What organic fungicides work against black spot?
Written by
Olivia Mitchell
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Black spot roses can wreak havoc in gardens, but there are non-toxic alternatives to chemical sprays. Organic alternatives are baking soda sprays, neem oil, and sulfur dust which are safe and environmentally friendly solutions. I have experience using these solutions in my 15 years of working in client gardens. These methods work if they are correctly applied at the correct time.
Baking Soda Spray
- Mix 1 teaspoon baking soda per gallon of water
- Add 1/2 teaspoon horticultural oil to help it stick
- Spray every 7 days during wet weather
- Avoid overuse to prevent soil sodium buildup
Neem Oil
- Apply at dusk to protect bees and beneficial insects
- Use 2 tablespoons per gallon for severe infections
- Combine with compost tea for boosted soil health
- Reapply after heavy rain washes off the oil
Sulfur Dust
- Dust leaves lightly every 10-14 days in humidity
- Wear gloves and mask during application
- Stop use when temperatures exceed 85°F (29°C)
- Pair with drip irrigation to avoid leaf burn
What impresses me most about neem oil is its dual action, it smothers any existing spores while making the leaves taste bad to future pests. Last summer, a client's ‘Double Delight' roses rebounded within three weeks using neem oil. Apply in the evening so the leaves have time to absorb before the heat of daytime burns them.
Sulfur is most effective in specific conditions. I save it for early morning applications when dew assists with powder deposition. Do not mix with oil sprays, since there are chemical reactions that can easily fry leaves. In trials during the Pacific Northwest season of the year, sulfur reduced infections considerably, 70%, when used with a resistant rootstock.
Soil health is important. Weekly compost tea drenches (1 cup/gallon) build rose immunity from the roots up. I incorporate worm castings into all planting holes, their enzymes inhibit the growth of fungus. Enriching the soil promotes healthy plants which are more resilient and recover faster from fungal attacks (even if the spores get past their defenses).
Read the full article: Black Spot Roses: Prevention & Treatment Guide
