News release: Dormant Oil Sprays for Fruit Trees

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640

Release Date: March 5, 2015

Headline: Dormant Oil Sprays for Fruit Trees

Sometimes I will receive a call in mid-summer from a homeowner who describes a problem on their fruit trees, and then asks, “What can I spray for it?” While I am always happy to give them an answer, it is often too late.  The disease or insect is well-established by that time, and the homeowner should have been spraying long before they called me, if they wanted to have a good crop.

For the best quality fruit crop, you need to start spraying in the next few weeks.  Then you need to keep spraying periodically, throughout the season.

Now this may sound strange, since fruit trees don’t have a leaf on them yet.  You may ask, “What is there to spray now?”  But while it may seem odd to be spraying a bare tree, conscientious orchard growers know that they need to start their spray program during the dormant season, if they want high quality fruit.

The first sprays of the season are called dormant oil sprays, and are applied in late winter or early spring, before the trees leaf out. The purpose of a dormant spray is to kill pests such as scale insects.  Dormant sprays also help control aphids and mites to some degree.  They can be used on fruit trees as well as many other deciduous trees and shrubs.

Sometimes referred to as “horticultural oils,” dormant sprays consist of highly refined petroleum oils combined with an emulsifying agent. To use them, mix horticultural oils with water and spray them on your tree.

These oils work by physically blocking the air holes through which the insects breathe.  With no access to oxygen, the insects suffocate and die.  This works particularly well for scale insect adults, but can also help control other insects in the larvae and egg stage.

Agricultural sprays vary greatly in their risks to people.  Fortunately, horticultural oils are among the safest to use, when applied properly.  They are also relatively safe for the environment, eventually dissipating through evaporation.

When you apply dormant oils, keep in mind that they do need to make good physical contact with the pest you are trying to control.  Thus, you should make sure that you cover the tree or shrub well.

One precaution should be noted.  Dormant oils should not be sprayed when temperatures are freezing.  Freezing temperatures can cause the emulsion to break down which leads to uneven coverage.  It’s best to wait until temperatures are at least in the 40's before spraying horticultural oils.

If you are growing fruit trees, there are a few other sprays that are applied very early in the season.  One example is Bordeaux mixture, which is used in apple trees to control fire blight.  Peach trees need to be sprayed with materials to control peach leaf curl.  Grapes need to be sprayed before bud swell to control a number of diseases.

For good quality fruit, you need to start spraying early, and so if you are growing fruit, I would encourage you to contact your local University of Missouri Extension Center and ask for guide sheet G6010, “Fruit Spray Schedules for the Homeowner.”