The Q&A Archives: powdery mildew on roses

Question: how do I get rid of the white powder stuff off of my roses? I've tried the baking soda and that didn't help.

Answer: Even though there are several types of powdery mildew fungi, they all produce similar symptoms on plant parts. Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth. Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. These are the cleistothecia or overwintering bodies of the fungus. The disease is most commonly observed on the upper sides of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. 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. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues. Several practices will reduce or prevent powdery mildews. Many plants, such as roses, are developed to be resistant or tolerant to powdery mildew. Inquire about resistant varieties before a purchase.

Once the disease becomes a problem: Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection. Avoid overhead watering to help reduce the relative humidity. Remove and destroy all infected plant parts. This decreases the ability of the fungus to survive the winter.

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). 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. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications.

This is probably more information than you ever wanted to know about powdery mildew, but if you are determined to grow healthy roses, you'll want to have a thorough understanding of this fungal disease. Best wishes with your garden!

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