Copper fungicide is a protective treatment used for managing various diseases in stone fruits, including peaches. It works by creating a barrier on the plant tissue, preventing fungal and bacterial spores from germinating and causing infection. Effective disease control relies on precise timing, as the product must be applied before the pathogen enters the plant. Knowing when to apply this protective layer is the most important factor in a successful disease management program.
The Essential Dormant Application
The most impactful copper fungicide application occurs during the dormant season, targeting fungi and bacteria that survive winter in bark crevices and dormant buds. This timing is important for preventing Peach Leaf Curl, caused by the fungus Taphrina deformans. The fungus infects the tree as new leaves emerge in early spring.
Best practice involves two separate applications during the dormant period. The first should happen in late fall, typically after 90% of the leaves have dropped. This initial spray eliminates overwintering inoculum before winter rains begin.
The second application is performed in late winter or early spring, just before the buds begin to swell. This pre-bud swell application establishes a final protective layer to intercept remaining fungal spores before the bud scales open. Once buds show any sign of green tissue or color, the fungus has already entered the plant, and copper application will be ineffective for preventing Leaf Curl.
Application must occur when conditions permit the spray to dry completely on the bark. Ideal weather includes temperatures above freezing (40°F or higher) with no rain expected for at least 24 hours. Applying the spray during wet conditions or immediately before a freeze reduces effectiveness and can damage the tree’s tissue.
Timing Applications During the Growing Season
Once the tree breaks dormancy, copper fungicide applications shift to managing secondary diseases, requiring caution due to developing foliage. An effective time for a post-dormancy spray is the Pink Bud Stage, just before the flower petals open. This application helps protect blossoms from Brown Rot (Monilinia fructicola), a fungal disease that can destroy flowers and spread to the fruit.
A subsequent timing is the Shuck Split Stage, when the tiny fruit begins to push through the remnants of the flower. This is a period for controlling bacterial diseases, such as Bacterial Spot (Xanthomonas arboricola pv. pruni). Applying a fixed copper bactericide at this stage and for a few weeks following can reduce infection and fruit loss.
Summer use of copper is avoided because peach foliage is susceptible to phytotoxicity, or chemical burn. The risk of damaging leaves and fruit increases when temperatures are high or when copper is applied to wet foliage. Therefore, most copper applications cease once the tree is fully leafed out, and alternative fungicides are used for mid-summer disease management.
A final application can be considered post-harvest, after all fruit has been picked but before the leaves drop. Spraying at this time reduces the number of fungal spores present on the tree going into winter. This targeted spray reduces initial inoculum, strengthening the effectiveness of the subsequent dormant season treatment.
Safe Mixing and Application Practices
The effectiveness of copper fungicide relies on proper preparation and thorough coverage. The spray concentration must strictly follow the product label, as exceeding the recommended dilution rate increases the risk of phytotoxicity. Some fixed copper formulations, like Bordeaux mix, require adding lime to “fix” the copper, making it less soluble and safer for the plant tissue.
When mixing the solution, use water that is not overly acidic, as a low pH can dissolve too much copper, leading to leaf burn. Adding a spreader-sticker agent is recommended to help the fungicide adhere better to the bark and buds, improving residual activity, especially in rainy climates. Copper acts only as a protectant and does not redistribute once it dries.
The application technique requires spraying the entire tree until the solution begins to drip, known as spray to run-off. Every surface of the trunk, branches, and twigs must be coated to ensure no overwintering pathogen is left exposed. Since copper is a heavy metal, wear appropriate personal protective equipment (PPE), including gloves, long sleeves, eye protection, and a mask, to avoid contact or inhalation.