Experts and BHG readers answer.
Tomato Plant leaves dying
Some yellowing of older leaves on tomato plants is natural. Do the leaves also have brown or black spots? If so, it likely is a fungal disease, either early blight or alternaria leaf spot. You can prevent the spread of these fungal diseases by spraying with preventive fungicides. If not spotting is seen on the leaves it may be an indication that the soil is drying too much between waterings.