Copper compounds are traditional, broad-spectrum fungicides used widely in viticulture, the cultivation of grapevines. They have been utilized for centuries, providing a reliable method for managing various plant pathogens. Their effectiveness stems from their multi-site mode of action, which makes it unlikely for pathogens to develop resistance. Successful disease management relies on the precise timing of these protective applications, determined by the grapevine’s growth stage and environmental conditions.
Identifying the Need: Diseases Copper Controls
Copper is primarily employed in vineyards because it creates a protective barrier on the plant surface, preventing fungal and bacterial spores from germinating and infecting the tissue. It functions strictly as a preventative measure, offering no curative properties once an infection has taken hold inside the plant. Copper is particularly effective against Downy Mildew (Plasmopara viticola), which is one of the most destructive diseases in warm, humid climates. If left unchecked, Downy Mildew can rapidly defoliate the vine and compromise the crop’s ability to survive the winter.
Copper also provides control of Black Rot (Guignardia bidwellii), a fungal disease that damages leaves and fruit clusters. While its efficacy against Black Rot is moderate compared to other diseases, it remains a component in integrated pest management programs. Copper also has activity against Phomopsis Cane and Leaf Spot (Phomopsis viticola), a pathogen that causes lesions on canes and leaves early in the season. Copper is considered to have fair activity against Powdery Mildew, but should not be the sole control method for that pathogen.
Phenological Timing: When to Apply Copper
The application of copper must be synchronized with the grapevine’s phenology, or growth stages, to ensure new, unprotected tissue is covered before infection occurs. Timing often starts with the delayed dormant application, an optional spray occurring just before bud break. This spray is primarily used to reduce the overwintering inoculum of diseases like Phomopsis.
The first critical window for copper application is pre-bloom, typically when new shoots are approximately five inches long. Spraying at this stage protects the rapidly expanding leaves and flower clusters from primary infection of Downy Mildew. Early applications lay the foundation for seasonal disease control, especially in years with high disease pressure.
The period from immediate pre-bloom through two to four weeks after bloom is considered the most important time for fungicide application. Copper sprays should be repeated immediately post-bloom, following fruit set, to protect the newly formed berries and maintain coverage on the expanding canopy. The young fruit is particularly susceptible to infection during this stage.
Subsequent applications, known as cover sprays, are applied every seven to fourteen days, depending on local weather conditions and disease pressure. High rainfall or fast canopy growth will wash the protective copper barrier off or expose new tissue, necessitating more frequent reapplication. Maintaining coverage during periods of rapid growth ensures the vine’s new leaves and developing clusters remain protected from secondary infections.
Copper Formulations and Application Guidelines
Copper compounds exist in various chemical forms, each with different properties that affect handling and application. Bordeaux mixture, one of the oldest formulations, is created by combining copper sulfate with hydrated lime and water. The lime acts as a safener by “fixing” the copper ions, which reduces the potential for phytotoxicity (plant injury) caused by free copper ions.
Modern “fixed coppers,” such as copper hydroxide or copper oxychloride, are pre-formulated and release copper ions slowly, offering a reduced risk of leaf burn compared to copper sulfate alone. Copper must be applied under good drying conditions, as slow drying (high humidity or cool temperatures) can increase the likelihood of phytotoxicity. Adding hydrated lime to the spray solution can decrease the severity of potential foliar injury, especially on copper-sensitive grape varieties.
Applicators must adhere to the product label’s specified rates to prevent damage to the foliage and fruit. Exceeding recommended rates can cause leaf burn (phytotoxicity), which stresses the vine and reduces harvest quality. Personal Protective Equipment (PPE) is necessary, as copper compounds can be corrosive and cause injury if mishandled. Due to copper’s long-term accumulation in soil, which can negatively impact soil organisms, many regions have established seasonal limits on the total amount of metallic copper applied per hectare.