I'm a big believer in dormant spraying. I think it's one of the easiest and most effective treatments you can do all year. Not only does it set back bugs and fungi in the spring, it also reduces the need for spraying with harsh chemicals throughout the growing season.
There are two classes of dormant chemicals in common application. They are both very useful, but have quite different purposes and the timing of application is important. The first class of dormant sprays are dormant oils that are specifically for insect control. The second class are dormant season fungicides such as lime sulfur that are primarily for disease control.
Dormant oil spraying is one of the most effective insect management tools we use to control overwintering insect populations. These are essentially highly refined petroleum products that are formulated to spray on trees and shrubs. The term "dormant" refers to the timing of application - they need to be used while the tree is dormant and before bud break in the late winter to early spring. Also, if the temperatures are too high or below freezing the application may not be effective - a 24 hour period of temperatures in the 40s and 50s is ideal. There are three types of insects that may be controlled with dormant oil: scale, mites, and aphids. Even if you don't see any insects, they are there and waiting for spring when they will make their presence known. Dormant oils work by coating the insect and blocking the spiracles (a small opening used for breathing). Some plants can be damaged by these oils so make sure to read the list carefully on the label. It should be applied as a fine mist, so buy a good quality sprayer and coat the entire tree including the trunk. Pay particular attention to thick barked trees as insects like to hide in the bark during the dormant season. You'll want to coat all of the nooks and crannies.
Lime sulfur spray is used to control fungal and bacterial diseases such as leaf curl, fire blight, scab, and anthracnose. The time to control such diseases is in the early spring when the buds are swelling as fungi come out of dormancy at the same time the plants do. If you spray too early in the season, the spores will still be dormant and the lime sulfur will not be effective. When applying both a dormant oil and a fungicide, you want to be careful about the timing of when you spray. There needs to be a space of about four weeks between the time you apply a dormant oil to control insects and the time you apply a fungicide for disease control. Here in Houston, our winters are only about 6-8 weeks long, and that doesn't allow much time for dawdling. I usually apply a dormant oil in about middle January and apply lime sulfur when buds begin to swell between late February and early March. If the buds have already opened its too late as the fungus will have already entered the bud and the dormant spray will be quite ineffective at that point. As with dormant oil, you'll want to thoroughly coat the tree, getting in all of the nooks and crannies.
Reducing the overwintering population of pests and and diseases during the dormant season, means that you'll have far fewer problems in spring you'll need to try and control. That means healthier trees and less spraying of dangerous pesticides and fungicides during the growing season - a good thing - and a far lower impact on the beneficial insects we'd like to encourage in our garden. Remember that these are still chemicals, so use the appropriate protective gear for application and read the label thoroughly. But, by all means, use those early spring days to get a head start on spring pests and diseases.
Scott
