How do I keep tomatoes from getting blight?

Tomato blight (early blight, Alternaria, and late blight, Phytophthora infestans) is mostly prevented by how you water, space and clean up, not by one spray. The main habits that help:

• Water at the soil, not the leaves. Drip or soaker hoses, and water early in the day so foliage dries fast. Wet leaves overnight favor both diseases.
• Mulch the base of the plants. A 2 to 3 inch layer of straw or shredded leaves keeps soil from splashing spores up onto the lower leaves.
• Give them air. Space plants widely, prune the lowest suckers and leaves that touch the ground, and stake or cage them.
• Rotate crops. Don't plant tomatoes, potatoes or peppers in the same bed for at least two to three years, since the fungus overwinters in debris.
• Clean up. Remove infected leaves as soon as you see spots (bag them, don't compost them), and pull up and destroy plants at the end of the season.
• Buy resistant varieties. Look for the letters V, F, N or T on seed packets, and for late blight, choose varieties with known resistance if your area has had it.
• Use a protective fungicide if needed. Copper or a labeled product such as chlorothalonil can protect healthy foliage when applied before or at first signs of disease. Follow the label, and check with your local extension office for what's allowed and recommended in your area.

Late blight spreads fast in cool, wet weather. If you see water-soaked, greasy-looking spots that turn brown and papery, especially with white fuzz underneath, act quickly and contact your county extension service to confirm the diagnosis. The USDA and most state extension services publish free tomato disease guides with photos, which is a good place to match what you're seeing.