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Fire blight risk high with warm temperatures: Reminders

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With warm temperatures projected fire blight temperature risk will be high to extreme in many areas starting Friday and over the weekend. Where moisture (including dew) occurs it will be critical to treat open apple and pear bloom for fire blight.

Remember to use high efficacy materials and concentrate on optimal timing. Consider your risk.

Reminders for conventional orchards:
  • Antibiotics work best when applied 12-24 hrs before a moisture event when warm conditions signal management. Save streptomycin products for 1 or less applications per year if you miss a preventative spray.
  • At best, antibiotics provide 3-4 days of protection. Applications every 2-3 days are necessary during high risk. Rotate modes of action. Cover blooms as they open.
  • Acidify your spray tank. Target pH 5.5 (new research shows 4.0 better) to improve efficacy and residual of antibiotics.
  • Consider antibiotic mixes in high risk blocks. A full rate of kasugamycin (100 ppm) with a full rate of oxytetracycline (200 ppm), as well as streptomycin (100 ppm) mixed with a full rate of oxytetracycline (200 ppm) have provided improved efficacy in some trials (Oregon 2015-2018). Actigard (2oz) plus an antibiotic applied during bloom has improved the efficacy of antibiotics an average of 10% in trials in Washington and Oregon (Smith and Johnson 2011-2014).
  • Rotate!
  • Good coverage is key. Product efficacy is based on thorough coverage of flowers. Use tree row volume to apply appropriate volumes to cover the tree architecture in your orchard. Products applied every other row or at high speeds may have insufficient coverage and lower efficacy.
  • Blossom Protect + Buffer Protect can be applied with antibiotics to extend the period of control.
Reminders for organic orchards:
  • Use your highest efficacy materials. Blossom Protect+ Buffer Protect averages 80% control and soluble coppers (e.g. Cueva and Previsto) 65-80% control in WSU trials. Other products may provide only 20-40% control and should be used in mixes, when risk is lower, or when marking risk is too high. For efficacy ratings see
  • Product sequence. Blossom Protect+Buffer Protect approaching full bloom followed by copper a few days later.
  • Always consider drying conditions. Most biologicals, coppers and biopesticides can mark.
  • If using Blossom Protect:
    • Apply to every row. Research has found better colonization of flowers by the yeast strains when it has been spray applied to the whole tree. That is, yeasts that colonize pome flowers do not appear to spread flower-to-flower as well as bacteria.
    • Apply in good drying conditions. Blossom protect can mark when it is wet.
    • Apply twice (or more). Two (or more) applications for good coverage of open flowers and sufficient time for the yeast to grow and colonize floral stigma.
    • Use buffer. Blossom Protect applications with the companion material, Buffer Protect, have shown significantly better control than with Blossom Protect alone.
    • Re-apply after lime-sulfur. Lime sulfur applications are both anti-bacterial and anti-yeast and will knock back the populations of an applied biological. Reapply the biological after lime sulfur treatment.

For more information

Example programs:

Risk models:



Tianna DuPontImg1380

WSU Extension Specialist, Tree Fruit

Office: (509) 293-8758

Mobile: (509) 713-5346

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