Answer: Rosemary is typically disease and insect pest free, however, under certain circumstances, it can develop a disease called powdery mildew. Powdery mildew is a fungal disease and the symptoms include white powdery spots or patches which develop on leaves and stems. Symptoms often first appear on the upper surfaces of the leaves. Heavily infected leaves turn brown and shrivel. But, the white cottony substance might also be the bubbles caused by a pest called a spittle bug. These are tiny little insects and they produce a bubbly substance that looks like spit. So, cut one of the affected stems and carefully inspect it. If it's froth, you've got spittle bugs and the best defense is to hose them off your plants. If it's a white powdery coating, it is powdery mildew.
Powdery mildew can be minimized by avoiding overcrowding 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 are horticultural oil and sulfur. Consult the label for dosage rates and safety precautions.
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