Dormant oils are specifically designed for use on trees during the dormant season. They have a heavier viscosity than other horticultural oils and they stay on the plant longer, as a result. This is great when you are trying to suffocate insect pests, but it can be devastating to foliage if temperatures rise, a condition called phytotoxicity. Summer or horticultural oils are lighter weight and evaporate more quickly, reducing the likelihood of burnt leaves.
Effectiveness of dormant oils
Dormant oil applications create barriers that block respiration of many, but not all, insects and limit the spread of infectious diseases. Spider mites, San Jose scale, leaf curl, powdery mildew, shot hole, and scab infestations are common reasons for applying dormant oil applications in winter or early spring. Pacific or two-spotted spider mites, peach twig borers, navel orangeworm, and oriental fruit moth are not affected by dormant oils alone. These oils are not recommended for aphid control.
Dormant oils can suffocate the eggs of many garden pests and deter females from laying eggs on specific trees. As a side benefit, the application of dormant oils has been shown to delay bud break, protecting tender new growth against potential frost damage. It has also been found to enhance the aroma of ‘Golden Delicious’ apples and increases the diameter of oranges.
How to use dormant oils
Dormant oil application should be done after pruning and is most effective after a foggy or drizzly day. Pump sprayers can be used to apply dormant oils, but it is important that every surface is sprayed: trunk, stems, twigs, and tops and bottoms of any leaves. If you have several trees, this can be a bit of work.
Problems associated with dormant oils
One of the biggest problems associated with dormant oil is the risk of hot weather. A single sunny day can transform fresh, young, green leaves into cinders. This is especially true up to 30 days after sulfur or other fungicides have been applied. [I speak from personal experience] Also, heavy dormant oils can harm beneficial insects right along with the pests. Dormant oils are not recommended for use on walnut trees.
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