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Sunday - June 20, 2010

From: Philadelphia, PA
Region: Mid-Atlantic
Topic: Non-Natives, Diseases and Disorders
Title: Powdery growth in hydrangea in Philadelphia
Answered by: Barbara Medford

QUESTION:

My hydrangea plants have a weird growth on their leaves that looks like white rice. It looks like it would be powdery if brushed, but I don't want to touch it for fear that it some type of mold. Any idea what it could be?

ANSWER:

There is one hydrangea native to Pennsylvania, Hydrangea arborescens (wild hydrangea), but we're betting you have a hybrid in your garden. Philadelphia, in the southeast corner of Pennsylvania, is in USDA Hardiness Zone 6b to 7. We think you probably have a form of Hydrangea macrophylla, also known as bigleaf, French, garden or florist's hydrangea. It is hardy to Zone 6, which means it can do well in your area. However, it is a native of Japan, and therefore out of our range of expertise. At the Lady Bird Johnson Wildflower Center we are committed to the growth, protection and propagation of plants native not only to North America but to the area in which that plant is being grown.

The best source we found for information on hydrangeas was The United States National Arboretum Hydrangea Questions and Answer. From that same site, you should read the part on Are hydrangeas bothered by diseases or insects?. Here is an excerpt from that portion that probably applies to your situation:

"For the bigleaf hydrangea, the major disease problem is powdery mildew (see image at right). It is most common on plants growing in shade and under high humidity conditions. Powdery mildew infested leaves are covered with a light gray powdery-looking substance. Purple splotches may also appear. Powdery mildew rarely kills plants, but is unattractive. Powdery mildew may occur on other hydrangea species, but is most severe on bigleaf hydrangea."

To us, mildew doesn't look like rice on a leaf, but it certainly isn't attractive. Another source for information on disease of hydrangeas is this one from the University of Florida Extension Service, Hydrangea macrophylla.

Native Hydrangeas from our Native Plant Image Gallery:

Hydrangea arborescens