A great tip from organic gardener Marion Hess is to use powdered milk to ward off tomato blight before it gets started. Her ingredients:
¼ cup plus 2 tablespoons powdered nonfat milk
½ cup Epsom salts (optional)
1 shovelful of compost (optional)
Sprinkle the powdered milk and optional ingredients into the hole before planting and mix with the surrounding soil. After planting your tomato, mix in an additional 2 tablespoons around the plant. Add a couple of tablespoons of powdered milk to the soil every few weeks taking care not to damage the developing root system.