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 101

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 breakdown like organic compounds. Therefore, too much copper fungicide use can lead to build up in the soil, negatively impacting soil health. Judicial 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 cells. Hence, the use of any copper product will come with cautions to avoid phytotoxicity. Copper products are the most effective on pathogens that need free water to infect the plant (like bacteria) and is one of the only crop protection materials that can help manage bacterial diseases such as fire blight and blister spot in apples (Figure 1). Copper can also provide some efficacy against certain fungal diseases such as apple scab.

Soluble vs Fixed Copper

There are two main types of copper products: soluble and fixed.

Soluble copper products have copper ions available in solution and are all available when sprayed. Residue is quicky removed from overhead irrigation or rain. Currently, there is only one type of copper (copper sulfate pentahydrate) that falls within the soluble category, and it is not registered on apples in Canada. In the US, Mastercop and Phyton are registered.

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 are the same level of insolubility, for example copper hydroxide (ex. Parasol WG, Kocide 2000) is more soluble in water than basic copper sulfate (ex. Copper 53W), while copper oxychloride (ex. Copper Spray) falls between the two. Copper octanoate (Cueva) is more soluble than fixed coppers, but approximately 26 times less soluble than copper sulfate pentahydrate.  

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.  Adding hydrated lime can make any copper product less soluble but some coppers are not compatible with lime, so always consult the product label.

Using Copper

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 apples, the max rate of Copper 53W is 3kg/ha so multiplied by the 53% metallic copper content means that there is 1.59 kg of metallic copper per hectare being delivered.  

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

Product	Registered on apple?	PCP #	Active ingredient	Metallic copper	Bee toxicity rank1
Copper 53W	yes	9934	basic copper sulphate	53%	III
Copper Spray	yes	19146	copper oxychloride	50%	II
Parasol Flowable	yes	25901	copper hydroxide	24.4%	II
Kocide 2000	yes	27348	copper hydroxide	53.8%	II
Cueva Commercial	yes	31825	copper octanoate	1.8%	III

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. However, there are cases of copper resistance in bacterial pathogens of vegetable crops developing when frequent and repeated applications were used. Fungicide resistance management guidelines should be used to maintain economical 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
• 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 OMAFRA 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

McGrath, M. (2020). Copper Fungicides for Organic and Conventional Disease Management in Vegetables. www.vegetables.cornell.edu

Abbasi, Khabbaz, S. E., Weselowski, B., & Zhang, L. (2015). Occurrence of copper-resistant strains and a shift in Xanthomonas spp. causing tomato bacterial spot in Ontario. Canadian Journal of Microbiology, 61(10), 753–761. https://doi.org/10.1139/cjm-2015-0228