Oregon State University Extension Service

Get a leg up on fruit tree problems with dormant oils

CORVALLIS, Ore. – Just when you’re ready for a long winter’s nap, it’s time to tend your fruit trees.

If you don’t, chances are they’ll struggle in the coming season. Giving them attention now helps ward off insects and diseases, said Ashley Thompson, an Oregon State University Extension Service fruit tree specialist and assistant professor in the College of Agricultural Sciences.

"You want to use products that have low toxicity, and won’t cause a lot of problems for the environment. Dormant sprays score pretty well."

Applying dormant sprays – horticultural oil, copper and sulfur – helps control nasty pests and diseases like codling moths and apple scab.

Superior oil, also called horticultural oil, is a highly refined miscible oil (up to 99.9 percent pure) that when mixed with water and sprayed on trees will smother overwintering insects and their eggs. It targets mites, aphids, leaf hoppers, mealy bugs, leaf miners and more.

Sulfur is a fungicide that controls fungal diseases like apple and pear scab and peach leaf curl.

Copper is a fungicide and bactericide that controls diseases like bacterial blight, fire blight and Nectria canker. It kills bacteria and fungal spores left in the trees, including Pseudomonas syringae, a common bacteria that can cause gummosis, which is oozing of bacterial infested honey-like sap from split bark. In a rotation of copper and sulfur, the copper will deal with bacteria and sulfur will target fungal diseases best.

With a spray regimen of all three – used in conjunction with good hygiene and pruning practices – most fruit tree problems can be nipped in the bud, Thompson said.

The trio of pesticides, which can be used in organic gardens, fit snugly into the realm of IPM or integrated pest management, a practice that uses a variety of low-risk tools to deal with pest problems and minimize risks to humans, animals and the environment.

“They are a really important part of good IPM,” Thompson said. “When you’re planning a program, you want to use products that have low toxicity, and won’t cause a lot of problems for the environment. Dormant sprays score pretty well. Their toxicity level for animals is pretty low if you follow the labels. Horticultural oil kills target insects, but beneficial insects are rarely around trees in the dormant season.”

Three-pronged spraying approach

A good reference for disease and pest control is Extension’s "Managing Diseases and Insects in Home Orchards," which has a list of cultural practices and least toxic products for various pests and diseases.

Thompson recommends a three-pronged approach to spraying. In fall around Thanksgiving, apply copper. Spray sulfur in early January and then at least two weeks later make a spray with horticultural oil. Then make another copper spray in mid-to late February. Don’t combine copper and sulfur or sulfur and oil in the same tank to minimize the risk of damage to tree bark.

If you don’t like to spray or forget the early spray, Thompson said the January application is the most important. This year, if you’ve missed the January timing, you’re still better off to make the third spray.

Some tips from Thompson:

  • Read the labels of all products you use and follow the instructions. Using any pesticide incorrectly is not only harmful to you and the environment, it can actually cause damage to the very plants you’re trying to benefit.
  • Apply horticultural oil during the dormant season to allow for greater coverage and a higher likelihood of getting to a majority of insects.
  • Spray when temperatures are above freezing but before buds break.
  • Don’t mix copper and sulfur in the same tank.
  • Prune trees to keep the branches separated for good pesticide coverage and good hygiene. The best time is in January so that the last spray or two will cover the pruning wounds.
  • Clean up fruit, leaves, weeds and debris under trees where coddling moth and other insects can overwinter. If you don’t want to rake leaves, mow over them a couple of times and leave them to decompose.
  • Prune diseased wood from your trees.
  • Add organic matter around trees for fertility and because enhanced microbial populations in the soil will help devour the remnants of orchard sprays that fall to the ground. Don’t spread mulch too close to the tree trunks.
  • Accept a little damage to fruit.
  • When planting fruit trees, consider dwarfs so you don’t need a ladder for spraying.

For more information on fruit trees, refer to Extension’s "Growing Tree Fruits and Nuts in the Home Garden" and "Training and Pruning Your Home Orchard."

Ashley Thompson
Assistant Professor of Horticulture

Source URL: https://extension.oregonstate.edu/news/get-leg-fruit-tree-problems-dormant-oils