May 6, 2021
May walnut orchard management considerations offered

Please note that the following are general recommendations intended to help you keep track of regular practices in a busy time. The optimal timing for management practices may vary based on specific location and conditions.

  • Irrigation: Before you start irrigating, consider plant water stress (pressure chamber) measurements and soil moisture sensor readings. Ongoing research in the Sacramento Valley has found irrigation can be delayed until June in some years, saving water and pumping costs without negative impacts to yield, size or quality. See the article in this newsletter for more information.

  • Water Sampling: If our relatively dry past winter means you may be relying on groundwater more than usual this year, or you think your water quality may have changed, take a water sample to evaluate for specific elements like B, Cl, and Na, if they can be a problem in your area. Water quality thresholds here.

  • Nitrogen: Apply the first round of nitrogen fertilizer in May, not before. Walnut trees only use stored nitrogen the first month after leaf-out, meaning N applied before May will likely be leached by rain and/or irrigation. Walnut tree nitrogen use is fairly steady over the growing season. Evenly dividing nitrogen applications in 3 to 4 doses between May and mid-August will improve N uptake compared to 1 to 2 applications.

  • Weeds: Survey weeds to see which weeds were not controlled by fall or winter treatment. The UC Weed ID Tool here can help with identification. Also see Herbicide Chart in this newsletter.

  • Pest Management: As we go into another lean price year, consider your IPM program through the lens of an IPM economic treatment threshold. With the value of the crop down, this may change the population level at which it makes economic sense to treat. See more here.

  • Codling moth: Continue monitoring codling moth traps to confirm flight activity and determine treatment thresholds and timings.

  • Aphids: Aphid sampling should begin this month and continue throughout spring and summer. Collect 5 first sub-terminal leaflets (one back from the last leaflet) from 10 trees, checking the top surface for dusky-veined aphids and the underside for walnut aphids. Make treatment decisions following guidelines here.

  • Walnut blight: Walnut blight infections remain a concern through May. Amidst lean prices, if you are trying to save a spray, consider trying the Xanthocast model, which uses leaf wetness and temperature for forecasting disease risk and timing re-treatment intervals. Model here.

– Katherine Jarvis-Shean, University of California (UCCE) Orchard Advisor, Sacramento, Solano and Yolo counties; Luke Milliron, UCCE Orchard Advisor, Butte, Glenn, & Tehama counties, and Janine Hasey, UCCE Farm Advisor Emerita, Sutter, Yuba, Colusa counties


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