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California Walnuts: Wrapping Up The Season – Key Management Points

Owen Taylor
From the University of California Cooperative Extension Service September 5, 2017 Updated

California Walnuts: Wrapping Up The Season – Key Management Points

By:

  • Luke Milliron UCCE Farm Advisor for Butte, Glenn and Tehama Counties Janine Hasey UCCE Farm Advisor, Sutter, Yuba, Colusa Counties
  • Emily Symmes, UCCE Area IPM Advisor, Sacramento Valley
  • Themis Michailides, UC Davis Plant Pathologist, UC Kearney Research and Extension Center

Ethephon: If you use an ethephon spray program, target sprays on Chandler for when the hulls achieve 100% packing tissue brown (PTB), which is typically around mid-September. See the Sacramento Valley Orchard  Source for more information: sacvalleyorchards.com/walnuts/ethephon-for-earlier-harvest/

Harvest Quality Considerations: Before harvest look for oilless nuts, which have white kernels and very dark pellicles (skin). These nuts often split and drop before healthy nuts, and can be destroyed when preparing the orchard floor for harvest. Aiming to harvest as early as possible can assist in reducing quality losses due to navel orangeworm, mold infestation, and darkening kernel color.

At harvest, only shake what you can pick up that same day. Walnut quality declines most rapidly during the first 9 hours after shaking. Finally, once harvest is complete clean hullers and driers of trash nuts that may be harboring moth larvae.

Hardening off young trees: The nitrogen fertilizer program for all walnut trees should be wrapped up by the end of August, however this is particularly critical for hardening off young trees for the winter To reduce the possibility of autumn frost damage, aim to cut off irrigation for young trees by mid-September to reduce growth and harden off the trees. Look to hold off on irrigation until a terminal vegetative bud has formed on the trunk.

Harvest Sample: Take harvest samples from the orchard floor to later inspect for worm damage and differentiate between codling moth and navel orangeworm (diagnostic crescent-shaped marking). Grade sheets only list percent insect damage. Performing a crack out of a representative collection of nuts from each production block tells a helpful story. Nuts may also be damaged by ants, walnut husk fly and sunburn. For more information please see the Sacramento Valley Orchard Source website: www.sacvalleyorchards.com/walnuts

October:

Prepping for cover crops: It is critical for any fall cover crop planting to be planned early, ensuring the correct seeding time by lining up seed and equipment before you are too deep into walnut harvest. Planting typically takes place in October and November, with young non-bearing orchards being seeded in October. In producing  orchards, plan to seed just after harvest but before significant leaf fall for best stand establishment.

Fertilizer: If July leaf sampling indicated potassium deficiency, plan to apply potassium (K2O) fertilizer in the fall. Apply potassium in a narrow band along the tree row in order to improve uptake efficiency, particularly on heavier soils. Information on potassium fertilizer forms and rates can be found at: apps.cdfa.ca.gov/frep/docs/Walnut.html

Time pruning to reduce Botryosphaeria/Phomopsis (BOT) infection: Often in August, black to dark brown lesions (the blight phase) start appearing on the hull from earlier, latent (symptomless) infections of the nut.

Botryosphaeria can also cause cankers which can result from pruning wound infections and infections that move from blighted fruit or leaf scars into the spurs. In research conducted by Themis Michailides, UC Davis Plant Pa- thologist at the Kearney Agricultural Research and Extension Center, winter pruning (February 9-10) resulted in higher infection rates than fall pruning (October 27-28).

Dr. Michailides found in 2015 that when pruning in February, wounds in medium-to-large wood (3- and 4- year old branches) are susceptible to Botryosphaeria infection for at least four months after the pruning cut is made.

Pruning wound infection rates were compared for winter vs. fall pruning. Pruning cuts were made in either February or October 2015, inoculated with Botryosphaeria spores, and evaluated over a year later (March 2016 and November 2016, respectively) for infection.

Winter pruning resulted in infection rates (cankers in shoots) from 78 to 99 percent compared to 28 to 75 percent in fall pruned shoots.

Higher infection rates were seen in three- and four-year old wood compared to one- and  two-year old wood, confirming 2015 results of longer cankers in older wood. It is suspected that the hollow pith inside older walnut branches provides a haven for Botryosphaeria infection and spore germination and pathogen growth since the pith holds water as a sponge.

Therefore, if pruning or hedging is planned this year, aim for as early in fall as you can and when weather is forecast to be dry. Deadwood removal however, is best done through the dry summer months.

November:

Sanitation for navel orangeworm (NOW): Keep in mind that the thicker shell of walnuts offers overwintering NOW more protection than the softer shell of almonds. There is less natural mortality in walnuts, even in wet years, and mowing or discing walnut mummies should always be done regardless of weather conditions.

Remember that winter sanitation mummy removal and destruction objectives for NOW are two-fold:

Increase direct mortality of the overwintering generation.

Reduce oviposition and development sites for early generations the following growing season to minimize population build-up.

Weed Management: A postharvest weed survey (ideally after first fall rains) allows you to identify weeds that  have escaped your control program this season, as well as newly emerging winter weeds.

For the weed survey form and weed identification links, click here.

Apply pre-emergent herbicides for winter weed control mid- to late fall. Watch the weather and apply shortly before rainfall so that rain will move the herbicide into the soil before seedlings emerge.

Owen Taylor
From the University of California Cooperative Extension Service September 5, 2017 Updated