Peppermint is an extremely popular mint that is useful for decoration, aromatherapy, alternative home remedies and cooking. It is a staple of most herb gardens. However, peppermint plants can succumb to a variety of diseases if they are not properly tended, and some of these issues can wreak havoc on your entire herb garden if the situation is not contained. Knowing the signs and symptoms of peppermint plant diseases will help you keep your peppermints healthy and happy throughout the year until you are ready to harvest and use them.
Rust spots form on the bottoms of mint leaves. They create lesions that may be orange, brown or black that are raised on the underside of the leaves. Rusts are fungal infections, and tend to grow on mint plants that are living in a humid environment as a result of overwatering, too much shade or a lack of air circulation.
If the rust infection is contained, you can use sterile pruning techniques to remove affected parts of the plant and dispose of them rather than allowing them to fall among the unaffected plants. However, if you find that the rust cannot be controlled in this manner, then you can use a sulfur fungicide on the rust. Remember that most fungicides will render plants inedible, though. Fight rust first by placing the plant in an area with good sunlight and air circulation and removing infected leaves.
Peppermint wilt establishes itself in the soil around the infected plant, so if you have an indoor herb garden that contracts this infection, you may have to start over. Verticillium fungal infections work their way through the roots and up into the leaves of the infected peppermint plant, causing leaves and stems to collapse and wilt since the roots can no longer function effectively to take up water and nutrients.
Verticillium cannot be treated once it is contracted. The peppermint plants must be removed along with surrounding soil. However, you can take preventative measures like fumigating the soil before you plant in order to stall the spread of the disease. Since Verticillium can lie dormant in soil for several years, you may need to simply plant resistant crops--like spearmint--until you are confident that the infection is completely gone.
Powdery mildew actually "looks" for peppermint plants in the winter because they provide a good host for this infection during colder months when fewer target hosts are available. Powdery mildew creates a chalky dust on the surface of the plant that may be gray, black or white. Generally it can be controlled by simply removing and disposing of affected parts of the plant and by placing the plant in a bright, well-circulated area. However, if necessary, you can treat powdery mildew with sulfur, but that will generally make the plant inedible after treatment.