The Q&A Archives: Rose leaf whitening

Question: What is the white dusting on my rose leaves and how do I get rid of it?

Answer: Sounds like the fungal disease called powdery mildew. Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth. Tiny, pinhead-sized, fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. The disease is most commonly observed on the upper sides of the leaves. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open. The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant, and weather conditions during the growing season. Powdery mildews are severe in warm, dry climates. Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection. If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary. These include: sulfur, neem oil (Rose Defense, Shield-All, Triact), triforine (Ortho Funginex), ornamental use only, or potassium bicarbonate (Kaligreen, First Step) Chemicals are most effective when combined with cultural controls. Apply fungicides at seven- to 14-day intervals to provide continuous protection throughout the growing season.

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