What is the White Powder on My Plants Leaves?

White powdery substances on plants can be a cause for concern for amateur and seasoned gardeners. White powder is a symptom of two common problems, the fungal disease powdery mildew and woolly aphids. Early identification and control are vital for treating these disorders.

Powdery Mildew

Causes

Powdery mildew if a fungal disease of indoor and outdoor plants caused by a variety of fungi species. Powdery mildew fungi do not require free moisture on plants leaf surfaces for germination. The fungus favors moderate temperatures between 60 to 80 degrees F for infection, shady growing areas and high humidity levels, reports the Ohio State University.

Symptoms

Symptoms of a powdery mildew infection consist of white powdery growth on leaf surfaces, leaf distortion, leaf death and leaf yellowing. The disease typically occurs late in the growing season and does not cause the serious symptoms listed above. However, severe infections warrant control and can cause serious aesthetic damage and reduce plant vigor.

Control

The best control for powdery mildew is to prevent disease spread and infection. Indoor and outdoor plants grown in close proximity to one another, especially in shady areas are at risk for powdery mildew infections. Because powdery mildew is a disease of nearly every ornamental plant, preventing favorable conditions is vital for control. Control for mild to moderate infections consists of doing nothing, spraying the fungus with a water hose to dislodge it and physically wiping the dust off leaf surfaces. However, fungicidal control is effective for severe infections.

Woolly Aphids

Causes

Woolly aphids are an insect pest of ornamentals, shrubs, hardwood trees and conifers. The aphid uses tiny mouthparts to suck nutrients from plant tissue, which can cause mild to severe symptoms on infected plants. The insect requires two hosts for infection, one that they overwinter and lay eggs in and another that they feed off nutrients during the summer months.

Symptoms

Symptoms of a woolly aphid infection consists of distorted leaves, leaf curl, leaf yellowing, reduced vigor and branch dieback, according to the University of Minnesota Extension. Woolly aphids measure about 1/8 inch long and have a fluffy fur appearance. Woolly aphids from a distance may appear as a white powder on leaves and branches, but upon closer inspection, you will notice the insect.

Control

Mild to moderate woolly aphid infections are not a cause for alarm. The aphid, like other aphid species, secretes honeydew when feeding on plant nutrients. This can cause sooty mold growth, which can cause reduced vigor and aesthetic damage. The University of Minnesota Extension website reports that systemic insecticidal control is effective for controlling severe woolly aphid infestations. However, natural predators typically manage populations well enough where insecticidal control is impractical.

Sources:

http://ohioline.osu.edu/hyg-fact/3000/3047.html

http://plantclinic.cornell.edu/FactSheets/powdery/powdery.htm

http://www.extension.umn.edu/yardandgarden/ygbriefs/e453woollyaphid.html