Q.Tomato Plant Problems.
They ( leaves) have a bright yellow line immediately followed by dark brown. Mostly on the bottom of the plant
Do you have any pics of plant on site? Watering issues (too much or wetting the plant leaves) and Soil nutrient issues (nitrogen, calcium, manganese, etc too low/high) and pests (cutworms, tomatoe worm etc) may all or part of reason Start simple: Are you watering too much? Watering at height of days heat and sun? Since you mention this is occurring mostly on leaves at bottom/ "soil level", are you splashing the water up as you water? Tomatoes do not like their foliage wet but rather should be Root watered (take spray head off and place hose on ground to water plants ) Most varieties need a deep watering every few days to once a week depending on sun/shade and soil moisture retention Based on just thrse 2