Answer: It sounds like a fungal disease called powdery mildew. 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!
Q&A Library Searching Tips