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.
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 insect 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.)
Here are common dormant sprays:
Horticultural Oils: These are lightweight oils, either petroleum or vegetable based. Petroleum oils used in horticulture are highly refined with a narrow range of distillation—which means they dry very quickly, commonly in about 5 minutes. These oils are also called narrow range oils and sometimes supreme oils or superior oils (because they are superior to horticultural oils of the past which were thicker and did not dry as quickly having a longer distillation). Some horticultural oils are called summer oils or all-year oils because they can be applied when foliage is present; today, unlike in the past, summer oils have the same properties as narrow range oils. Dormant oil, now, simply refers to the time of application—not the property of the oil. Vegetable oil used as horticulture oil is derived from the seeds of some oil seed crop including soybeans, canola, and cottonseed.
Neem oil from the seeds of the neem tree ( Azadirachta indica ): It can be used as both an insecticide and a fungicide. Neem oils are effective at killing insect eggs and soft-bodied pests such as whiteflies and aphids. Neem oil also prevents powdery mildew and black spot on roses, fruit trees, and vegetables. Neem oil also kills fungi.
Fixed copper fungicide containing elemental copper, such as tribasic copper sulfate, copper oxychloride sulfate or cupric hydroxide: The metal ions in copper are antimicrobial materials; they kill the living cells of fungi, bacteria, and viruses by a chemical reaction that punches a whole in the microbial cell walls draining them of vital nutrients and water causing them to die.
Lime-sulfur is a mixture of hydrated lime (calcium hydroxide) and sulfur. Sulfur, not lime, is toxic to fungi spores; it disrupts the transfer of electrons in cellular proteins. Lime is added to the mix because it reduces the toxic effect of sulfur on plants. Lime-sulfur is the earliest synthetic chemical used in agriculture–first used in France to control pests and diseases on grapes in the 1850s. Lime sulfur spray will control fungi, bacteria, and insects. It can only be used on the bark of dormant plants, but it will burn the leaves of plants so it can’t be used on foliage or evergreens.
Synthetic fungicides containing chlorothalonil, daconil, iprodione, thiophanate methyl: These commercially manufactured organic compounds weaken molecules within fungi until they can no longer perform tasks necessary to keep the fungi alive.
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 Control 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.
Diseases Controlled Using Liquid Lime-Sulfur Spray:
- Anthracnose twig and leaf fungus on grapes, blueberries, gooseberries, blackberries, and raspberries.
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 new generation of pest. As well, many diseases overwinter in garden debris and weeds and become airborne when it’s windy or rainy in late winter, spring, and summer.