Bacillus subtilis is a naturally occurring soil bacterium used as a biological fungicide. It works by colonizing plant surfaces and outcompeting disease-causing fungi for space and nutrients. Also produces antifungal compounds (lipopeptides) that directly inhibit fungal growth. Organic approved.
Powdery mildew, botrytis (gray mold), bacterial leaf spot, fire blight suppression, Rhizoctonia, Fusarium, Pythium, downy mildew, rust, Sclerotinia. Works best as a preventive - much less effective curatively. Best for vegetables, fruits, ornamentals, and greenhouse crops.
Cease (BioWorks), Serenade Garden Disease Control (Bayer - most popular consumer brand), Rhapsody (BioWorks), Companion (Growth Products), Subtilex (Becker Underwood). Available as liquid concentrate and wettable powder.
Essentially non-toxic. OMRI listed for organic production. No REI (re-entry interval), no PHI (pre-harvest interval). Safe around children, pets, wildlife, and beneficial insects including pollinators. Can be applied up to and including the day of harvest.
Preventive, not curative: B. subtilis must be present on the plant surface BEFORE the disease arrives. Think of it as a living shield. Once disease is established, it cannot cure it. Begin applications early and maintain regular coverage (every 7-14 days).
Compatibility: B. subtilis is a living organism. Do not tank-mix with fungicides that kill bacteria (copper, chlorothalonil). If using both, apply copper first, wait 5-7 days, then begin B. subtilis applications.
Best results: Apply in early morning or late afternoon to avoid UV degradation. The bacteria need time to colonize the plant surface before strong sunlight kills them. Adding a spreader-sticker improves adhesion and coverage.
Soil health bonus: When B. subtilis washes off leaves into the soil, it continues working as a beneficial soil microbe - suppressing root rot fungi (Pythium, Rhizoctonia, Fusarium) and promoting plant growth through nutrient cycling.