Garden Detective

Garden Detective: Mildew left her London planetree frosted

I have a London plane tree that is about 6 years old. It has developed something on the outer leaves of the lower branches. The leaves have white moldy splotches and are curling up. Can you tell me what it is and what to do for it?

Cathy Yokoyama, West Sacramento

That looks like powdery mildew, a common problem for sycamores and their relatives. London plane trees are a hybrid of American native sycamores (which are very susceptible to mildew) and Asian plane trees.

According to UC master gardeners, powdery mildew is a fungal disease. It can become quite common in late summer and autumn, especially if there’s high humidity.

Triple-digit heat usually kills off most powdery mildew infestations, but temperatures this summer were milder than usual. Coupled with recent high humidity, that led to a lot of unwanted white “frosting” on leaves. An outbreak can look like leaves were dusted with powdered sugar, but there’s nothing sweet about this mildew.

Powdery mildew particularly thrives in mild weather with temperatures in the 70s and 80s, making early autumn high time for outbreaks in our area. It tends to attack plants in shady areas, too.

Seemingly overnight, small patches of dusty white “powder” appear on leaves and twigs. The leaves become distorted as the powder builds up. Eventually, the leaves turn yellow and fall.

Within the white patches are black spots that look like pepper. Those are the fungus’ insurance that it can survive the winter. These spore factories survive even on fallen leaves. Those black spots will produce reproductive spores that are distributed by wind. New infections start when the spores land on susceptible plants – such as sycamores and London plane trees. Other plants that often get mildew outbreaks are roses, crape myrtle, begonia, dahlias, mums, zinnias, sunflowers, grapes and lilacs (among many more).

Not all powdery mildew is the same; different species prefer different plants.

Some London plane tree hybrids are resistant to mildew including the Yarwood, Columbia and Liberty varieties.

Treatment of powdery mildew aims at containment. Fungicides need to be applied before any mildew is seen or very early in the outbreak. And a London plane or sycamore tree – which can reach 75 feet tall – may be too big to effectively spray.

The good news: Powdery mildew rarely does significant damage to London plane and sycamore trees. The leaf damage is mostly aesthetic. The infected leaves will eventually whither and fall off. The important thing to remember: Pick them up and discard them – or the mildew will come back next spring.

To prevent a return of this fungi, make sure to thoroughly clean up after your tree this winter. After the tree drops all its leaves, rake them up and discard. Also, change out any mulch under the tree; more mildew spore factories may be lurking in that old mulch.

For more tips on powdery mildew, check out Pest Note 7493, “Powdery Mildew on Ornamentals,” on the University of California’s Integrated Pest Management website, www.ipm.ucdavis.edu. This pest note is also available by mail. Send a stamped self-addressed, business-size envelope to: Pest Note 7493, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827-3823.

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