October 2016 Plant Disease Advisory

The artillery fungus is common in home and urban landscapes where it decays organic debris and is most commonly associated with wood mulch. The genus of this fungus is Sphaerobolus, which translates to “sphere thrower”, and gives a clue regarding its unique adaptation. If not for the production of the sticky spore masses that it discharges forcibly about 15’ high and 20’ distant, this fungus would likely be overlooked because its fruiting body or spore-producing structure is less than 1/20 of an inch in diameter.

Artillery Fungus
The tiny dark objects on this rhododendron leaf are the spore masses or peridioles of the artillery fungus. (Photo credit: Bruce Watt, University of Maine, Bugwood.org)

Each year the Virginia Tech Plant Disease Clinic receives inquiries from clients anxious about numerous, tiny, dark dots that adhere to the siding of homes, vehicles or plants. These strongly adhesive “peridioles” are the spore masses of the artillery fungus (Fig. 1), which are most often projected toward light surfaces. The sticky peridioles do not harm plants. At best, they are a cosmetic problem on plants, houses, and vehicles and, at worst, when peridioles cannot easily be removed, they cause a stain or damage to the finish upon removal. Last year a diagnostician in the Plant Clinic experienced repercussions from the artillery fungus when a stump near her driveway was ground up and used for mulching a bed near her driveway. Subsequently the wood mulch became an ideal habitat for the artillery fungi. She easily identified the tiny, dark dots on her car, but removing them was another matter. 

There are no fungicides registered to control this fungus. The best approach for the artillery fungus is avoidance: locate any wood mulch away from objects that you do not want subjected to “sphere throwing”. If you must mulch, you may want to use synthetic mulch, which is not subject to habitation by the artillery fungus. Mulch containing mostly bark (e.g. 85%) vs. wood deters colonization by the artillery fungus, but does not guarantee the artillery fungus will not appear. The internet is full of information on this fungus, along with lots of anecdotal approaches for peridiole-removal; however, try them at your own risk!