May Prune Orchard Management Considerations

Franz Niederholzer, UCCE Farm Advisor, Colusa, Sutter and Yuba Counties
Luke Milliron, UCCE Farm Advisor, Butte, Glenn and Tehama Counties
Katherine Jarvis-Shean, UCCE Farm Advisor Sacramento, Solano & Yolo Counties

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.

  • Rust: Monitoring commences with the start of the month, surveying 40 trees every 1-2 weeks. Pay close attention to non-bearing replants, exceptionally vigorous trees, and previous hot spots. Consider treating when the first leaf with rust is found. For more on rust see UC IPM for rust in prunes.
  • Peach twig borer (PTB) and Oblique-banded leaf roller (OBLR): These worms feed on the fruit surface later in the season, damaging the fruit skin and “opening the door” for fruit brown rot infection later in the season. Don’t assume earlier sprays worked to control these pests. Inspect fruit at 400 degree days after the first PTB biofix. In the orchard, look for larval entry points on the fruit (ideally 15 fruit from 80 trees), especially at fruit to fruit or fruit to leaf contact points. Treat if 2% or more (24+ of 1,200) of the fruit have damage. For OBLR, begin fruit inspections at 930 degree days after biofix for that pest, following the same sampling protocol and treatment threshold. More on PTB at UC IPM for Peach Twig Borer and on OBLR at UC IPM for Oblique-Banded Leaf Roller.
  • Aphids: Leaf curl plum aphids move to summer hosts in May, but mealy plum aphid stay in orchards until mid-July. Heavy infestation of mealy plum aphid can limit flower bud development this year, which can mean less crop next year.
  • Irrigation: Continue monitoring pressure chamber, soil moisture and/or weekly ET to manage irrigation and maintain adequate orchard moisture. May and June are the most critical months for end-cracking, which occurs when very dry orchards are irrigated.
  • Fertility: Continue with nitrogen and potassium fertilization program if a good crop is set. More than 50% of annual N budget should be applied before June 1st.


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