Answer: Early blight is caused by a fungus that can live on foliage, fruit, even seeds. The best you can hope for is to get your plants through the season without too much damage. Pick off the first damaged leaves that you see to slow down the disease begin spraying compost tea on your plants every 2-3 weeks during the summer. You can make the tea by adding 1 part aged compost that contains some manure to 5 parts water and letting the mixture sit in the shade for 2 weeks. Then strain through cheesecloth and spray the liquid on your plants. The tea contains fungi that outcompete the blight fungi.
As general precautions, avoid planting tomatoes where either tomatoes or potatoes grew the year before, and choose varieties with flat, uncurled leaves, which are less susceptible. Good luck with your crop!
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