Now is the time of year when many home orchardists start a winter spray program to control summer fruit tree pests and diseases. Instead of immediately reaching for a chemical solution, here are some recommendations from the University of California Statewide Integrated Pest Management program, http://www.ipm.ucdavis.edu
Prevention: Prevention is the first step in controlling diseases such as peach leaf curl in home orchards. Many problems can be avoided by choosing resistant fruit tree varieties and providing them with proper care. That care includes removing all dropped fruit and leaves that might be harboring pests. Proper timing and placement of pruning cuts can also help remove or control disease.
Resistant Varieties: Apricots are immune to peach leaf curl and a few peach varieties are available that are resistant or partially resistant to leaf curl. Currently available resistant varieties include Frost, Indian Free, Muir, and Q-1-8. The peach cultivar Frost is reportedly very disease-tolerant but must receive fungicide applications the first 2 to 3 years. Redhaven peach and most cultivars derived from it are tolerant to peach leaf curl, meaning the disease can be present but damage is limited.
Pruning Management: Although symptoms of leaf curl are seen primarily in spring as new leaves develop, there is little you can do to control the disease at this time. Some people remove diseased leaves or prune infected shoots, but this has not been shown to improve control. Normally, diseased leaves fall off within a few weeks and are replaced by new, healthy leaves.
If a tree is severely affected with peach leaf curl this can stunt its growth, so consider thinning fruit later in the season. Pruning in fall prior to applying any fungicides can reduce spore numbers overwintering on the tree and reduce the amount of fungicide needed. If leaf curl symptoms occurred on your trees in spring, be sure to treat the following fall and/or winter to prevent more serious losses the following year.
Chemical Treatment: Historically, the most commonly used fungicides available to home gardeners have been fixed copper products. For all copper-containing products, the active ingredient, copper, is listed as “metallic copper equivalent,” or MCE, on the label. Various product formulations differ widely in their metallic copper content. The higher the MCE, the greater the amount of copper and the more effective the product will be. Read labels carefully and follow directions exactly.
Currently only liquid products containing copper ammonium complex products with 8% MCE (e.g., Kop R Spray Concentrate [Lilly Miller brands] and Liqui-Cop [Monterey Lawn and Garden]) are available to consumers. The copper ammonium complex products can be made more effective by adding 1% horticultural spray oil to the application mix; the oil also aids in controlling some aphids, scale insects, and mites.
Be aware that repeated annual use of copper products over many seasons can result in a buildup of copper in the soil, which eventually can become toxic to soil organisms, and if it moves into waterways, can harm some aquatic species.
Please contact your local county U.C. Cooperative Extension office for gardening and agriculture advice. This article adapted from the UC Integrated Pest Management program.