During cold winter weather, one doesn’t often think about spraying fruit trees and ornamental shrubs for spring insects and diseases. It’s just not on the radar, but it turns out that January and February are the best time to apply dormant sprays to combat insect and disease issues. Many ask, “What are dormant sprays” ?
Dormant sprays act on insects or disease pathogens differently. Many insects overwinter on trees and shrubs, either as eggs or immobilized in a protective shell (scale insects). Horticultural oils applied during cool dormant conditions work by smothering the eggs of some insect species or encapsulated scale insects. Since they cannot breathe, they die.
On the other hand, dormant sprays containing copper or sulfur actually kill latent fungal spores that are ready to infect the moment weather warms. They also burn tender young plant tissue, so can only be used when the plant is not actively growing. These preventative sprays can delay disease incidence in early spring and allow for reduction or elimination of regular fungicide applications. The old adage “an ounce of prevention is worth a pound of cure” surely applies to these situations.
There are several products on the market for dormant applications.
Dormant oil is a type of horticultural oil, made of refined petroleum products, for application on trees or shrubs when the trees are not actively growing. It has been in use for over 100 years. They are effective in the suppression of scale insects and mites. Care must be used to not apply them when daytime temperatures are above 75 degrees or night temperatures below 28 degrees. Other horticultural oils exist that may be applied during the growing season to control soft-bodied insects, but not during extremely hot weather. Many different brands exist, some are certified organic. They can be purchased at most garden centers, but the best selection is usually found at your independent nursery or farm store.
Dormant fungicides can be classified into two groups. Those that contain copper and those that contain sulfur. The most common preventative remedy for fungal disease had been lime-sulfur. It is no longer available in small home garden quantities due to shipping container restrictions, it can be purchased online in larger quantities for use in Florida. When applied to dormant plants, lime sulfur actually works by sanitizing the stem, killing all fungal spores. It cannot be used during the growing season since it burns leafy tissue. Caution must be taken when mixing and loading since, being an acidic product, can burn the skin. Wear chemical resistant gloves when applying (bought at your local hardware store for $4.00-$10.00), safety goggles and follow all label directions carefully since it is caustic and labeled DANGER. Also, never apply lime-sulfur within one month of horticultural oil applications. It should be applied in early to mid February, avoiding hard freezes for the 24 hours around application time. Sulfur based fungicide sprays may also be used instead of Lime-Sulfur as a dormant application.
Dormant copper sprays are effective on both bacterial and fungal pathogens and used primarily on fruit crops for the suppression of many fruit diseases including fire blight, bacterial leaf spot, powdery mildew, downey mildew and anthracnose. There are several different brands of copper fungicide preparations on the market, most nurseries and garden supply centers will have some in stock. Always read the label for proper personal protective equipment and dosage rates, to avoid copper buildup in the soil over time.
For more information, contact your local extension agent or consult these extension publications from: Disease Management Strategies, Florida Blueberry Disease Guide and Pest Control Using Horticultural Oils .
This article was originally published on January 2016, and has been modified to reflect current information.