California Walnuts: What To Remember Over The Next 4 Months In The Sac Valley

APRIL

  • Walnut blight: new spray rotations may be available for blight control this year (see article on Kasumin 2L, this newsletter). Stay on top of spray timings. For basics of blight management, see: sacvalleyorchards.com/walnuts/diseases-walnuts/walnut-blightmanagement
  • For varieties susceptible to pistillate flower abscission (PFA) (especially Tulare or Serr), apply first ReTain® spray at 30-40% female flower bloom. The percent PFA and rate of bloom determines if a second spray is needed. ReTain® cannot be applied within 2 days of a copper application.
  • 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.

  • Bot: Limbs that have been killed by Bot canker are easy to identify between budbreak and full leaf expansion, but wait to prune until rain is no longer forecast. If timing Bot treatment based on the Leaf Wetness Model, watch for storms that bring ≥¼” rain and temperatures ≥50 F.
  • Perform irrigation system maintenance now, before irrigation is necessary and system problems cause tree stress. Check for broken or clogged filters and emitters. See: micromaintain.ucanr.edu for more tips on maintaining micro-irrigation systems.
  • Codling moth: Traps for codling moth should have been deployed in early- to mid-March to establish biofix. Check traps twice weekly until biofix (moths found on consecutive trap checks AND sunset temperatures above 62F) and weekly thereafter. After biofix, begin accumulating degree days to track development and inform application timing(s) if population densities necessitate treatment. Many effective mating disruptants are available for codling moth; if using mating disruption, hang or apply disruptants ahead of historical biofix in your orchard. In mating disruption orchards (or orchards in proximity to disrupted orchards), make sure to use combination lures to track the population (CMDA), rather than pheromone lures only (1X or L2), as pheromone traps will be shut down or far less sensitive.
  • Navel orangeworm: Sanitation activities (removal and destruction of mummy nuts in the orchard and surrounding areas) should be completed by mid-March. Data shows that increasing levels of mummy destruction have the greatest impact on NOW mortality (shredding > double-disking > intact mummies in weeds > intact mummies on bare berms). Consider placing pheromone and kairomo orchard to monitor adult male and female flights and abundance, respectively.

MAY

  • Nitrogen fertilization applications should begin in May. Walnut trees only use stored nitrogen the first month after leaf-out, so N applied before May will likely be leached. Walnut tree nitrogen use is fairly steady over the growing season. Evenly dividing nitrogen application in 3-4 doses between May and mid-August will improve N uptake compared to 1-2 applications.
  • Codling moth: Continue monitoring traps. Look for 1B flight peak approximately 600 to 700 degree days (DD) after first biofix. If populations are high, or the orchard has a history of significant codling moth damage, consider treating according to UC IPM guidelines (link below) if populations are high, the orchard has a history of significant codling moth damage, and/or the residual period from the 1A treatment (if applied) has elapsed. Ideal treatment timings will depend on material (roughly 650 to 700 DD after first biofix – confirm with trap activity).
  • Navel orangeworm: Continue monitoring trap activity. Research still indicates that best treatment timing for NOW is late season (husk split). Trap data can provide information regarding relative population abundance and flight activity as husk split approaches.
  • Aphids: Begin sampling for aphids in May, examining upper leaf surfaces for dusky-veined aphids and lower surface for walnut aphids. Walnut aphid natural enemies (parasitoid wasps) tend to provide good levels of control if not disrupted. Evaluate the level of parasitism in your orchard (abundance of aphid mummies relative to non-parasitized). Consider treatment only when the number of non-parasitized aphids exceeds an average of 15 per leaflet in a sample of 50 leaflets from 10 trees.
  • Weeds: Survey weeds to see which weeds were not controlled by fall or winter treatment. The UC Weed ID Tool at weedid.wisc.edu/ca/weedid.php can help with identification.

JUNE

  • Bot: If applying only one fungicide spray for Bot canker, a mid-June to mid-July spray timing significantly reduced blighted shoots compared with a no spray treatment. Prune out dead branches to reduce inoculum now that threat of rain has passed.
  • Codling moth and NOW: Continue monitoring traps and developing nuts for evidence of infestation.

  • Spider mites: Start looking for spider mites and predators (especially predatory mites and sixspotted thrips) in late spring and map areas of concern for summer monitoring. Begin summer monitoring in June or early July (erring on the early side if warmer temperatures). Good predator abundance early in the year can provide significant natural control later in the year IF not disrupted by broad-spectrum pesticides or miticides.
  • Husk fly: Hang traps by June 1. Yellow sticky traps charged with an ammonium carbonate lure work best. Check traps 2 to 3 times per week and treat based on detection of eggs in trapped females or increases in trap catches. For more details on treatment decision-making, see sacvalleyorchards.com/walnuts/insects-miteswalnuts/walnut-husk-fly-biology-monitoring-and-spray-timing/

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