Chlorothalonil is a broad-spectrum contact fungicide that has been the backbone of disease management in turf, vegetables, and ornamentals since the 1960s. Its multi-site mode of action makes resistance development extremely rare. Recently banned in the EU but remains legal in the US.
Nearly all common fungal diseases: brown patch, dollar spot, leaf spot, anthracnose, powdery mildew, downy mildew, botrytis (gray mold), early blight, late blight, Septoria leaf spot, rust, scab. One of the broadest-spectrum fungicides available. The 'swiss army knife' of fungicides.
Daconil (GardenTech — the iconic homeowner brand), Bravo (professional turf), Echo (professional), Chlorostar, Quali-Pro Chlorothalonil. Available as liquid concentrate and ready-to-use spray.
Moderate toxicity — EPA Category I or II depending on formulation (some formulations carry "Danger" signal word). Eye irritant, skin sensitizer. Wear protective eyewear and gloves. Classified as a "likely human carcinogen" by EPA at chronic exposure levels — follow label strictly.
The resistance advantage: Unlike triazole fungicides (propiconazole, myclobutanil), chlorothalonil attacks fungi at multiple sites simultaneously. This makes resistance development extremely unlikely. It's the ideal rotation partner for site-specific fungicides.
Contact vs systemic: Chlorothalonil is strictly a contact/protectant fungicide — it does NOT enter the plant. It must be on the leaf surface BEFORE the fungal spore arrives. This means it's preventive only, not curative. Reapply every 7-14 days and after rain.
Vegetable garden hero: For tomato blight (early and late), chlorothalonil (Daconil) is one of the most effective homeowner options. Begin spraying before disease appears and maintain coverage throughout the season.