Answer: Early Blight is caused by the Alternaria fungus. It overwinters on infected plant material, even seeds, so it's hard to completely remove the spore reservoir from the garden by cleaning up all the vines and fruit. Early Blight works slowly, whereas Late Blight (caused by the Phytophthora fungus) may kill plants within a week. The fungus is always growing somewhere and releasing spores into the air, which moves on wind currents. Phytophthora doesn't overwinter in your soil, so if late blight infected your plants you can reuse the soil next year. Even though you can use the soil, I'd recommend rotating your tomato crops into different containers and planting something other than tomatoes in the containers where you grew tomatoes this year.
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