Plant Health Problems
The disease problems on mint are similar to those of other herbs. See Herbs for a detailed discussion of these diseases.
Diseases caused by Fungi:
Rust symptoms appear in early summer as light yellow or brown spots on stems, petioles, and mid-veins of leaves. Upon close examination, the spots are powdery with rust spores. This disease can result in some defoliation, especially when plants are crowded.
This disease can be minimized by cleaning up plant refuse in the fall and by adequate spacing of the plants to promote good air circulation.
Infected plants are slow to grow in the spring and often have a bronze cast. Lower leaves yellow and wilt during warm or dry weather and plants can die. The fungus is soilborne and enters through the roots and becomes systemic in the water transport system of the plant.
One of the key strategies for control of Verticillium wilt is prevention. Therefore, it is important to avoid planting mint in infested soil. It is also helpful to maximize plant vigor by good cultural care and watering. Chemical controls are not effective for this soilborne fungus.
Powdery mildew, Erysiphe.
White powdery spots or patches develop on leaves and stems. Symptoms often first appear on the upper surfaces of the leaves. Heavily infected leaves turn brown and shrivel.
Disease can be minimized by avoiding overcrowded spacing of plants and by carefully picking off affected leaves as soon as symptoms are evident. Symptomatic leaves can be placed into a plastic bag in order to avoid spreading the spores of the fungus to other plants. Use of fungicides is usually not necessary. However, for culinary plants, applications can be made as soon as symptoms are visible. Among the compounds registered for use in Connecticut are horticultural oil and sulfur. Consult the label for dosage rates and safety precautions.