How to stop a yucky black mess

Citrus leaves show sooty mold growth. Scale, aphids and other sucking insects

produce the honeydew that the fungi grow on. (Photo courtesy UC Integrated Pest Management)

Sooty mold forms on honeydew left by sucking insects

By Debbie Arrington

What's this yucky stuff all over my oranges? Icky black gunk coats the leaves of the crape myrtle, too. And ants seem to love this sticky mess.

It's the curse of the sooty mold. Expect to see a lot of it this fall.

According to the UC Cooperative Extension master gardeners, sooty mold isn't one fungi but an assortment, depending on the plant and the insects involved. They all have one thing in common: Honeydew.

Not the melon, but the sugary secretion deposited by aphids and other insects including leafhoppers, whiteflies, soft scales and mealybugs. These insects feed on plants, and excrete honeydew as waste. This honeydew sticks to everything -- leaves, twigs, flowers, fruit, trunks, even lawn furniture and pavement.

When the weather is right (like now), black fungi starts forming on the honeydew. That's the sooty mold.

Ants love honeydew and further complicate the situation. They'll herd aphids on to plants, and harvest their honeydew to feed their nest. The more honeydew, the more ants -- and the more sooty mold.

The mold itself usually doesn't do much harm to the host plant, according to the UC integrated pest management pest notes on sooty mold. If particularly heavy, it can interfere with the leaves' ability to photosynthesize, depriving the plant of food and energy to grow. Heavily coated leaves will die and drop off early.

On fruit, sooty mold can be washed off with a little soap and water. It doesn't harm the interior of citrus, apples or other fruit, which is still edible. Likewise, vegetables coated with sooty mold can be washed and eaten. But it can make a major mess on patio furniture, pavement and any car parked under an infected tree.

The solution is prevention. Sooty mold needs honeydew, which means sucking insects are at work. Control the little suckers and you get rid of the mold.

That means being observant. Watch out for aphids, whiteflies and other insects that create honeydew as well as ants that may introduce them to a plant. By stopping them before they create a major infestation, you can stop the honeydew-sooty mold cycle.

For more information and tips, consult the UC IPM Pest Notes: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74108.html