Clearing the Cobwebs: Copper Use for Grape and Stone Fruit Diseases

Katie Goldenhar, OMAFRA Plant Pathologist – Horticulture & Wendy McFadden-Smith, Tender Fruit and Grape IPM Specialist

Do you really know how copper fungicides work? With so many on the market, if you’re considering using a copper product in your disease management program, there are some important factors to consider.

Copper is one of the original fungicides and yet there are constantly new copper products coming to the market. Copper is an inorganic compound that does not break down like organic compounds and therefore too much copper fungicide use can lead to build up in the soil, negatively impacting soil health, so judicious use is required.

Copper is a general, non-selective biocide, meaning it works as a bactericide, fungicide and when used incorrectly, herbicide. When copper particles degrade in water, they release ions that inhibit critical enzymes in not just pathogen cells, but also plant ones. Hence, the use of any copper product will come with cautions to avoid phytotoxicity. Copper products are one of the only crop protection materials that can help manage bacterial diseases such as bacterial spot in peaches. Copper can also provide some efficacy against certain fungal and oomycete diseases such as peach leaf curl and downy mildew.

Fixed or insoluble copper products contain copper that releases ions at slower rates that continue after application when there are wetting events. Particles can persist on the leaf after drying and continually release ions when there is moisture present. Not all fixed coppers have the same level of insolubility. For example, copper hydroxide (Kocide 2000-O, Parasol WG) is more soluble in water than basic copper sulfate (Copper 53W), while copper oxychloride (Copper Spray, Cobranza, Guardsman Copper) falls between the two. Copper octanoate (Cueva) is more soluble than fixed coppers, but its concentration of copper is significantly lower than other copper products.  Typically, there is longer residual control with fixed coppers. The challenge is that there needs to be enough ions present to kill the target pathogen without injuring the crop. Generally, fixed copper products reduce the chances of phytotoxicity since not all the ions are present at once.  If the water used 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. Adding hydrated lime can make any copper product less soluble but some coppers are not compatible with lime, so always consult the product label.

An important factor to consider when using copper is that copper does not move within a plant – it stays where it lands and has no post-infection activity. Spray coverage and preventative applications are important when applying a copper product. Copper particle size is another factor influencing efficacy, primarily determined by how finely the product is ground. Large particles will easily be removed by wind or rain after application has dried whereas small particles will provide better coverage of the leaf, adhere to plant surface, and provide longer residual control.

In Canada, the metallic copper content is present on the label as the percent available elemental copper. Table 1 shows some copper products registered in Canada and their corresponding copper content. If you want to compare the amount of copper being applied in each product, multiply the metallic copper content by the rate per hectare. For example, in peaches, the max rate of Copper 53W is 2kg/ha so multiplied by the 53% metallic copper content means that there is 1.06 kg of metallic copper per hectare being delivered.  

Table 1. Copper products registered in peaches and their corresponding metallic copper content. Always consult the product label before use.

Product	PCP#	active ingredient	metallic copper	Bee toxicity rank1
Copper 53W	9934	basic copper sulphate	53%	III
Guardsman Copper Oxychloride 50	13245	copper oxychloride	50%	II
Copper Spray	19146	copper oxychloride	50%	II
Parasol Flowable	25901	copper hydroxide	24.4%	II
Cueva Commercial	31825	copper octanoate	1.8%	III

¹from University of California – Ranking of pesticides according to whether (I) the product should not be applied to flowers, (II) the product should not be applied to flowers except between sunset and midnight, or (III) no bee precaution except what is listed on label.  https://ipm.ucanr.edu/bee-precaution-pesticide-ratings/

Copper fungicides belong to the FRAC group M1. The “M” stands for multi-site and is thought to be at low risk for resistance development. This is the case for fungal pathogens. However, there are cases of copper resistance in bacterial pathogens, like Erwinia amylovora, the fire blight pathogen, developing over years when frequent and repeatedapplications of copper were used. Fungicide resistance management guidelines should be used to maintain economic control for the future.

Other factors to consider include:

• pH  – generally, the lower the pH, the more soluble copper becomes, which increases the chance of phytotoxicity
• tank mixing (compatibility and phytotoxicity) – it is well known that foliar fertilizers and phosphorous acid products cannot be used with copper.  Always check the label of all products in a tank mix for compatibility issues.
• weather factors – slow drying will increase the chance of phytotoxicity and heavy rain may reduce residue
• application rate and frequency

The more questions you ask the better, so reach out to your OMAFA specialist, agronomist, or copper fungicide supplier for more information on disease management using copper.

References

Peter, K. (2023). Optimizing copper and biologicals for bacterial spot in peach. Ontario Fruit and Vegetable Convention, February 23. www.ofvc.ca

Shane, B. & Sundin, G. (2011). Copper formulations for fruit crops. https://www.canr.msu.edu/news/copper_formulations_for_fruit_crops