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Copper Fungicide: Why, When, Where and How To Use It

Copper Fungicide: Why, When, Where and How To Use It

You may have come across suggestions from experienced gardeners to use copper-based fungicides as a remedy for plant fungus. While some have praised its effectiveness, others have warned about its potency.

Before we address its uses and effectiveness, let’s start from the beginning –

What is copper fungicide? Copper fungicide refers to any plant product with copper sulfate as an active ingredient. When dissolved in water or combined with other compounds, copper can penetrate diseased plant tissue and prevent fungi and bacteria from spreading further since these organisms are sensitive to copper ions.

The main snag of copper fungicide is that it doesn’t break down in the soil as quickly as other fungicide products do, so knowing precisely how, where, and when to apply the stuff is key to treating the early stages of plant disease without killing the plant itself!

Let’s look at how it can be used and how safe it is, and we’ll share our pick of the best copper fungicide products and more.

How Does Copper Fungicide Work?

When converted into a liquid state, copper fungicide kills harmful organisms in plant leaves by “denaturing enzymes and other critical proteins, which kill the pathogen cells,” according to crop specialist Gordon C. Johnson at the University of Delaware.

Copper Fungicide Uses

Ideally, copper fungicide should be applied to plants before the signs of fungal infection are visible as it is predominantly a deterrent product, but you can also use it immediately on plants that display early signs of infection.

Many gardeners also apply copper-based fungicide to plants or trees that are nearby a diseased plant as a preventative measure.

Is Copper Fungicide Safe for All Plants?

Every copper-based fungicide product has different strengths and applications, but as long as the directions are followed correctly, it will be safe for all plant types.

When used in large quantities, the copper element can cause “phytotoxicity,” killing or severely damaging plant tissues.

Other factors can make copper applications a little more risky, such as using it on new leaves, wet leaves, or during wet weather periods when the product hasn’t been able to dry sufficiently.

Gordon C. Johnson explains that “copper ions are released in small amounts when copper residues are wetted, sending them to other areas of the plant to penetrate cells. Copper injury is also most likely on new leaves and leaf edges that have thin, undeveloped cuticles (the outer protective waxy layer).”

What Plants Can You Use Copper Fungicide On?

Copper fungicide can be used on most houseplants, vegetables (particularly squash and cucumbers, tomatoes, eggplants, and potatoes), fruit plants/trees, herbs, ornamentals, and your lawn.

Avoid use on copper-sensitive plants, however, such as bromeliads, ivy, and annual geraniums.

Copper Based Fungicide for Trees

Copper-based fungicide products for trees are commonly in a “fixed-copper” spray form, but it can also come in a blue paste known as Bordeaux mixture, which combines copper and lime and is often applied to trunks with a wide paintbrush.

Preventative sprays are normally applied to trees during winter to take advantage of cool, dry periods and the tree’s dormant state.

How Do You Apply Copper Fungicide to Indoor Plants?

Firstly, remove badly infected sections of the plant using sterile pruning shears (making sure to clean your tools after use to prevent spreading mold spores).

Allow the plant to completely dry in a warmer room with good air circulation before taking it outside and spraying it all over with the fungicide.

Only bring it indoors once it has dried again. Keep affected plants isolated to avoid spreading fungal spores to healthy ones.

When Should You Not Use Copper Fungicide?

As copper fungicide is a deterrent and not a cure, it won’t always be effective on plants with a late stage of blight.

Conversely, copper fungicide should not be something you reach for when minimal marks/spots appear on leaves as this could be controlled by trimming or pruning the affected areas and monitoring for possible spread.

So be sure you have correctly identified the disease on your plant before using copper fungicide in haste.

Is Copper Fungicide Safe for Humans?

When used in small amounts and as directed on the label, copper fungicide is perfectly safe for humans.

Products often recommend wearing protective gloves and clothing, masks, and eyewear when applying as it has been known to cause eye/skin irritation.

Is Copper Fungicide Organic?

Though copper fungicide has been approved under the USDA National Organic Program, it is technically classified as a synthetic.

According to the Organic Program list, copper-based plant control “must be used in a manner that minimizes accumulation in the soil.”

This has divided many organic growers since copper sulfate is still a much-relied-upon fungicide.

Is Copper Fungicide Safe for Bees?

Copper fungicide is generally safe for bees but only when it is applied to plants or trees late in the day when bees are not actively pollinating or foraging.

This allows the product to dry before bees return to pollinate.

Copper Fungicide vs. Neem Oil

While copper fungicide products combine copper with added compounds such as salt and acid, neem oil is a naturally-occurring fungicide found in the seeds of the Azadirachta indica, or neem tree, native to India and Africa.

Here’s how they compare:

How Often Can You Apply Copper Fungicide?

Generally, most spray products suggest applying copper fungicide weekly or every 10 days until symptoms disappear.

Whenever possible, at least 12 hours of dry weather should follow a copper fungicide application.

When To Apply Copper Fungicide to Fruit Trees

For best results, copper fungicide should only be applied as a preventative measure in the fall or before leaf emergence, according to the Tree Fruit Advisory Extension at Utah University.

As the young foliage and flowers on fruit trees can be damaged by copper, applying fungicide specifically at bud break or immediately after planting is best.

As for dealing with early symptoms of disease on mature plants, copper fungicide can safely be applied every 7-10 days up until harvest.

Copper Fungicide Spray – How To Use

Typically, most copper fungicide products come in a ready-to-use spray.

Otherwise, they will come in a bottle of concentrated liquid copper that normally advises mixing 0.5 to 2 ounces of the liquid per gallon of water in a spray bottle (taking care to use protective gloves and clothing).

You can then spray the solution directly onto your plants when you first notice the symptoms of fungal infection – targeting the tops and undersides of the leaves – and repeat weekly.

Always follow the guidelines set by the manufacturer as each product may vary in terms of dosage and may mention certain weather or temperature limits for the fungicide to be used to best effect.

Best Copper Fungicide

There are many different copper fungicide formulas out there in liquid, paste, and powder forms – each one containing varying levels of copper, different active ingredients, etc.

If you’re not sure where to begin, here are three we’d recommend:

Bonide Copper Fungicide

This 16-ounce liquid concentrate has a low concentration of copper soap (10%) and can be used right up to the day of harvest to tackle many common and uncommon plant diseases and can even be effective on late-stage blight.

The product mixes instantly with water so you can use it immediately with a hose-end/tank sprayer.

Bonide Copper Fungicide also comes in a dust product to mix with water or apply using a dust miser and a ready-to-use spray too!

Southern Ag Liquid Copper Fungicide

Southern Ag’s 32-ounce liquid copper concentrate contains over 27% copper diammonia diacetate, making it super tough on mildew, rust, black spot, and many more stubborn fungal infections.

It can even be effective on algae on your lawn and kill moss on live oak trees.

Simply mix in with water, and use as a cover spray using a hose-end sprayer to apply to your favorite fruits, vegetables, and ornamentals.

Monterey Liquid Copper Fungicide

This 32-ounce, ready-to-use spray by Monterey allows you to apply diluted fungicide solution directly and safely onto your plants from the bottle thanks to the easy trigger spray nozzle.

Use as a preventative dormant spray or apply during the growing season on a wide range of vegetables, fruits, and flowering plants – this aims to control many listed plant diseases from peach leaf curl and fruit rot to rust, downy mildew, and more!


Copper fungicide can be a very effective preventative when it comes to keeping many of your plants and trees disease free.

It’s also very strong, risking overkill when used in large amounts, so be sure to use it sparingly, and make sure you have correctly identified a fungal infection on your indoor or outdoor plant before applying it.