The leaves of a healthy mint plant can be used to make tea or as an ingredient in many recipes. If an indoor mint plant exhibits black or brown spots on its leaves, however, the plant will be inedible and will require the attention of a gardener to keep it from dying.
The area in which a mint plant grows inside plays a major role in its health. Plants grown without access to natural light or a comparable artificial grow lamp may produce brown, unhealthy mint leaves and be more susceptible to pest and disease problems. Keep the mint plant where it receives an adequate amount of light during the day to keep it healthy and green; mints grow best in full light or partial shade.
Pest problems can lead to the formation of dead spots on the leaves of a mint plant that become brown or black as the mint plant is weakened. Scale insects, aphids and spider mites are pests that affect indoor houseplants and feed on the leaves, which stops the mint from producing nutrients properly. Apply a mild pesticide or insecticidal soap to the mint plant according to the directions to keep the mint plant healthy.
Anthracnose is a fungal disease that can affect mint plants. It causes small black or swollen spots to appear on the leaves of the mint plant, and the leaves will eventually die if the plant remains untreated. Prune healthy plants in the fall to prevent the disease from spreading and using the plant as a winter host, and remove affected leaves from sick plants to stop the anthracnose from producing spores. Keep the mint's planter drained.
Mint rust is a disease that affects mint plants by creating reddish brown spots on the underside of the leaves and infecting the stems of the plant. Eventually, the mint plant will experience leaf loss as the fungus produces spores and kills off the leaves. Apply fungicides that contain the chemical chlorothalonil to the mint plants to kill the fungal spores and stop the disease from spreading; follow all directions on the fungicide for the best results.