Plant Health ProblemsThe disease problems on rosemary are similar to those of other herbs. See Herbs for a detailed discussion of these diseases.
Diseases caused by Fungi:
Powdery mildew, Sphaerotheca.
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.