ipm.ucanr.edu/). Yet the other three fungi, Alternaria, Aspergillus niger and Fusarium were not considered pathogens on walnut. By performing a variety of tests both in the lab and in the field, the Michailides lab discovered these fungi are responsible for walnut mold. Additionally, walnut blight, caused by Xanthomonas arboricola pv. juglandis, may exacerbate the problem, leading to larger lesions on the hull and the potential for greater damage to the hull and nut.

What can be done?

One year of field experiments in the southern San Joaquin Valley show that applications of Merivon at 6.5 fl. oz/ac (FRAC group 7 and 11, a.i. fluxapyroxad and pyraclostrobin, respectively) or Rhyme at 7.0 fl. oz./ac (FRAC group 3, a.i. flutriafol) at three weeks prior to hull split are most effective at controlling walnut mold. Applications made between hull split and three weeks prior to hull split are also effective. Unfortunately, applications made to control Botryosphaeria and Phomopsis are not effective in controlling walnut mold caused by Alternaria, Fusarium, and/or Aspergillus niger. *Note: Please refer to current label recommendations and restrictions when applying pesticides.

Fig. 2. (Right) Moldy, off color nuts which lead to economic loss due to downgrading.

Figure provided by Themis Michailides.