White Mold Diseases on Roses

Powdery mildew can affect all visible parts of the rose plant.

Although the gems of an ornamental garden, roses are subject to many diseases that can affect their health and appearance. One common rose disease, powdery mildew, causes a white mold to form on buds, flowers, leaves and stems. At best, powdery mildew detracts from the beauty of your roses. At worst, the disease can kill your plants.

Cause

Powdery mildew is a common problem in vegetable and ornamental plants. In roses, the fungus Sphaerotheca pannosa var. rosae causes the disease; powdery mildews found on other plants will not infect roses. During ideal conditions, a single spore can grow into the distinctive fuzzy white mold that signals the disease. This mold releases further spores that, under the right conditions, spread to infect other parts of the plant or other rosebushes.

Symptoms

As the name suggests, powdery mildew causes a powdery white mold to form on all above-ground parts of the rosebush, especially young tissue. As the disease progresses, the leaves become twisted and distorted and drop from the plant. New shoots and stems display stunted growth and may also become distorted. Infected buds fail to open. Often, the mold forms first on the undersides of leaves. Because the fungus feeds by extracting sap from the plant, you may also notice that the plant fails to grow properly and shows poor vigor.

Growing Conditions

Powdery mildew requires high humidity levels in order to grow and spread, so humid weather often predicts outbreaks of powdery mildew. During damp weather, the fungus produces spores and, during dry weather, it disperses its spores. When cool summer days are followed by humid nights, powdery mildew tends to become a problem.

Prevention

Several varieties of roses have demonstrated resistance to powdery mildew, so plant these whenever possible to control outbreaks. Resistant cultivars include Duet, Camelot, Golden Slipper and All That Jazz. Avoid wetting the foliage during watering, and space plants to allow adequate air circulation to dry their leaves. Weekly applications of a preventative fungicide should focus on new growth most susceptible to infection, especially during weather favorable for development of the disease.

Treatment

Several fungicides are approved to treat powdery mildew in roses. Since approved fungicides vary by state and change often, you should contact your local extension office or a local nursery for recommendations. Remove and destroy infected stems, leaves and blossoms immediately to limit further spread of the disease. Rake up dead leaves from under the plant in the fall, as these can harbor the fungus and lead to infections the following season.

References

• University of Minnesota Extension: Rose Diseases
• Clemson Cooperative Extension: Rose Diseases
• Ohio State University Extension: Controlling Rose Diseases
• Clemson Cooperative Extension: Powdery Mildew