You may be considering a copper spray to control or prevent certain diseases, particularly bacterial diseases in your crops.  Here's a quick review of how copper controls pathogens.   Copper is usually applied in the "fixed form" which lowers its solubility in water. 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., Cueva). 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. The slow release of copper ions from these relatively insoluble copper deposits reduces risks of ­phytotoxicity to plant tissues.  Copper ions denature proteins, thereby destroying enzymes that are critical for cell functioning. Copper can kill pathogen cells on plant surfaces, but once a pathogen enters host tissue, it will no longer be susceptible to copper treatments.  A copper spray acts as a protectant fungicide/bactericide treatment, but lacks post-infection activity.

Because copper products come in different formulations and have different properties, it is important to read all the information on the labels. Besides rates, you will want to know about compatibility with other pesticides, adjuvants, and fertilizers.  Many growers are tank mixing biological fungicides and plant activators with coppers, while many are compatible, some are not, so make sure to check both labels for compatibility or call the manufacture/distributer for technical assistance.

The effectiveness of copper sprays has been correlated with the amount of elemental copper applied. 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. Professor Emeritus Tom Zitter of Cornell University suggests that for vegetable crops "Begin by choosing a copper product with at least 20% or more copper as the active ingredient to insure the greatest release of copper ions".

There are several suggestions for avoiding phytotoxicity (plant injury) with copper sprays. Limit the copper ion concentration on plant surfaces by using copper products that are relatively insoluble in water, i.e. fixed copper.  Copper can accumulate to high levels on plant tissue when sprayed repeatedly to cover new growth and there is no rain.  In this situation, after a rain event, a large amount of copper ions may be released leading to phytotoxicity. Check the pH of your water source. Solubility of fixed coppers increases under acidic conditions. Copper sprays will become more phytotoxic if they are applied in an acidic solution. Most copper products are formulated to be almost insoluble in water at pH 7.0. As the pH of water decreases the solubility of the copper fungicides increases and more copper ions are released. If the water /solution in spray tank is too acidic (below pH 6.0-7.0, depending on the copper formulation) excessive amounts of copper ions could be produced which may cause damage to fruit and foliage. Formulations vary in solubility — hydroxides are more soluble than oxychlorides which are more soluble than tribasic copper sulphates and cuprous. Less soluble formulations are usually more persistent. Copper sprays generally cause more phytotoxicity when applied under slow drying conditions, such as when it's wet and cool.   Always read the label and follow copper tank mix partner label instructions.

For a comprehensive list of Copper Products Used for Vegetable Disease Control see:

http://vegetablemdonline.ppath.cornell.edu/NewsArticles/CopperFungicides2012.pdf

and for specific information on copper fungicides in organic disease management see:

http://vegetablemdonline.ppath...

Sources:  Dr. T. A. Zitter, Cornell University and  Dr. David A. Rosenberger, Cornell University

This article was published in the June 7th 2018, ENYCHP Vegetable News.  Click here to view the full newsletter.