One of the most common problems with roses in East Texas is the fungus powdery mildew. This is a plant pathogen known as an obligate parasite that can grow and reproduce only on or in a living host. Sphaerotheca pannosa var.rosa, or powdery mildew, can infect any green tissue, so it may be found on leaves, green stems, and flower parts. Although it rarely kills a plant, the infection reduces plant vigor and lowers its aesthetic value. It is named for the grayish-white, powdery mat of fungal mycelia and spores present on the surface of the plant tissues.
Newly unfolded leaves are the most susceptible to infection, with the lower leaf surface first infected and then later the upper leaf surface with the leaves being somewhat curled and purpled in the advanced stage. This coating reduces the ability of the leaves to photosynthesize. Mature leaves are resistant to mildew and usually show no symptom development or, at most, only small local lesions.
The environment plays a major role in the development of powdery mildew. The disease occurs during cloudy, humid conditions when days are warm and nights are cool, which usually means mid to late spring in our area. In other words, day temperatures in the 80s and high night-time humidity induce mildew formation. Unlike most foliar blights or leaf spot diseases, however, powdery mildew does not require free moisture on the foliage to infect the plant. Powdery mildew can spread rapidly, with the disease cycle completed in as little as 72 hours, though more commonly it takes 7 to 10 days. One other important factor is that with reduced air movement, the disease is more likely to be a problem. Therefore, a good prescription for good healthy roses is the following:
- Select powdery mildew-resistant rose varieties
- Plant roses in full sunlight and in an area with good drainage
- Do not crowd plants; allow for good air movement
- Fertilize roses adequately, but avoid stimulating succulent growth
- Apply a specific fungicide for powdery mildew at the onset of the disease and repeat as necessary
- Prune infected canes, and rake and discard infected leaves and flowers during the growing season
Craig Reiland, Smith County Master Gardener
Texas AgriLife Extension Service