Controlling Powdery Mildew

What Is This Disease?

Powdery mildew is a common fungal disease of roses and landscape plants. It’s actually the name given to a group of diseases caused by several closely related fungi. This disease is easily identified by the white or light gray moldy powder that appears on the surface of young leaves, shoots and flower buds. This unsightly mold causes abnormal growth, leaf distortion, discoloration, and inhibits flower development. Powdery mildew is not usually considered fatal, but plant damage can definitely occur when infections are severe and left untreated. That’s where the Blue Bottle comes in. BioAdvanced offers do-it-yourself solutions to control powdery mildew and protect your roses, ornamentals and susceptible plants.

Signs of Infection

Powdery mildew can cause abnormal, stunted plant growth before the telltale white powdery fungus becomes visible. When powdery mildew does become visible, it usually forms on the upper surface of plant leaves. It begins as small, circular, powdery white spots, which expand and eventually join together as infections progress. Dark "pepper-like" spots appear in the powder (the spots are spore-producing bodies). Leaves curl, twist or become discolored and fall off, buds fail to mature, flowers are deformed or fail to open, and fruit and vegetables may be reduced, misshapen or covered with powdery patches.

What Causes Powdery Mildew?

The fungus survives the winter attached to plant parts and plant debris, such as fallen leaves. When weather warms in the spring, the fungi begin to produce spores (conidia). Humidity is a key factor in the onset and spread of powdery mildew. Unlike most fungi, powdery mildew does not require free water to germinate, only a high level of humidity. Under optimal conditions, this fungus can produce a new generation every 72 hours.

Spores are then spread by the wind, so lower humidity unfortunately encourages the spread. Infections can increase in periods of heavy dew. Powdery mildew is usually more severe in shady areas and areas with poor air circulation. This disease occurs all season long, but is less common during the heat of summer, unless you’re in an area with consistently high humidity, especially in the evenings.

Where Is Powdery Mildew Found?

Powdery mildew is one of the most common diseases in the garden as there are approximately 11,000 species of this fungi. They’re similar, but different species target different host plants. In other words, if you find powdery mildew on roses, it should not be cause for concern for nearby zinnias. However, both plants may become infected with powdery mildew if conditions are favorable for development of this disease.

Powdery mildew appears on many ornamental plants, including everything from azaleas and rhododendrons, to dogwoods and lilacs. It infects flowers like roses, beebalm, zinnias, along with oak trees, English ivy and many vegetable crops.

Tips For Prevention

Preventing powdery mildew begins with:

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