Horticultural oils are an effective method of controlling insect and disease problems, with a low level of toxicity to humans, but should always be used with a good understanding of their benefits and limitations.
Some species of landscape plants are sensitive to oil products and may develop damage after the application of oil products. These include azalea, beech, black walnut, Douglas fir, hickory, Japanese holly, juniper, maple (particularly Japanese and red), redbud, smoketree, and spruce (particularly dwarf Alberta).
Phytotoxic symptoms of oil damage may appear as marginal leaf burning or a general yellowing of the foliage. Leaves of sensitive plants may also develop a water-soaked appearance, then turn purple and drop from the plant. Flowers may appear spotted, or as in the case of begonia and coleus, the leaves may be pitted or speckled.
In the case of blue spruces, oils remove the waxy coating that gives the foliage its blue appearance resulting in a green tree, instead of blue. The tree will eventually put out new foliage with the bluish color, but several years will be required for the tree to regain its full original color.
To get the best results from a horticultural oil application, follow these precautions.
- First, read and follow all label directions, paying particular attention to which plants are listed on the label as sensitive to the product. Do not apply oils to sensitive plants or allow the product to drift onto them.
- Do not combine oil applications with sulfur or pesticides containing sulfur like Captan. Sulfur reacts with the oil to form phytotoxic products. Because elemental sulfur can persist for long periods, label directions on most oils prohibit their use within 30 days of a sulfur application.
- Damage to plant tissue usually occurs when the oil does not have a chance to evaporate in a timely manner. Therefore, do not apply oil when humidity is expected to remain above 90% for a period of 36 or more hours. Do not apply just before rain or when the leaves are wet.
- Do not apply oils to quickly expanding shoots in spring.
- Do not apply oil when temperatures are above 100 degrees. Plants under drought stress may have increased risk of injury
- Thorough spray coverage of plants is essential to effective control since oils have no residual effect.
- Finally, allow plants to harden off well in fall before making dormant oil applications; this avoids increased susceptibility to winterkill.
- Do not apply dormant oil if a heavy freeze is expected. The oil product may break down under freezing temperatures and uneven coverage or damage to the tree could occur. Oil sprays must have ample time to dry before freezing weather- usually at least 10-12 hours.
Finally, oils also are useful against the fungal disease powdery mildew. Diluted horticultural oils, often mixed with a small amount of baking soda, can be an effective control for this common plant disease. Mix 1 heaping teaspoon baking soda, 1 tablespoon dormant oil and 1/2 teaspoon insecticidal soap or dish soap. (In this instance the insecticidal soap or dish soap is used as a surfactant.) Mix these products well in one gallon of water. Spray on plants for powdery mildew control, making sure the plants are well hydrated before spraying.
Sarah Browning is a Horticulture Extension Educator with University of Nebraska- Lincoln Extension in Dodge and Saunders Counties. She can be contacted by phone at 727-2775: by mail at 1206 W. 23rd Street, Fremont, NE 68025: or by e-mail at sbrowning2@unl.edu
