Green algae grows on the surface only of leaves, twigs, and trunks. Wet, humid weather favors development of these microscopic, moisture-loving, primitive plants. The colonized holly is not harmed unless sufficient algal growth develops on the leaves and interferes with the photosynthetic process. Algal growth can cause plants to appear somewhat dirty or unhealthy. Typically, however, algae is only an aesthetic concern.

Management Options

Non-Chemical Management

Space plantings and prune to provide good air circulation and reduce humidity.
Avoid overhead watering or limit it to times when foliage can dry quickly.
Do not plant in very damp areas or densely shaded sites.

Select non-chemical management options as your first choice!

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Use as preventative treatments. Do not apply sulfur products after April 1 or on a warm day, or defoliation may occur. Do not apply sulfur products within 2-4 weeks of a horticultural oil application. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Images

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Caption: Green algae on holly Photo by: B.M. Johnson

WSU Extension

Hortsense