Sooty Mold

March 19, 2005

By Mary Giambalvo,
Master Gardener

It is not enough we gardeners have to deal with aphids and whiteflies and o
ther sucking insects on our plants.  Often we are also left with fruit and leaves and garden furniture that look like they spent time in a coal scuttle.  This is sooty mold, a calling card from those insects.

Sooty mold is actually a fungus that grows on honeydew.  Honeydew, not
the melon variety, is a sweet substance that aphids, whiteflies, scale, psyllids, mealy bugs and other insects with sucking mouth parts excrete after slurping out sap from plants.  This syrupy substance, sticking to plant parts or dripping down on patio furniture, is an ideal growing ground for sooty mold.

Is it ugly?  Yes.  Is it dangerous to
the plant?  Probably not, if it is present in reasonably small amounts.  A large, heavy infestation of sooty mold fungus, though, can keep vital sunlight off leaves and could make them drop prematurely and stunt the plant.  Fruit coated with sooty mold, however, can be washed carefully and eaten safely.

The important way to get rid of or prevent sooty mold from desecrating our plants is to control
the insects that expel the honeydew on which the sooty mold fungus feeds.

This can often be done by hosing down
the leaves when the insects are feeding.  If this doesn't work, one can also use neem oil or insecticidal soap, applying it over a period of several days.

Also, since honeydew is a favorite food of ants,
they will keep the insects producing it in protective custody.  It is important to rid the tree or plant of these pesky jailers.  Applying a coat of a sticky barrier, purchased in garden centers, around the trunk of the tree and trimming any branches that touch other ant thoroughfares will prevent more ants from moving up and guarding the insects.  Baits can be used if they are not in reach of children or pets.

the ants are gone and the sucking insects have been eaten by good bugs, have moved on or have perished from the sprays, the sooty mold will slowly disappear.  If you can't wait, try soap and water for the branches you can reach.

While sooty-black plants and garden benches are less than savory to
the eye, there are worse things for gardeners to face.  Let us hope to avoid those, too.

For more information on sooty mold, call or email and ask for Pest Note 74108.  For fur
ther information on ant control, ask for Pest Note 7411.

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request .Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM.  You may also call the Paso Robles office at 237-3100 on Wednesdays from 9 AM to 12 PM.  The San Luis Obispo Master Gardeners website is at Questions can be e-mailed to: