By Paul Oliver, Founder, TreeHelp.com

There’s nothing more disheartening than watching a healthy-looking tree or garden suddenly develop spots, blights, wilts, or rots. Fungal and bacterial diseases can strike quickly, and if left untreated, they can cause lasting damage—or even kill your plants. That’s why I frequently recommend Monterey Liquid Copper Fungicide to both homeowners and professional gardeners alike.

Copper-based fungicides have been a staple of plant disease management for over a century. But Monterey’s formulation takes that proven effectiveness and delivers it in a user-friendly, versatile liquid concentrate that’s safe for use on a wide range of trees, shrubs, flowers, fruits, and vegetables.

Why I Like It: Broad-Spectrum, OMRI-Listed Protection

What sets Monterey Liquid Copper Fungicide apart is its combination of broad-spectrum protection and organic approval. It uses copper ammonium complex—a form of copper that works as a contact fungicide and bactericide. It disrupts the cellular processes of pathogens on plant surfaces, helping to stop infection before it can take hold.

And here’s the big plus: it’s OMRI-listed for organic gardening, which means it meets the standards for use in certified organic growing. That’s huge for gardeners who want effective disease control without turning to synthetic chemicals.

It’s also very versatile—effective against a wide range of plant diseases, including:

• Leaf spots 
• Blights 
• Downy and powdery mildew 
• Anthracnose 
• Black spot 
• Fire blight (on apples and pears) 
• Peach leaf curl 
• Bacterial canker 
• Rusts and molds 

It works on ornamentals, fruit trees, nut trees, and vegetable crops alike.

How It Works

Unlike systemics, copper fungicides like this one work on contact. That means the product needs to be on the surface of the plant tissue before fungal spores land or bacteria begin to spread. It's a preventative, not a cure—so timing is everything. 

The copper creates a protective barrier on leaves, stems, and buds, killing fungal and bacterial pathogens as they try to establish themselves.

How to Use It

Application is simple, but there are a few best practices to get the most from each spray:

1. Dilute properly: Monterey Liquid Copper is a concentrate. You’ll need to dilute it with water according to the label directions. For most applications, that’s between 0.5 to 2 oz per gallon of water, depending on the severity of the problem and the type of plant.
2. Use a sprayer: A handheld or backpack sprayer works well. Make sure your sprayer is clean and adjusted to create a fine mist for good leaf coverage.
3. Apply evenly: Spray all plant surfaces, including undersides of leaves where diseases often take hold. Aim for thorough coverage, but avoid excessive runoff.
4. Reapply as needed: Copper is rainfast once dry, but you’ll need to reapply every 7 to 10 days, or after heavy rain, during periods of high disease pressure.

When to Apply

It Early and preventive applications are key. Apply before symptoms appear or at the first sign of disease for best results.

Here’s when to use it on common plant types:

• Fruit trees (apple, peach, cherry): Apply during the dormant season to prevent issues like leaf curl and bacterial canker, and again at pre-bloom and post-bloom as needed. 
• Vegetables (tomatoes, cucumbers, squash): Start early in the season and repeat regularly, especially during humid weather when blight and mildew thrive. 
• Roses and ornamentals: Apply at the first sign of black spot, mildew, or rust, and continue throughout the season as needed. Avoid applying during high heat (above 85°F) or midday sun, as copper can sometimes cause leaf burn in sensitive plants. Early morning or late afternoon applications are ideal.

Avoid applying during high heat (above 85°F) or midday sun, as copper can sometimes cause leaf burn in sensitive plants. Early morning or late afternoon applications are ideal.

Helpful Tips for Best Results

1. Shake well before mixing: Copper products can settle in the bottle, so give it a good shake.
2. Test a small area first: If you’re unsure how a plant will react, test a small section and wait 24 hours before full application.
3. Don’t mix with other chemicals: Copper can react with other garden sprays. Apply it separately unless the label states it’s safe to combine.
4. Clean your sprayer after use: Copper residue can build up and corrode metal parts. Rinse thoroughly after each use.

Responsible Use

Although copper fungicides are organically approved, it’s important to use them responsibly. Excessive copper use can build up in the soil over time, so avoid overapplication. Stick to label rates and timing intervals. Also, be mindful not to spray near water bodies, as copper is toxic to aquatic life.

Used properly, however, Monterey Liquid Copper Fungicide is an excellent, low-impact tool for managing disease in your garden or landscape. 

Final Thoughts

Fungal and bacterial diseases are among the most common threats to home orchards, vegetable gardens, and ornamental landscapes. But with Monterey Liquid Copper Fungicide, you have an effective, organic-approved way to protect your plants before problems escalate.

Whether you’re preventing leaf curl on peaches, black spot on roses, or early blight on tomatoes, this is a reliable tool every gardener should keep in the shed. If you ever need help identifying a disease or planning a spray schedule, reach out to us at TreeHelp.com. We’re here to help your trees and gardens thrive—naturally and effectively.