California Walnuts: Orchard Management Considerations: Budbreak through Early Summer

APRIL

  • Stay on top of your blight sprays, especially if we have rain around “prayer stage” of leaf out. More details here.
  • The first flight of codling moth typically starts between mid-March and mid- April and will have two peaks (1A and 1B). Male biofix is the first day male moths are consistently found in traps and sunset temperatures reach 62°F. In orchards not using mating disruption with more than 3% damage last season, treating 1A and 1B flights may be warranted if pheromone traps (1X or L2) are catching more than an average of 2 moths/trap/night. Treat 1A larvae about 250 to 300 DD after biofix, depending on material. Some newer “reduced-risk” chemistries are more effective applied on the early end. If 1A treatment is not needed based on orchard history and/or trap catches, delaying treatment until 1B or second flight will minimize interference with walnut aphid parasitoids. For more information, click here.
  • Consider putting out navel orangeworm (NOW) pheromone traps (Biolure or L2), at least in more susceptible varieties and/or blocks with a history of NOW damage. See the Winter 2016/17 NOW Update article for more information.
  • Limbs that have been killed by Bot canker are easy to identify between budbreak and full leaf expansion. But wait to prune out infected limbs once rain is no longer forecasted, i.e. early summer. For more on Bot, see the article in this newsletter.
  • If timing Bot treatment based on the Leaf Wetness Model, watch for storms that bring ≥¼” rain and temperatures ≥50o F. See the Bot article in this newsletter for more on treatment timing.
  • For orchards with standing water, take pictures to document the flooding. This will help with diagnosing problems later in the season and is needed if applying for disaster relief programs. Photos taken on your phone will usually have a “created” file date that will help with record keeping.
  • Apply Foliar Zinc (if needed, based on leaf sample analysis) when shoots are 6 to 10 inches long, when zinc can be easily absorbed through the leaf surface. If the deficiency is severe, additional sprays can be applied two more times every 2 to 3 weeks.
  • Perform irrigation system maintenance now, before irrigation is necessary and system problems could cause tree stress. Check for broken or clogged filters and emitters. See http://micromaintain.ucanr.edu/ for more tips on maintaining micro-irrigation systems.
  • Scout for scale crawlers by putting out double-sided sticky tape in late April, if scale has been a problem and you didn’t treat for scale during the dormant season.

MAY

  • Watch traps for the codling moth 1B peak about 600 to 700 DD after the first biofix. If a significant 1B flight peak occurs (and damage exceeded 3% last season), consider treating when residual effectiveness of the first treatment has elapsed. If a 1A spray was not applied, 1B treatment should be approximately 650 to 700 DD from first biofix (confirm with trap activity; ideal timing depends on material). More information here.
  • 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 application in 3-4 doses between May and the end of August will improve N uptake compared to 1-2 applications.
  • Apply a fungicide spray for Bot canker and blight around mid-month in orchards with moderate to heavy infection.
  • Survey weeds to see which weeds were not controlled by fall or winter treatment. The UC Weed ID Tool at  can help with identification.
  • 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. Here are guidelines for making treatment decisions.

JUNE




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