Black Mildew on a Climbing Hydrangea

Lady beetles eat aphids and scale insects.

Climbing hydrangea (Hydrangea anomala petiolaris) is a clinging vine that uses specialized roots along its stems to grab hold of vertical surfaces. Although it does not look like other hydrangea species, such as bigleaf, oakleaf and paniculata, it is susceptible to the same insect pests and diseases. Powdery mildew appears as white patches on leaves, but it is sooty mold that appears on leaves as a black, powdery coating.

Sooty Mold

Commonly known as black sooty mold, Capnodium is a fungus that hinders photosynthesis by spreading across plant leaves and shading them from the sun. Sooty mold retards plant growth and reduces plant vigor. The fungal spores grow on a sweet, sticky substance called “honeydew,” which is excess plant sap excreted by aphids and scale insects. The appearance of sooty mold is an indicator that climbing hydrangea is hosting insect pests.

Aphids and Scale Insects

Aphids are soft-bodied insects that pierce plant parts with their mouthparts and feed on sap. They begin attacking plants in spring when tender new growth emerges. They rarely kill plants but severe infestations can compromise plant health and, left unchecked, can kill them. Scale insects can be soft-bodied or armored. They form protective coverings under which they live and feed.

Leaf Spot Fungus

Cercospora leaf spot initially might be mistaken for sooty mold. It is a disease that leaves dark spots on hydrangea foliage. The dark spots are either dark brown or purplish instead of black, depending on the hydrangea species infected. Although it is not a mold or mildew, it may give a similar appearance in its early growth stages. Cercospora leaf spot can spread rapidly among hydrangeas, although it rarely leads to plant death.

Sooty Mold Control

Woody stems of climbing hydrangea are usually tough enough to withstand streams of water directed at aphids to dislodge them. Since the blasted-off aphids do not find their way back on plants, this treatment can be repeated daily until populations are under control. The protective coating on scale insects is impervious to many pesticides. Horticultural oils and insecticidal soaps suffocate and kill the insects. Label directions will indicate if a specific product is safe to use on hydrangeas. Removing the insects removes the food source for sooty mold and it eventually disappears.

References

• University of Florida IFAS Extension; Black, Sooty Mold on Landscape Plants; Alicia R. Lamborn; 2009
• Mississippi State University Extension Service; Hydrangeas for Mississippi Gardens; Dr. Wayne C. Porter
• North Carolina Cooperative Extension; Sooty Molds; Steven D. Frank, et al.; November 2002
• University of Missouri Extension; Aphids, Scales and Mites on Home Garden and Landscape Plants; Bruce A. Barrett; June 2008