holly shrubs

Asked November 10, 2016, 1:11 PM EST

While trimming my holly shrubs I noticed a black film on the woody stems. Is this normal or do I need to do something? Thanks, Ruth

Jefferson County Kentucky holly bushes

1 Response

It sounds like sooty mold. Most likely due to a scale infestation; look for small, white or gray ovals on the stems or underside of the holly leaves.

UK plant pathologists say this, "Sooty molds are dark‑spored fungi which grow on the sugary “honeydew” excretions of certain insects. Aphids are the most common honeydew producers, but other sucking insects (such as white flies and scale insects) may also leave honeydew deposits. Honeydew may drip from the site of insect activity onto objects or plants below (e.g. cars, houses, signs, ground covers). The dark‑colored fungi growing on these excretions give plant surfaces or other objects a black, sooty appearance. Sooty mold fungi derive their nutrition from the honeydew and not from the plant. When sooty mold growth is heavy, the shaded tissue may turn yellow. Sooty mold growth can be prevented by controlling the insect or insects responsible for leaving honeydew deposits. While the black growth may be considered “unsightly”, it will rarely endanger the plant’s health."

Scale are insects that have a shell-like covering under which the insect feeds off of plant sap. To control scale, dormant oil can be used in the winter to coat and smother the insect. insecticidal soaps can be used in the growing season. There are various types of scale, with differing times of egg hatch. You are welcome to bring a branch into the office for identification of the scale to narrow down the timing for spray controls.

Feel free to contact our office if you have other questions.

Let me know if I can help you further!

Carol Wilder
Horticulture Technician
Jefferson County Cooperative Extension Service
200 Juneau Drive
Louisville KY 40243