Sooty Mold

Sooty mold appears as a dark gray to black coating on leaves, stems, and even nearby structures. The fungal growth sits directly on honeydew excreted by scale insects, creating a layer that can completely cover leaf surfaces. You may notice this problem on hazelnut, photinia, or viburnum in the Puget Sound region, especially mid to late summer when scale insect populations are at their peak.

Sooty mold itself doesn't infect plants, but it does block sunlight and can interfere with photosynthesis if coverage is severe. The real issue is the scale insects underneath, so tackling this problem means controlling the insects first. You can wash honeydew off leaves with a strong stream of water or wipe affected branches, but unless you address the scale population, the sooty mold will return quickly.