Fall & Winter Prune Orchard Management Considerations

Luke Milliron, UCCE Orchard Advisor, Butte, Glenn, and Tehama Counties
Franz Niederholzer, UC Farm Advisor, Colusa, Sutter and Yuba Counties
Dani Lightle, Former UCCE Orchards Advisor
Drew Alonso Wolter, Former UCCE Horticulture Intern

Irrigation

  • As the days grow shorter and cooler (soon!), orchard irrigation needs decrease but don’t vanish. Manage post-harvest irrigation to minimize water stress, as water stressed prune trees are more susceptible to Cytospora canker spread compared to well-watered trees. Following harvest, water stress measured as stem water potential using a pressure chamber should be mild to moderate (-12 to -16 bars). Learn more about pressure chamber use at: com/manuals/stem-water-potential.
  • Alternatively, use ET estimates, which are available for North and South Sacramento Valley at: com/et-reports). A final option is soil moisture evaluation with electronic sensors or by hand using an auger, to determine when and how much irrigation water to apply.

Orchard Clean-up

  • Cytospora cankers will continue to grow unless they are cut out. Dead wood with Cytospora pycnidia are sources of new infections (see photo of pycnidia in the article on Cytospora in this newsletter). Remove existing Cytospora cankers and spore sources by cutting branches several inches to a foot below the symptoms and burn prunings (if permitted). If an orchard has a lot of dieback/dead wood, consider a special crew or employee to just cut out diseased and damaged wood. This lets them focus on the cuts to clean up the orchard and not go back and forth between regular pruning cuts and orchard clean up. See the pruner’s pocket guide at: com/prunes/pruners-pocket-guide-for-cutting-out-cytospora.
  • Clean up “barked” trees damaged at harvest. Trunk/limb damage from harvest can result in Ceratocystis canker infection and possible tree death. Cut away any loose or damaged bark back to “tight” bark with a sharp knife or chisel and hammer. Painting the wound with commercial wound sealer can help protect damaged trunks while healing.
  • Flag dying or weak trees for removal. Backhoe out old trees, making sure to get as many roots out of the hole as possible.

Pruning

  • Avoid pruning when rainfall is imminent. If there is rain forecasted during the 30 days after pruning, strongly consider a fungicide spray (Topsin-M® or Topsin-M + Rally®) before the rain event to protect the pruning cuts.
  • Remove and burn prunings (if permitted) to reduce in-orchard inoculum levels of bark cankers like Cytopsora. If you can’t burn, consider a powerful shredder to pulverize larger prunings in the field.
  • Skipping pruning for a year is a significant cost-savings but be prepared to shaker-thin if needed next spring. If thinning isn’t done when needed, the savings from skipping pruning may turn into a net-loss after setting a large crop of small fruit and suffering increased sunburn, broken limbs, and increased Cytopsora damage.
  • If you are training a young orchard, consider intermediate or long pruning for earlier yields, but be prepared to tie your trees to support the longer growth. Learn more at: sacvalleyorchards.com/prunes/horticulture-prunes/long-pruning-for-greater-early-yields.

Orchard Topping

  • If you are going to top your orchard in fall, top when there is no rain in the forecast. If rain is in the forecast, be sure to protect fresh wounds from infection by rain-splashed spores with a fungicide spray (Topsin-M® or Topsin-M® + Rally®). This must be done before the rain!
  • Young, well-irrigated trees topped before mid-October will show some regrowth before leaf fall. Topping young, vigorous trees before a big wind will reduce the risk of blow-over.

Nutrition

  • Tree nitrogen (N) uptake is limited in the fall (there is nothing to feed) and trees will not take up N once leaf drop has begun. N should not be soil applied after September to avoid N leaching by winter rains.
  • A fall foliar nitrogen (N) spray of 10-20 lbs N/acre may help reduce bacterial canker incidence next spring in young orchards, especially if summer leaf N levels are low. One 50 lb bag of good quality standard urea (or low biuret urea) contains 23 lb N.
  • Soil applied potassium (K) should be banded in the fall. One dry ton of crop removes 26 lbs of potassium from your orchard. A common maintenance rate is 400-500 lbs/ac of K fertilizer (potassium sulfate). You can also opt to apply fertilizer K through a drip irrigation system (fertigation) and/or multiple foliar applications once a crop has been set next spring. Learn more at: com/prunes/horticulture-prunes/potassium-nutrition-maintaining-optimal-levels.
  • Foliar zinc (Zn) to correct zinc deficiency can be applied at the beginning of leaf drop (late October/early November). 20 lbs of 36% zinc sulfate at 100 gallons water/ac may also hasten leaf drop, reducing risk of blow over and/or disrupting aphid reproduction. [A low rate of pyrethroid insecticide can be tank mixed with this zinc sulfate application rate and timing for good aphid control next year.]
  • A lower rate of zinc sulfate (5 lbs material/acre in 100 gallons water per acre) is an effective zinc foliar fertilizer spray when applied ahead of natural leaf drop (late September or early October). This lighter rate will not accelerate leaf drop. This timing is too early for a pyrethroid tank mix to control aphids next year.

Insects

  • Aphids: If there is a history of aphid problems and no need for a dormant scale or peach twig borer treatment, low rates of pyrethroids have been shown to be effective when applied between mid-October and mid-December. Treating aphids in the fall, or during the dormant season in conjunction with scale/PTB treatments, has less impact on natural enemies than spring aphid treatments. Not every orchard needs to be treated for aphids. Detailed articles on prune aphid management can be found at com/prunes/insects-mites-prunes.
  • Scale: Obtain dormant spur samples looking for San Jose scale and European fruit lecanium, as well as evidence of parasitism in both species. If significant parasitism is noted, treatments may not be necessary. More information on dormant sampling and treatment thresholds at: ucanr.edu/PMG/r606900511.html.
  • Peach twig borer: During the dormant period, a moderate rate of pyrethroid is effective on aphids and PTB (6-8 oz/acre of Asana) without the need to use the maximum labeled rate (14+ oz/acre). Alternatively, Peach twig borer can be controlled at bloom, by tank mixing B.t. (DiPel®, Javelin®; doesn’t harm bees) with bloom fungicides.
  • Visit sacvalleyorchards.com/prunes/insects-mites-prunes/aphid_management for a table of timing, efficacy, and other considerations for treatment of these three pests.

Weeds

  • Conduct a post-harvest weed survey (ucanr.edu/PMG/C606/prune-fallweeds.pdf) to evaluate your 2019 weed control program efficacy. More details on the value and keys to scouting at: sacvalleyorchards.com/almonds/weed-control/post-harvest-weed-scouting.
  • Pre-emergence herbicide (combined with a post-emergence burn-down material if winter weeds have already germinated) should be applied shortly before a moderate rain event (0.25”) to move material into the soil. Avoid application prior to a large rain event (> 1”), which can move the product too deep into the soil for good weed control. Avoid spraying root/trunk sucker leaves with any spray containing systemic herbicides such as glyphosate (Roundup, etc.) since those herbicides can enter the tree and cause damage next spring. More information on fall weed management at: com/prunes/fall-weed-management.

Gophers

  • Late fall to early winter is a prime gopher control time because populations are low and can be controlled before adults breed and create more gophers. See gopher control strategies at: ucanr.edu/PMG/r606600311.html.

Replants

  • Before replanting in winter, consider the probability of a successful replant by evaluating why the previous tree died and if management lessons can be applied for the entire orchard. Also consider the longevity of the larger orchard and whether a replant will have enough time to become profitable. Replanting with a bare root, instead of a potted tree will give you a larger and an easier tree to irrigate.
  • Pruning replants exposes them to Cytospora infection from inoculum already present in mature orchards. Consider protecting new pruning wounds with a fungicide spray (Topsin-M® or Topsin-M + Rally®). Risks are particularly high in interplant situations, with a high number of newly planted trees: com/prunes/diseases-prunes/interplanted-orchard-hazards.
  • Was the tree lost to a soil-borne disease? If so, is spot fumigation an option? Could a different rootstock choice increase the chance of success? See the latest update on rootstock performance and survivability in UC trials at: com/prunes/horticulture-prunes/update-on-rootstocks-2019.

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