Why are my tomato plant's leaves turning yellow with brown spots?

Yellowing leaves with brown spots on a tomato plant can be a sign of a fungal disease called early blight. This disease is caused by a fungus that thrives in warm, humid conditions. To prevent the spread of early blight, it is important to remove and destroy infected leaves, provide good air circulation around the plant, and avoid overhead watering. Applying fungicides can also help control the disease.