Why do black spot treatments fail?
Written by
Olivia Mitchell
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Black spot roses can make a comeback despite treatments, because of four major mistakes gardeners make. Last season, a customer's ‘Queen Elizabeth' roses relapsed on three occasions; spray fungus preventer once a month instead of weekly. It doesn't matter which product we use, what matters is being consistent with fungicide use! Let's analyze why most treatments fail.
Sporadic Spraying
- Missing 7-day intervals lets spores reproduce
- Rain washes off protectants, reapply within 24 hours
- Stop sprays once leaves harden (late spring)
Foliar Watering
- Wet leaves create 8+ hours of spore-friendly moisture
- Use drip lines 12 inches from stems
- Water before 10 AM for daytime drying
Cane lesions can be silent killers. I've found black streaks underneath thorn clusters that clients have missed before discovery. Make it a weekly habit to check the new growth with a magnifying glass. If you see purple-black blisters, prune 6 inches below where you see the lesions then torch those cuttings. If you only prune some of the damage, you will only overlap the damage and cause re-infection.
Tool Hygiene
- Sterilize pruners with 70% alcohol between cuts
- Replace blade sharpeners yearly
- Avoid carbon steel tools (rust harbors spores)
Product Checks
- Test fungicide efficacy with pH strips (ideal: 6.5-7.0)
- Shake concentrates before mixing
- Store unopened products below 77°F (25°C)
When using any type of application for weed and pest control, above-ground water application will damage even the best programs. A client of mine lost 90% of his leaves due to an automated sprinkler system running. Switching out to soaker hoses saved this garden. I often ask clients to gauge rainfall with a simple $5 gauge and adjust their sprays accordingly when necessary. Dry leaves will be your best ally.
Using fungicides after their expiration date is equivalent to throwing money in the trash. I keep a Sharpie in my toolbox so that I can date products when I open them. Most fungicides lose effectiveness after 18 months. So, throw them out! Last year, I was able to stop outbreaks in 80% of my potato rehabilitation cases using fresh batches of copper fungicide.
Read the full article: Black Spot Roses: Prevention & Treatment Guide
