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Dormant season spraying

  • Martha Proctor
  • November is the month to begin to spray deciduous fruit trees. Photo: Martha Proctor
    November is the month to begin to spray deciduous fruit trees. Photo: Martha Proctor
    After a few years of watching and waiting, a peach tree I planted several years ago finally bore fruit this year. Unfortunately, it was unable to produce during those first few years due to peach leaf curl, a fungal disease. Peach, apple, pear, and many other kinds of deciduous fruit trees are vulnerable to diseases and insects that can impact crop production and, ultimately, the tree’s health. Applying dormant oils can help protect trees from soft-bodied insects and leaf curl, shot hole fungus, powdery mildew, and other fungi.

    Dormant oil sprays (e.g., horticultural, narrow-range, or superior oils) are non-chemical pesticides. These highly refined petroleum products or vegetable oils (soybean, cottonseed, sesame, neem, or other oils) are mixed with water and applied to trees and shrubs during the winter when the trees are dormant.  When applied to the point of run-off to the branches and trunks of fruit trees, these sprays plug the pores through which insects, e.g., aphids, spider mites, psyllids, thrips, and scale, breathe, thus suffocating them. Thorough application and coverage are essential as many insects prefer the undersides of leaves or reside under sepals or buds where they are hard to reach.  Wear protective gear, long sleeves, and long pants when spraying. Take care not to swallow, inhale or expose your skin and eyes to the oil.

    Proper timing is critical.  Dormant oils should be applied at the beginning of dormancy in late November and again during leaf bud swell, just before buds begin to open in February or early March.  Associating the timing for spraying with Thanksgiving and Valentine’s Day can be a helpful mind jogger.  The timing coincides with the increasing activity of the larvae of overwintering insects. These oils are temperature sensitive, so apply on a clear, non-windy day in the 50o to 70o F temperature range when the temperature is not expected to drop to freezing for 24 hours after application.

    A good time to apply dormant oil is right after a period of rain or foggy weather. Do not apply to drought-stressed trees or to any tree during fog, rain, or during or prior to hot or freezing weather.  If it rains within hours of application, reapply the treatment.  Do not treat within 30 days before or after applications of sulfur or certain other fungicides to avoid damage to trees.  Dormant oil applications are not appropriate on citrus or avocado trees as these species do not enter winter dormancy.

    When mixing up spray solutions, mix less than you think you’ll need as disposal of leftovers is difficult.  Estimate the amount your trees will need for good coverage by doing a test spray using water.  Always follow the directions on the container label.  Although horticultural or dormant oils leave no residue, so have a limited impact on pets, wildlife, and beneficial insects, they are toxic to fish.  Carefully follow listed precautions with regard to protecting bees by only spraying in the early morning or late evening.

    In addition to the use of dormant sprays, there are several other important non-chemical cultural management options.  Avoid overhead irrigation; plant fruit trees in full sun.  Rake and destroy (do not compost) the fallen leaves around your fruit tree—space tree plantings and prune to provide good air circulation and light penetration.

    Incorporating good cultural practices into your garden maintenance program will set the stage for more successful pest management.  By applying dormant oils annually as directed to well-maintained fruit trees, you will help to protect your trees from several overwintering insects and prevent damage to the flower and foliage tissue. Most actively growing plants are not negatively affected by horticultural oils, especially if they are not under environmental stress. If used properly, horticultural oils can be a less toxic approach than chemically-based insecticides.