Answer: The problem could be a fungus, perhaps grey leaf spot. Fungus diseases like warm, moist conditions, which could be met if you are growing several tomato plants close together under lights. A couple of preventative measures can help: move your plants further apart to increase air circulation and set up a small fan near the plants. When you water, take care not to get the leaves wet, and always water in the morning so any moisture will have time to evaporate quickly. An excellent antifungal spray can be made from mature compost that contains some manure. Mix one part compost to five parts water and let the mixture sit in your garage for a couple of weeks. Strain the solution through cheesecloth and dilute it till it's the color of weak tea. Spray thison your plants at 2-week intervals at the first sign of the disease. This compost tea can be used to control many types of disease, and some commercial greenhouse operations routinely use it on their plants. I hope it works for you.
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