Bacterial spots and blight are two of the most common issues that can cause brown spots on a tomato plant's leaves. Particular details of the brown spot can help you identify the cause of the spotting so you can determine how to stop it from spreading.
Identify the specifics of the brown spots can help determine the specific problem and how to treat it before it causes more serious issues for the plant or spreads to others. Wet leaves cause one of the most common issues that leads to brown spots. Tomatoes, like other members of the nightshade family, do not like having wet leaves. If this is the cause, water the plant in the morning so it has time to dry out before night.
If a tomato plant has a brown spots that expand to black, the plant may have blight, a type of fungus. Humidity, wet conditions or an already weakened plant can succumb to blight. Prevent this problem by giving the plants enough space for air circulation and keep water on the roots instead of the leaves. If the plant is already infected, remove the spotted leaves and try applying a fungicide to the plant. Look for appropriate fungicides at a gardening center or a hardware store.