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By Kym Pokorny
OSU Extension 

Get a leg up on fruit tree problems with dormant oils


December 28, 2023 | View PDF

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 Steve Renquist, a horticulturist for Oregon State University Extension Service who has taught hundreds of gardeners the basics of managing fruit trees.

Applying dormant sprays – such as Superior dormant spray oil, copper, and sulfur – helps control nasty pests and diseases, including codling moths and apple scabs.

* 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 the oozing of bacterial-infested honey-like sap from bark split. 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, according to Renquist.

The trio of pesticides, which can be used in organic gardens, fit snugly into the realm of integrated pest management (IPM), 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,” Renquist 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. Superior or horticultural oil kills target insects, but beneficial insects are rarely around trees in the dormant season.”

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 the least toxic products for various pests and diseases.

Renquist recommends a three-pronged approach to spraying. In the fall around Thanksgiving, apply copper. Spray sulfur in early January and then at least two weeks later make a spray with dormant 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, Renquist said the January application is the most important. This year, if you’ve missed the January timing, you’re still better off making the third spray.

Some tips from Renquist:

* 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 cause damage to the very plants you’re trying to benefit.

* Apply Superior or 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, and debris under trees. They can harbor insects and diseases. If you don’t want to rake leaves, mow over them a couple of times and leave them to decompose.

* Clear weeds from around the trunk and under the tree where insects and rodents can hide.

* Add organic matter around trees for fertility because enhanced microbial populations in the soil will help devour the remnants of orchard sprays that fall to the ground.

* 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.

Author Bio

Kym Pokorny, Communications Specialist for Oregon State University

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Public Service Communications Specialist


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