How does copper fungicide work?

Last updated: October 2026

Copper fungicide releases copper ions that denature proteins and enzymes in the fungal and bacterial cells they touch before those pathogens infect the plant¹. It is a protectant only: it has no post-infection activity and does not move inside the plant, so it has to coat leaves before disease starts¹. The ions are released only while the spray residue is wet³.

What is copper fungicide made of?

Copper fungicides are inorganic copper compounds that kill both bacteria and fungi, which is why they are one of the few tools growers have against bacterial leaf diseases¹. Copper sulfate, the simplest form, has been registered in United States pesticide products since 1956⁷. The copper binds to proteins in fungal cells, damaging the cells so they leak and die⁷.

Almost all copper fungicides sold today contain a fixed copper as the active ingredient, and copper hydroxide is the most common¹. Fixed coppers dissolve poorly in water, so the dried spray deposit releases a small amount of copper ions every time the leaf is wetted, which gives them residual activity^1,2. Copper sulfate pentahydrate is different: it is highly soluble¹. The amount of active ingredient on a label does not tell you how much copper a product holds; the metallic copper equivalent (MCE) in the ingredient section does¹.

In North America, interest in copper fungicide peaks in June, when warm, wet weather favors tomato leaf diseases; peach leaf curl sprays go on after leaf fall in late November or December. In the Southern Hemisphere, the same stages fall about six months later, with the tomato season around December and January.

How does copper stop plant disease?

Copper ions kill pathogen cells by denaturing proteins and enzymes, but only in pathogens they contact before infection; copper has no activity once a pathogen is inside the leaf¹. Copper also kills plant cells if it is absorbed in sufficient quantity, so the cuticle, the leaf's waxy outer layer, is what protects the plant².

Water drives the whole process. UC IPM notes that copper is active only when it is wet, when the ions are in solution³. Dew releases more ions from the deposit, while heavy rain can wash much of it off: Cornell University reports that a 2-inch downpour can remove a lot of residue¹. Acidity raises the solubility of fixed copper, which makes it more effective at first but shortens its residual activity and increases the chance of plant injury¹.

Because copper does not move inside plants, the spray protects only the surfaces it covers¹.

How do you use copper fungicide on tomatoes?

On tomatoes, copper is used mainly against bacterial spot (Xanthomonas species) and bacterial speck (Pseudomonas syringae pv. tomato), and as one option for fungal leaf spots such as Septoria leaf spot^4,5,6,8. University of Missouri Extension reports that home gardeners control the bacterial diseases primarily with copper products, which come in formulations approved for organic production⁸.

Start before disease does. NC State Extension states that copper, chlorothalonil or mancozeb will help reduce Septoria leaf spot in gardens with a history of the disease, but they must go on before it appears because they give only preventive protection and will not cure the plant⁴. For bacterial spot, all labeled products work best applied preventively, and there are no curative products⁵. Cornell University places the best start before the first infection, which occurs at least 5 days before symptoms show¹.

Coverage and weather decide how well a spray works. Cornell lists the factors: drying time before rain or overhead irrigation, rain amount and intensity, dew, how thoroughly the sprayer covers the plant, the rate used (higher label rates do better than lower ones) and how often applications are made¹. Follow the product label for the rate, interval and days to harvest on tomatoes.

Spraying works alongside cultural controls. For bacterial spot, NC State Extension stresses disease-free seed and transplants, rotation away from tomatoes and peppers, and not handling plants while they are wet⁵. For Septoria leaf spot, it suggests rotating beds, mulching to stop soil splashing onto lower leaves, and removing the first affected leaves⁴.

When is the best time to spray copper?

Timing depends on the disease. For tomato leaf diseases, sprays protect new growth through the wet weather that favors infection; NC State Extension notes that bacterial spot develops fastest at 75 to 86°F with high rain and humidity⁵.

Peach leaf curl is the clearest dormant season use. UC IPM advises treating susceptible peach and nectarine trees just after leaves fall, usually in late November or December, with a possible second application in late winter before buds swell³. Treating after symptoms appear in spring will not work, and fungicides applied during the growing season can harm foliage³. UC IPM adds that a 1% horticultural oil mixed with copper ammonium or copper soap improves effectiveness³.

Why does copper sometimes stop working?

Bacteria adapt. Cornell University notes that bacterial pathogens have proven adept at developing resistance to copper, which can make copper fungicides ineffective¹. A systematic review of 133 publications found that 78.5% of Pseudomonas syringae pv. tomato isolates and 54.0% of Xanthomonas isolates screened were copper tolerant⁶.

Field results follow the same pattern. In the same review, only 38% of studies that tested copper alone, rather than mixed with other products, reported highly effective control of bacterial speck and spot; the rest reported variable, weak or no control⁶. For commercial growers, NC State Extension recommends full rates of fixed copper combined with mancozeb for bacterial spot⁵.

Weak results can also come from the application itself: spraying after infection has started, poor coverage, or rain before the deposit has dried¹.

What are copper's risks to plants, soil and water?

Copper can burn the crop. Injury shows up most on new leaves and leaf edges, where cuticles are thin and copper can enter through pores, and it is worse when copper goes onto wet leaves or dries slowly². Research has shown that adding surfactants to copper greatly increases leaf injury, and copper should not be used with spray acidifiers or other acid forming products².

Copper does not break down. As an inorganic element, it can accumulate in soil when used intensively over many years¹. UC IPM warns that this buildup can harm soil microorganisms and, through runoff, aquatic organisms³. Studies with several aquatic species have found copper highly to very highly toxic to fish and aquatic life, with trout and juvenile fish particularly sensitive⁷.

For people, copper sulfate can cause severe eye irritation⁷. The U.S. EPA considers copper practically nontoxic to bees, though Cornell University notes that copper sulfate is considered more toxic to native bees than other coppers^7,1. Follow the product label for protective equipment, re-entry time and disposal.