You can also thin out the stems on perennials to allow air to pass through the clump. Sphaerotheca pannosa var.rosa, or powdery mildew, can infect any green tissue, so it may be found on leaves, green stems, and flower parts. This is a plant pathogen known as an obligate parasite that can grow and reproduce only on or in a living host. Powdery mildew is a common disease of roses, crepe myrtles, dahlias, zinnias, calendulas, sweet peas, cucurbits and vine crops. Treatment and Prevention Natural prevention Plant roses in full sun. When you prune roses concentrate on four of five good stems and aim for a cup shape, so that the air can circulate. Rose powdery mildew spreads during the growing season by means of microscopic, air-borne spores produced on the powdery growth. Powdery mildew on roses. The good news is, you can manage this disease; sometimes fungicides are needed. The fungal spores are spread by wind. Powdery Mildew on Roses. One of the most common problems with roses in East Texas is the fungus powdery mildew. rosae) forms powdery, gray-white mats on the leaves and flower surfaces, often making rose bushes look like they've been dusted with powder. Rose powdery mildew is a disease of roses caused by the fungus Sphaerotheca pannosa var. The powdery mildew fungus (Sphaerotheca pannosa var. Powdery mildew of roses, a disease thought to have been first described by Theophrastus in 300 BC, is a problem worldwide, in greenhouses or outdoors, wherever roses are grown. rosae.It forms what looks like a white powder over the surface area of the rose leaves, and may also spread to the stems and new rose buds. When it comes to diseases that may befall your plants, powdery mildew tops the list as one of the most common culprits. They should receive a full six to eight hours of sun daily. Nearly 40% of the fungicide sprayed on roses is to control powdery mildew. Powdery mildew fungus favours high humidity and dry conditions. Take out any branches in the middle, if you can. Moderate temperatures of 60° to 80°F and shady conditions are the most favorable for powdery mildew development, and will spread. To prevent powdery mildew on roses prune them carefully. Powdery mildew is one of the most destructive diseases of roses (Rosa spp.). Plants will grow more robustly and be able to resist powdery mildew better. Water splash and air currents spread the spores of powdery mildew which grows on the surface of the leaf only. This is a fungal disease called Powdery Mildew; one of the most common diseases of roses, peonies, phlox (and many other plants).