Dormant spraying fruit trees in winter
Diseases and insect pests that often attack fruit and nut trees, grapes, and brambles in spring and summer can be headed off during the winter by using dormant sprays.
Dormant sprays – which kill overwintering pests and some diseases – are best applied when plants are in the dormant stage – after leaves have dropped in the fall and before leaf buds open in spring.
“Dormant spray” is a generic term. Some dormant sprays are refined oils – sometimes called horticultural or insecticidal oils. The oils smother overwintering insects such as aphids, mites, scale, and thrips, as well as their eggs.
Other dormant sprays contain copper or a synthetic fungicide. Copper and fungicides limit infection and prevent the spread of bacterial and fungal diseases such as peach leaf curl, fire blight, powdery mildew, shot hole, and brown rot.
Trees commonly sprayed with horticultural oil, copper, or synthetic fungicide include apples, peaches, pears, apricots, cherries, nectarines, crabapples, almonds, quince, pyracanthas, and roses.
A third type of dormant spray is liquid lime-sulfur. It is used mainly on small fruit plants such as grapes, blueberries, blackberries, and raspberries. Lime-sulfur sanitizes stems and kills overwintering fungus and bacteria.
Dormant sprays are relatively easy to apply and they are generally not disruptive to beneficial insects and the environment when applied properly, and they do not harm humans or other warm-blooded animals when applied following label directions.
For the backyard gardener, dormant spraying may not be necessary every year. It’s warranted when pests or diseases have plagued trees and plants and affected the quality of the harvest the previous growing season.
To be effective, dormant sprays must be applied thoroughly – every square inch of a stem, trunk, and branch. Insects and disease organisms overwinter in the cracks and crevices of tree bark and plant tissue. The objective is to coat insects, insect eggs, fungi, and bacteria, and allow the spray to smother the organisms or disrupt their cellular activity.
Often, dormant spray will suppress, but not totally control a pest insect or disease. Additional insecticidal or fungicidal sprays may be necessary during the growing season – especially as insect populations and diseases increase during warm summer months.
Dormant sprays are best applied during the day when temperatures are above 40°F. Warm temperatures allow dormant sprays to spread easily and evenly. To ensure good coverage, avoid spraying on windy days. And delay spraying if a freeze is coming; dormant sprays are commonly mixed with water and water can freeze on plants and harm them. (Sprays are cut with water and usually contain three to four percent oil. They also include soap-like emulsifiers that allow water and oil to mix for spraying.)
Almost as important as using a dormant spray is cleaning up fallen leaves under trees and in the garden. Many pests overwinter or lay eggs in fallen leaves and garden debris under trees. In spring, adults will fly up into the trees and start a new generation of pest. Many diseases overwinter in garden debris and weeds and become airborne when it’s windy or rainy in late winter, spring, and summer.
Common dormant sprays
Horticultural oil: These are lightweight oils, either petroleum or vegetable based, and are used to control overwintering insects and insect eggs. They can be found at garden centers and may also be called supreme oil, superior oil, all-season oil, and summer oil.
Neem oil: This oil is made from the seeds of the tropical neem tree. It can be used as both an insecticide and a fungicide.
Fixed copper fungicide: This product contains copper sulfate, which kills fungi, bacteria, and viruses.
Lime-sulfur spray: A mixture of hydrated lime and sulfur. Lime sulfur spray will control fungi, bacteria, and insects.
Synthetic fungicides: These products contain chlorothalonil, daconil, iprodione, or thiophanate methyl. These commercially manufactured organic compounds control fungi.
Insects controlled using dormant spray oils
- Codling moth on apple, pear, plum, and walnut.
- Mites on peaches, nectarines, apricots, and to some extent plums.
- Peach twig borer on apricot, nectarine and peach, and sometimes almond, plum, and prune.
- Soft scale on many kinds of landscape trees.
- Diseases controlled using dormant spray fungicides (synthetic fungicides and fixed copper fungicides)
- Downy mildew fungus on grape and rose.
- Fire blight bacteria on apple, crabapple, pear (ornamental and fruiting), pyracantha and quince.
- Leaf curl fungus on nectarine and peach.
- Powdery mildew fungus on grape and rose, and occasionally apricot, cherry, nectarine and peach.
- Rust and black spot fungi on rose.
- Shot-hole fungus on almond, apricot, nectarine, and peach.
- Anthracnose twig and leaf fungus on grapes, blueberries, gooseberries, blackberries, and raspberries.
Steve Albert is the author of The Kitchen Garden Grower’s Guide available at Amazon.com. He teaches in the landscape design program at the U.C. Berkeley Extension. He lives in Oakmont.