Answer: It sounds like Early blight fungus to me. The fungus is in the soil and as soon as the rains comes and splash soil onto the leaves in summer, the fungus starts attacking. First the lower leaves, then it moves up the plant. Eventually it can kill all the leaves. The best precaution is to mulch your plants early in the season, keep them healthy with regular watering and fertilizer, pick off thr lower leaves as they get infected and discard them, and if you're really in a crisis, spray the leaves with copper sulfate early in the season.
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