Copper is a potent spray chemical, useful on many stone and pome fruits. It is active against bacterial diseases like fireblight, bacterial canker, and bacterial spot, and fungal diseases like cherry leaf spot, peach leaf curl, and apple scab as well.
Used correctly, copper sprays, especially very early in the season before plant growth starts, prevent overwintering disease inocula from infecting new tissue and set the stage for lower disease pressure when life resumes in spring.
But its ability to kill is not limited to disease organisms. It can damage plant tissue, russet fruit, suppress earthworms, and accumulate in the soil. It is toxic to aquatic organisms and some higher animals, especially sheep, but humans have a high tolerance to it.
Copper is highly reactive and comes in many forms—oxides, hydroxides, sulfates, and others.
“There are at least 37 labels for coppers that can be used in agriculture in New York State,” Dr. David Rosenberger told growers during a presentation at the Great Lakes Fruit, Vegetable, and Farm Market Expo last December. Rosenberger is tree fruit pathologist at Cornell University, working at its Hudson Valley Laboratory in Highland, New York.
He believes copper compounds have a place in the disease control program, but growers have to learn how copper performs best for them. It can be used at high rates early in the season and at low rates throughout the season. It is one of the few consistently effective fungicides organic growers can use for disease control.
“Copper can poison both the crop and the target pathogen,” he said. “For many crops, one must balance the benefits of maximizing efficacy versus the risks of phytotoxicity.”
Read the label
Because different formulations have different properties when used as spray materials, growers need to learn how to read and interpret labels.
“The effectiveness of copper sprays is highly correlated with the amount of elemental copper that is applied,” Rosenberger said. “The metallic copper content varies widely by product.”
Potency also varies by how the product is prepared. Finely ground copper products are more active than coarsely ground ones.
“Copper sprays control plant pathogens because copper ions denature proteins, thereby destroying enzymes that are critical for cell functioning,” Rosenberger said. “Copper can kill pathogen cells on plant surfaces, but once a pathogen enters host tissue, it will no longer be susceptible to copper treatments. Thus, copper sprays act as protectant fungicide/bactericide treatments, but lack postinfection activity.”
One way to avoid plant injury is to limit the copper ion concentration on plant surfaces by using copper products that are relatively insoluble in water. “Because of its high solubility in water, copper sulfate can cause phytotoxicity even at relatively low application rates,” he said. The high solubility also means that copper sulfate residues can be rapidly removed by rainfall, he added.
“Copper products registered for tree fruits are almost all fixed coppers that have low solubility in water,” Rosenberger said. “When mixed with water, the spray solution is actually a suspension of copper particles, and those particles persist on plant surfaces after the spray dries. Copper ions are gradually released from these copper deposits each time the plant surface becomes wet. The gradual release of copper ions from the copper deposits provides residual protection against plant pathogens. At the same time, the slow release of copper ions from these relatively insoluble copper deposits reduces risks of phytotoxicity to plant tissues.”
Fixed coppers include basic copper sulfate (e.g., Cuprofix Ultra Disperss), copper oxide (e.g., Nordox), copper hydroxide (e.g., Kocide, Champ), copper oxychloride sulfate (e.g., COCS), and copper ions linked to fatty acids or other organic molecules (e.g., TennCop, Cueva).
In spray guides, the rate per acre for application of copper compounds varies widely. The Midwest Spray Guide advises using copper before bud swell to prevent peach leaf curl. Recommended rates are 8 to 16 pounds for copper hydroxide (Kocide) or copper oxychloride (COCS), which would translate to 2.5 to 5 pounds of elemental copper. Kocide 3000 is 46 percent copper hydroxide and has 0.3 pounds of elemental copper per pound. Kocide 2000 is 53 percent copper hydroxide and contains more copper. For cherry leaf spot, Kocide 2000 at 5 pounds per acre is recommended at first and third cover. No rate is specified for use of copper hydroxide, copper oxychloride, basic copper sulfate, or Bordeaux mixture on apples for suppression of fireblight, but the timing should be before growth starts.
When used on stone fruits, Kocide 2000 is labeled at 6 to 12 pounds per acre with a maximum use rate of 51.4 pounds per season. When high rates are used year after year, copper can accumulate to high levels in orchard and vineyard soil.
Coarse or fine?
The finely ground coppers may be preferable for delayed dormant and dormant applications on apples and for fall and spring applications on cherries where the objective is to control bacterial canker, Rosenberger said.
Most copper labels list a broad range of rates. In general, the upper end of labeled rates is suggested for applications that are made in spring or fall on cherries or for applications made at silver tip or green tip on apples. The lower labeled rates may be more appropriate for spring applications if spring weather has been delayed and one might therefore expect trees to advance rapidly from bud break to bloom, he said.
Using excessive rates of copper, especially finely ground coppers that have good residual properties, could result in fruit russetting on some apple cultivars if copper ions are splash-dispersed to developing fruit tissue after flowers reach pink or bloom.
Copper products such as Cueva and MasterCop contain very low concentrations of elemental copper, Rosenberger said. These products are better suited for applications during summer (e.g., for bacterial leaf spot on peaches or cherry leaf spot on tart cherries) when repeated applications with low rates of copper are desired to minimize phytotoxicity.