Dormant sprays are horticultural oils and some insecticides and fungicides that are sprayed on dormant fruit trees during the late winter or early spring. Some dormant sprays suppress overwintering fungal diseases; others smother the eggs of the coddling moth and kill aphids, mites and scale insects.
Dormant oil is also called insecticidal oil, narrow-range oil and superior oil. It kills soft-bodied insects including aphids, mites, scales and thrips. It also smothers the eggs of the coddling moth and other insects. It must have contact with the eggs or insects, and it is not effective after it dries. It may be applied on dormant plants or on foliage when plants are not dormant. It can be mixed with lime sulfur when plants are dormant.
Applying Dormant Oil
Apply dormant oils with a compressed air or pump sprayers. To prevent burning of tissue, water trees well before applying. Do not spray if cold north winds or rain is expected. A clear day with no breeze is best. Apply at temperatures between 40 and 70 degrees F. The temperature should remain higher than 50 degrees F for at least 24 hours so the oil can spread and cover all crevices and crooks of a tree.
When to Apply
Apply dormant oils when all of the leaves have fallen in late November or early December and again in early February.Cover the entire tree with a layer of oil. When dormant oil is mixed with lime sulfur it should be shaken, not stirred, a horticultural variation of James Bond’s vodka martinis.
Copper Compounds Plus Sulfates
Copper is a trace element that makes fruit sweet. Copper and sulfur in combination is used as a dormant spray or in the spring to control a variety of bacteria and fungi including leaf blight, shot hole and peach leaf curl. It must be applied before the fungi have infected a tree.
The Bordeaux mixture, a combination of copper sulphate and hydrated lime, originated in the Bordeaux vineyards in France more than a century ago and is still useful as a dormant spray. The University of California, Davis, offers instructions on how to make the Bordeaux mixture (see Resources below).
Copper sulfate is approved for use for certified organic agriculture. Combinations of copper and sulfur can damage eyes and cause skin irritation. It is highly toxic to fish.
Fixed copper fungicides (copper oxychlonde sulfate, cupric hydroxide, tribasic copper sulfate) have some of the same ability to control diseases as the Bordeaux mixture. They can be used as a dormant spray, but they are less able to withstand winter rains. They should not be mixed with oil and are best used after leaves have broken in the spring. Mixing fixed coppers with other pesticides can injure a plant.
Sulfur is applied on dormant trees to control brown rot, powdery mildew, rust and other fungal diseases. It also helps suppress spider mites. Unless the plant is dormant, do not combine sulfur with dormant oil or within 30 days after treating with oil. It must be applied before the fungi have infected a tree.
Lime sulfur is mixed with dormant oil to control apple scab, powdery mildew and other fungal diseases. It will control mites and other overwintering insects. Lime sulfur can damage trees if it is applied after the buds break in the spring or before Nov. 15 in most growing areas.