Copper is an effective tool for managing bacterial spot and bacterial canker on stone fruits. These diseases, caused by Xanthomonas arboricola and Pseudomonas syringae organisms, overwinter in trees and can build up quickly in orchards. Applying the proper rate of copper at the right time can prevent disease establishment, reduce pathogen populations and prevent injury to the tree itself.

Dr. Kari Peter, Penn State Fruit Research and Extension Center, recently explained the methodology behind the use of copper sprays during the New England Winter Fruit Seminar Series. “All copper products, to a degree, can cause injury on peach and nectarine trees,” Peter said. The danger is phytotoxicity, which causes defoliation.

How Copper Works

Copper is a general biocide and non-selectively kills cells, including plant cells. It has to be carefully used in adequate amounts to kill microbes, but not damage plants. Copper products come in soluble and insoluble forms, and the percentage of metallic copper – which is the number growers need to know – varies dramatically across products.

Copper works in conjunction with moisture. When water and copper combine, copper ions are released. The ions are the active compounds which kill cells. Copper is most effective on diseases which thrive when free water is present, Peter said. The ions destroy critical enzymes that are important for cell functions.

Because copper “sticks where it hits,” the ions, once released, will kill any cells they latch onto, including plant cells.

In soluble copper products, the copper ions are released together en masse when water is present. In Bluestone (copper sulfate pentahydrate), all of the copper ions are ready to go at once, because it is a 100% soluble product. Any residue from soluble copper is rapidly removed so there is no residual effect.

But mixing soluble copper with lime (Bordeaux mixtures) will change it into a fixed copper. The lime limits the release of copper ions so that they’re not all activated at once.

Insoluble or low-soluble copper formulations (known also as fixed coppers) release ions gradually. When insoluble sprays are used, a suspension of copper particles is dispersed. When wetting occurs, ions are released. If re-applied, layers of ions will be available to the plant and released when moisture is present.

“With each event of water being applied to the leaf, you’ll have more copper ions present,” offering residual protection, Peter said.

The insoluble or fixed copper is safer, as the ions are released in lower concentrations, decreasing leaf damage and the danger of phytotoxicity. Fixed coppers do have different water solubility. The greater the solubility, the more ions released when water is available. Copper hydroxide is more soluble than copper sulfite, for example.

It is the percentage of metallic copper that is important, no matter the form of the product – granular, powder or aqueous solution. The percentage of metallic copper per unit of product determines efficacy. Particle size can also be a factor, as larger particles are more easily dispersed after the surface dries and can result in loss of residual activity as well less uniform coverage. When using soluble copper, there are not particles, just copper ions.

Acidity impacts copper efficacy, and more acidic pH levels will increase phytotoxicity. Adjuvants can change the pH of a mixture, as can water pH. Peter cautioned against mixing copper products with foliar fertilizer sprays due to acidity and the ability of the fertilizer to pull copper into plant cells, increasing the risk of injury. Testing the pH of copper spray solutions is recommended, and the pH of copper products themselves can vary.

After application of copper, slow drying conditions can cause copper to remain active. If too much is re-applied, older leaves can become impacted from copper damage, while younger leaves will still be uninjured. Thus, in really wet seasons, copper may not be recommended.

Bacterial Spot & Bacterial Canker

Timing copper applications to the lifecycle of the disease-causing pathogen is needed.

Blossom blast and bacterial canker are both caused by P. syringae strains. These are cold weather diseases, and factors including nutritional deficiency, winter pruning, spring freeze, severe winter freeze or high ring nematode populations can contribute to disease severity, Peter said. The bacteria overwinter in plant cells and cause the cells to freeze at a higher temperature than they otherwise would. The bacteria feed off the nutrients released by injured plant cells.

Effective copper treatment occurs the day prior to a projected spring frost or freeze when trees are in bloom. Because the copper application will not have prolonged residual effects, the protectant must be on the blossoms immediately prior to conditions being conducive for bacteria to activate.

Bacterial canker can infect the fruit, leaves and the entire tree. On fruit, sporadic water-soaked lesions or chocolate-colored lesions occur. Necrotic lesions with chlorotic rings can give leaves a “shot gun” appearance. If it’s in the tree, amber gummosis, sunken bark and girdled branches and trunks occur.

Bacterial canker infection can be managed by reducing the number of pathogens in the trees before the active period of the bacteria. Copper can’t do this itself, but in conjunction with vegetable oil, it can be used effectively. The oil will help penetrate into the plant cells, taking the copper with it, which is what is needed for control, Peter said.

The Bordeaux mix plus vegetable oil should be applied in succession, with increasing amounts of copper at each application, from September – November, as the pathogen population increases as cooler weather arrives. It should be applied again in spring, at a high rate, when conditions favor disease emergence.

Bacterial spot is a warm weather disease, preferring hot and humid conditions, with the ideal temperature of 86º F. If there is 100% humidity for three days, the conditions are optimal for bacterial spot, Peter said.

Different cultivars have various susceptibilities. Those most susceptible to bacterial spot have early season lesions which occur about three weeks after petal fall. Less susceptible varieties have fruit with shallow pits and skin cracks, which appear late in the season, and the fruit from these trees can still be saleable, depending on your market.

Bacterial spot pathogen populations will overwinter. A dormant copper spray at the rate of two pounds of metallic copper per acre is recommended. Removal of dead wood and cankers is needed. Seasonal applications of 0.5% metallic copper, or 1% metallic copper, starting at shuck split and repeating at 10-day intervals until three weeks prior to harvest is effective. Copper should be rotated with other products. If a crop is lost due to spring freeze, treatment is still needed to prevent pathogen populations from growing.

Phytotoxicity from copper usage can look similar to bacterial spot, and growers can confuse the two and exacerbate the problem. To differentiate, Peter offered comparisons. Copper damage “looks like Swiss cheese” on the leaves, which remain on the branch. Bacterial spot causes angular lesions, always bordered by a vein, and yellowing around the lesion. Defoliation occurs even with a low number of lesions present. Copper damage is a concern, but copper is an effective tool when used properly.