California Walnuts: Production Check List Through This Summer


  • Walnut Husk Fly (WHF) traps should already be out, 2 traps for every 10 acres on the north side of the tree, hung as high as possible. Monitor traps at least twice weekly until first treatment. Treatment timing can be based on when females with eggs are found, or simply when trap catches increase sharply. For more on monitoring and treatment, see the Sac Valley Orchards article at
  • If only applying one Bot canker and blight fungicide spray, the second part of June to first part of July had the most effect in recent UC trials, when tested in both a low and high disease situation. For more on timing, see the Sac Valley Orchards Bot article at


  • Codling moth third flight occurs in late July to early August (on average, 1100 Degree Days after the second biofix). Check traps to look for the third flight. Treatment decision is based on a combination of factors including previous treatments, number of nuts infested in the previous generation, trap catches, and the ability to harvest early. See for more details.
  • Monitor weekly for WHF. If spray residue from the previous spray has run out, harvest is more than 3 weeks away and eggs are present in trapped females, an additional treatment may be prudent. For more, see
  • Take July leaf samples to assess nitrogen, as well as potentially potassium and zinc deficiencies, and boron toxicity, depending on your circumstances. Sample a total of about 60 terminal leaflets from at least 16 trees on the same rootstock scattered throughout the orchard.
  • Watch for spider mites by monitoring weekly through mid-August. Once a week, randomly select 10 trees per orchard, and from each tree take 5 leaflets from low branches and 5 leaflets from high branches. If more than half the leaflets with spider mites do not also have predaceous mites, this is cause for concern. Monitor again in 3-4 days to determine if populations are increasing and treatment is warranted. For more, see


  • Consider ethephon as a way to increase color quality and minimize the chances of multiple shakes in a modest price year. Ethephon application should take place when 100% of sampled nuts have reached maturity, as indicated by “packing tissue brown” (PTB). This occurs around mid-August for the earliest varieties and around mid-September for Chandler in most years. For more on sampling to track PTB and ethephon application, see the Sac Valley Orchards article at
  • Monitor for Navel Orangeworm. Healthy, intact walnuts are only susceptible to NOW damage at and after hull split. Consult with your PCA for monitoring and treatment options, bearing in mind pre-harvest intervals, duration of residual activity, and impacts of materials to your overall IPM program. See the Sac Valley Orchards article at for more information.


  • Collect a sample of at least 100 nuts at harvest for each block and freeze them. Compare them after harvest with grade sheet to evaluate different potential sources of damage and how to improve your IPM program for next year. For help evaluating the source of damage see photos and notes at


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