March Prune Orchard Management Considerations

Franz Niederholzer, UCCE Farm Advisor, Colusa, Sutter & Yuba Counties
Luke Milliron, UCCE Farm Advisor, Butte, Tehama & Glenn Counties
Katherine Jarvis-Shean, UCCE Farm Advisor Sacramento, Solano & Yolo Counties
Emily J. Symmes, former UCCE Area IPM Advisor, Sacramento Valley
Dani Lightle, former Orchard Systems Advisor, Glenn, Bute & Tehama Counties

It is finally looking a like spring outside, and prune bloom is picking up. Below are some orchard tasks to keep in mind for prune growers. 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.

  • Forecast = cold at bloom: a closely mowed orchard floor is warmer than one with tall weeds/cover crop, while freshly disked soil is the coldest.
  • Forecast = hot at bloom: If temperatures climb above 81-82oF during or soon after full bloom, fruit set may be reduced and crop loss can occur. To cool the orchard as much as possible when hot weather (80oF plus) at bloom is predicted, run sprinklers during bloom (especially full bloom and the next 2-3 days after full bloom) when temperatures reach 75oF and keep them on until they drop below 75o Evaporation of sprinkler water as it moves through the air provides some small temperature reduction (usually just one or two oF). If the weather is hot at bloom, taller orchard floor vegetation may keep the orchard cooler during the day.
  • Fungicides: Have your air-blast sprayer ready to apply bloom fungicides. Check calibration and do general maintenance (check sprayer filters, replace nozzles as needed, etc. Reference the fungicide timing and efficacy document as you plan your fungicide programs for the year. Remember to rotate FRAC groups for resistance management.
  • Brown rot: A single bloom spray for brown rot, applied at 25-40% bloom, is recommended to protect flowers from brown rot even when skies are clear during bloom. Use locally systemic fungicide(s) (FRAC Group 3, 9, and/or 11) in a single-spray brown rot program. A scab material can be included with this single brown rot spray. Dew can wet the flowers long enough to allow infection, even if there is no rain, so treating at least once for brown rot is recommended. If the weather outlook changes and rain is forecast during bloom, spraying twice is recommended; once at white bud (5% bloom) and again at full bloom. The full bloom spray is the most critical.
  • Russet scab: This disorder develops when significant rainfall occurs during or immediately after bloom. If a single bloom spray is applied for brown rot, before 50% bloom, scab material can be included in that spray. Once the fruit is through the jackets, the risk of scab is mostly gone. The suggested full bloom timing of captan or chlorothanil (Bravo®/Echo®) can harm bees so spray in the late evening or night (bees are back in the hive) or arrange with your beekeeper to remove the hives before spraying. The recommended timing for bee removal is when 90% of flower are open.
  • Peach twig borer (PTB): Monitor during and after bloom. Chewing damage on buds during bloom indicates PTB activity and may warrant treatment. To protect bees, avoid any insecticide in the spray tank at bloom, except Bt (Bacillus thuringiensis formulations such as Dipel®, Javelin®, etc.). Begin post-bloom monitoring with pheromone traps (minimum 2 per block) no later than mid-March to determine biofix (moths caught on two consecutive trap checks) and begin accumulating degree days to inform when to begin fruit inspections. More on PTB here.
  • Aphid: If control measures were not taken during fall or winter, two 440 oil sprays (4 gal/acre/spray) at bloom can be effective against mealy plum and leaf-curl plum aphids if applied at slow ground speeds (for example 1.5 mph) 7-10 days apart. Oil has a level II precaution for bee safety, meaning it should only be sprayed between sunset and midnight, ideally when temperatures have dropped below 55oF to avoid foraging bees. The safest option for bees is to consider utilizing an alternative management timing (spring, fall, or winter) for aphid control. Finally, oil should not be applied with or shortly before/after captan, chlorothalonil, or sulfur because the combination can be phytotoxic. More leaf curl plum aphid info here. More mealy plum aphid info here.
  • San Jose Scale (SJS): If San Jose scale dormant treatments were not applied, not effective, and/or SJS pressure is high, treatments targeting the late spring crawler stage can be effective. Pheromone traps need to be placed ASAP if not out already. Apply crawler treatments 600-700 degree days after biofix (defined as males caught on consecutive trap checks).
  • Bees: Your bees should already be ordered by now, so if they aren’t lined up now is the time. Generally, you want to install one hive per acre.
  • Weeds: Rotating and/or mixing herbicides with different modes of action (MOAs) is critical to good weed management, particularly of herbicide-resistant populations. But MOAs and labeled crops are not always easy to keep track of. Brad Hanson, UCCE Weed Specialist, has organized a chart to help, with herbicide name, a common trade name, the site of action group and the crops for which an herbicide has been labeled for use. This chart is a helpful tool, but remember that labels change often. Always check the herbicide label before use.
  • Irrigation: Check the uniformity of your irrigation system and perform maintenance before the system is needed for frost protection, orchard cooling at bloom, or the irrigation season starts.

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