Copper Fungicide vs. Chlorothalonil: Which Is Used for Which Tree Diseases?

Copper fungicide controls both fungal and bacterial tree diseases, while chlorothalonil only controls fungal diseases. That makes copper the right call for bacterial problems like fire blight and bacterial leaf spot, while chlorothalonil is a solid foliar protectant for fungal-only issues like leaf spot and blight on ornamentals.

What's the difference between copper fungicide and chlorothalonil?

Copper is an inorganic, multi-site fungicide and bactericide that's been used on tree crops since the Bordeaux mixture was first developed in the 1880s. Because it works against bacteria as well as fungi, it's one of the few sprays that can knock back both types of infection with the same application.

Chlorothalonil is an organic, broad-spectrum contact fungicide. It sits on the leaf surface and protects new growth from fungal spores, and it's known for lasting through rain better than many other foliar sprays. What it can't do is touch a bacterial infection — it has no bactericidal activity at all.

Liquid Copper Fungicide Spray is TreeHelp's go-to copper option, and it's worth having on hand before disease season starts rather than after symptoms show up.

Which tree diseases does copper fungicide treat?

Copper's dual action makes it the better pick whenever bacteria might be involved, and it still holds up against plenty of fungal problems too.

Fire blight. This is a bacterial infection, so chlorothalonil won't touch it. Spraying Liquid Copper Fungicide in early spring, before bud break, reduces the bacterial spores overwintering on the bark. Trees in the Fire Blight collection — apple, pear, hawthorn, cotoneaster, firethorn, and mountain ash — are the most common hosts.

Leaf spot. On oak, maple, magnolia, and birch, leaf and tar spot is usually cosmetic and doesn't need treatment. But in severe cases, spraying copper fungicide when spots are first noticed, then again in 14 to 20 days, helps clean things up. Browse the Leaf Spot collection for the full treatment routine.

Anthracnose and bacterial leaf spot. Copper controls anthracnose and bacterial leaf spot on a wide range of tree species.

Secondary infection on cold-damaged palms. Copper doesn't prevent freeze injury or protect a palm from frost. What it does is guard cold-damaged tissue against palm bud rot — the secondary bacterial and fungal infection that attacks a bud already weakened by cold. Applied to the bud right after a freeze, a copper fungicide is a common preventative against that follow-on infection, a useful second job for growers in the products for palm trees category. (This use is based on field observation of cold-injured palms rather than controlled trials — see UF/IFAS Extension: Cold Damaged Palms and TreeHelp: Palm Cold Protection.)

One caution worth passing along: copper can discolor foliage or blossoms on sensitive plants, so test spray a small area first before treating the whole tree.

Which tree diseases does chlorothalonil treat?

Chlorothalonil is a strictly fungal-only fungicide, so it belongs in the toolkit for foliar fungal diseases where no bacteria are involved — things like leaf spot, blight, and rust on ornamentals and shade trees. It's a good rotation partner because it works through a different mode of action than copper, which helps slow down resistance building up in a fungus population over repeated seasons.

What it won't do is help with fire blight, bacterial leaf spot, or any other bacteria-driven problem. If a tree is showing bacterial symptoms, reaching for chlorothalonil instead of copper just means the infection keeps spreading while the spray does nothing to it.

If you're not sure whether what you're seeing is fungal or bacterial, our Fungicides collection has options for a range of fungal-specific issues once you've confirmed the cause.

Copper fungicide vs. chlorothalonil: which one should you use?

Start by identifying whether the disease is bacterial or fungal. If there's any bacterial involvement — fire blight, bacterial leaf spot, oozing cankers — copper is the only one of the two that will help. If it's a confirmed fungal-only issue and copper's phytotoxicity risk is a concern on a sensitive tree, chlorothalonil is a reasonable alternative for foliar protection.

Because copper and chlorothalonil work through different modes of action, some growers rotate between the two across a season to reduce the chance of resistance building up in a fungal population. That said, always match the treatment to the actual diagnosis rather than defaulting to whichever spray is already in the shed.

For most home tree care situations across oak, maple, magnolia, birch, and palm, Liquid Copper Fungicide Spray covers the widest range of both bacterial and fungal problems in a single product.

Frequently Asked Questions

Does copper fungicide treat fire blight?

Yes. Fire blight is caused by bacteria, and copper is one of the few fungicides with bactericidal activity. Spraying Liquid Copper Fungicide in early spring before bud break reduces the bacterial spores overwintering on the bark.

Can chlorothalonil treat bacterial tree diseases?

No. Chlorothalonil is a fungicide with no bactericidal activity, so it won't control fire blight, bacterial leaf spot, or other bacteria-driven infections. A copper-based product is needed for those.

Is copper fungicide safe to spray on all trees?

Copper can cause discoloration on the foliage or blossoms of sensitive plants. Test spray a small area first, and avoid contact with metal or masonry surfaces like galvanized roofing, since copper can react with them.

Can you rotate copper fungicide and chlorothalonil on the same tree?

Growers sometimes rotate the two across a season since they work through different modes of action, which can help reduce fungal resistance. Match the choice to the actual diagnosis first — copper for anything with a bacterial component, chlorothalonil for confirmed fungal-only issues.