The Latest Dirt - July 2024

Black Dot Mystery Perplexes Help Desk Team

By Susan Heckly

Client’s conundrum
In May, a client from Trilogy in Brentwood called and described some sort of pest all over a neighbor’s walls, windows, and plants. She described it as small black dots of stuck-on material. She had been to several nurseries and talked to “master gardeners” at those nurseries who had no idea what it was, even looking at it under magnification. She said it stuck to walls so securely that it damaged the wall underneath when they tried to remove it.

The client also sent some camellia leaves with the black dots on them so we could examine the dots, but the envelope didn’t reach the Help Desk for over two weeks.

Help Desk response
While talking with the client, I thought this sounded like it might be artillery fungus, something I learned about somewhere in the long-ago past. I looked at websites from several universities (I couldn’t find anything from UC while I was talking with the client), and they accurately described the damage. See the links below.

Emma Connery looked at the samples under our microscope and found the black dots to be pretty uniform throughout. When flipped over, they clearly showed that these were not insects.

Even after doing more internet sleuthing, I could find nothing from UC IPM on this fungus, but there is a lot of information from the Midwest and Eastern states. Clearly, it’s a bigger problem in those areas, but also clearly, we have it here in Contra Costa.

Artillery fungus (Sphaerobolus stellatus), also called shotgun or cannonball fungus, grows on moist wood mulch or other rotting wood in cool weather (below 75 degrees). Some sources said the low-cost recycled wood mulch made from chipped pallets is more likely to harbor this fungus. Tiny mushrooms that look like birds’ nest fungi open and shoot their even tinier (only 2mm in diameter) sticky black spores far and wide. Researchers found the spore mass could shoot as far as 6 meters at a speed of 10 meters per second! The artillery fungus shoots its spores toward sunlight. In shadier backyards, they will shoot toward bright reflective surfaces such as white house siding or a white car, where the spores will be easier to see than if they land. Artillery Fungus, also known as shotgun or cannonball fungus, grows on moist wood mulch or other rotting wood in cool weather.

The solution is to remove the wood mulch and replace it. To prevent a recurrence, bark chips or gravel can be used. A thick layer of fresh wood chips on top of the old mulch can also work. Or you can learn to love this fungal marvel.

For more information about artillery fungus:

University of Wisconsin:
https://botit.botany.wisc.edu/toms_fungi/jul2005.html
Michigan State University:
https://www.canr.msu.edu/resources/artillery-fungus-sphaerobolus-stellatus
JGI Fungal Genomics Resource (a UC site):
https://mycocosm.jgi.doe.gov/Sphst1/Sphst1.home.html