Preventing Blackspot on Roses

One of the most common questions I receive each year is: "How do you prevent blackspot?" The key is diligence and maintaining a consistent spray program. To keep blackspot at bay, you need to spray regularly, every 10-14 days.

Steps to Get Started

  1. Water Your Roses: Ensure your roses are well-watered the day before you spray. This is best practice.

  2. Spray at the Right Time: Spray in the morning when temperatures are below 80 degrees to prevent leaf burn.

  3. Protect Yourself: Wear safety glasses, long sleeves, long pants, and nitrile gloves for safety.

  4. Use a Dedicated Sprayer: Use a sprayer like the Ortho 1-gallon sprayer from Home Depot. Remove the cone on the sprayer wand, as it is unnecessary for spraying roses. Only use this sprayer for fungicides, insecticides, or miticides—never for herbicides like Roundup. Cross-contamination can be fatal to your roses.

Recommended Products for Blackspot Prevention

These products can be purchased from Amazon or other sources:

  • Hi-Yield Spreader Sticker (16 oz)

  • Infuse - Systemic Disease Control by Bonide

  • Cleary's 3336 - Fungicide (16 oz; an 8 oz bottle may be available, as only 1.5 teaspoons per gallon are needed, making it last a long time)

  • Mancozeb by Bonide – Always keep this on hand.

Tip: Label each container with the date opened and indicate the correct amount to use per gallon.

Formulations for a 1-Gallon Mix

Rotation #1

  1. Fill the sprayer with approximately 1/2 gallon of water.

  2. Add:

    • 1/4 teaspoon - Spreader Sticker

    • 2 1/4 teaspoons - Infuse

    • 1 Tablespoon - Mancozeb

  3. Add additional water up to the 1-gallon mark.

  4. Close the sprayer and swish it around to mix well.

  5. Spray the shrub thoroughly.

  6. Dispose of leftover spray: Spray any excess in the yard or flower bed—DO NOT POUR DOWN THE DRAIN.

Rotation #2

  1. Fill the sprayer with approximately 1/2 gallon of water.

  2. Add:

    • 1/4 teaspoon - Spreader Sticker

    • 1 1/2 teaspoons - Cleary's 3336F

    • 1 Tablespoon - Mancozeb

  3. Add additional water up to the 1-gallon mark.

  4. Close the sprayer and swish it around to mix well.

  5. Spray the foliage and canes of the shrub thoroughly (avoid spraying blooms).

  6. Dispose of leftover spray: Spray any excess in the yard or flower bed—DO NOT POUR DOWN THE DRAIN.

Additional Tips

  • Track Your Sprays: Mark a calendar to log what was sprayed each week. I personally use an Excel spreadsheet.

  • Rotate Fungicides: Rotating fungicides prevents blackspot from developing resistance to chemicals.

    • Infuse (MOA 1) and Cleary’s 3336 (MOA 2) are systemic fungicides.

    • Mancozeb is a contact fungicide—it kills blackspot on contact but does not provide long-term prevention.

  • Consistency is Key: By following this routine diligently, you will effectively prevent blackspot and maintain healthy, beautiful roses.

Following these best practices will help keep your roses thriving and blackspot-free throughout the season!