Get a leg up on fruit tree problems with dormant oils

When you’re ready for a long winter’s nap, it’s time to tend your fruit trees.

If you don’t, they’ll likely 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 with low toxicity and won’t cause many environmental problems. Dormant sprays score pretty well."

Applying dormant sprays – horticultural oil, copper, and sulfur – helps control nasty pests and diseases like 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, leafhoppers, mealy bugs, leaf miners, and more.

Sulfur is a fungicide that controls fungal diseases like apple, 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 split bark. In a rotation of copper and sulfur, copper will deal with bacteria, and sulfur will best target fungal diseases.

Thompson said that a spray regimen of all three—used in conjunction with good hygiene and pruning practices—can nip most fruit tree problems in the bud.

The trio of pesticides, which can be used in organic gardens, fits snugly into the realm of IPM or integrated pest management. IPM is 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 an essential 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

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

Thompson 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 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.

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

Some tips from Thompson:

* Read the labels of all products you use and follow the instructions. Misusing any pesticide is not only harmful to you and the environment, but it can also 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 reaching most insects.

* Spray when temperatures are above freezing but before buds break.

* Don’t mix copper and sulfur in the same tank.

* Prune trees to separate the branches for good pesticide coverage and hygiene. The best time is in January when the last spray or two will cover the pruning wounds.

* Clean up fruit, leaves, weeds, and debris under trees where coddling moths and insects can overwinter. If you don’t want to rake leaves, mow over them several times and leave them to decompose.

* Prune diseased wood from your trees.

* 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. Don’t spread mulch too close to the tree trunks.

* Accept minor damage to the 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

Public Service Communications Specialist