Answer: There are many causes of yellowing leaves in tomatoes. Fortunately, the causes of lower leaves yellowing and dropping are rarely anything to worry about. It's almost always a case of water stress but if you plant your tomatoes in the same garden spot year after year, they could be showing signs of a fungal or bacterial disease. For now, try mulching over the soil so water doesn't splash from the ground to the plant. Next year move your plants to a different spot in the garden - just in case.
Best wishes with your tomatoes!
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