Fly Specks on the Window or the Cannon Fungus Strikes Again
General Principles Information Note 1 (GPIN-001)
L.F. Grand, Plant Pathologist
R.K. Jones, (Retired) Extension Plant Pathologist

The Problem...

In recent years there have been an increasing number of complaints aobut small (less than 1/10 inch), dark brown to black, "flyspeck-like" objects that stick tenaciously to house siding, windows, cars and a variety of other surfaces. Removal of these sticky objects has proven quite difficult, especially from light-colored cars where attempts to remove the spots have resulted in a marring of the paint finish. Speculation on the identity and source of these structures has implicated insects, fungi and even tar splashed from road resurfacing activities.

Small, dark "eggs" stick to the surface of this house.

The Source...

The culprit is a fungus, aptly described as the cannon fungus, Sphaerobolus stellatus. The fruiting bodies of this fungus are 1-3 mm (0.04-0.12 inches) broad, roundish in shape and off-white to a buff or orange-buff color. At maturity the fruiting body splits in a star-like pattern exposing a dark brown, roundish "cannon" or "egg" (termed a peridiole) that is 1-2 mm (0.04-0.08 inches) broad and contains the spores of the fungus. This structure is forcibly ejected or shot from the fruiting body, hence the name "cannon" fungus. This fungus is also called the "sphere thrower" and the "artillery fungus". The "egg" which may be ejected up to 14 feet from the fruiting body has an oily or sticky surface that enables it to adhere to most surfaces it encounters. Once stuck to a surface the "egg" dries to a disk shape and adheres tenaciously. Removal of the "egg" often leaves an oily stain or discoloration on the surface. Fruiting body development is correlated with high moisture and temperatures in the 70s and low 80s degrees F (10-20 degrees C). A change in temperature to 90 degrees F will stimulate ejection of the "egg" from the fruting body. The fungus also is phototrophic and the "egg" is shot toward a light source.

Upclose, you see distinct, small brown/black, hard structures that are hard to pick off from the surface. The fungus is not picky on where the "eggs" land.

The Habitat...

Sphaerobolus stellatus grows primarily on decaying woody substrates (especially wood chips), twigs, corn cobs, and dung. The fungus can also be found on rotted wood in greenhouses and on wood chip mulch and some bark mulch in potted indoor plants. The apparent increase in occurrence of this fungus is attributed to the increased use of wood chips as mulch in landscapes. Under the proper environmental factors large numbers of fruting bodies can develop over a substantial area of wood chip mulch in a landscape. Conditons of high moisture and temperatures in the 70-80 degrees F range that favor fruting body development (growth) occur in the spring and fall.

The Control...

Avoid using wood chips as mulch when this problem occurs; pine or hardwood bark mulch does not seem to harbor the fungus as much and can be used as an alternative. Spores are known to survive up to 11 years so covering mulch which contains fruiting bodies with fresh wood chips may only solve the problem temporarily. No fungicides are recommended nor labeled for this use.

Other Links

Plant Disease Information Notes Home Page
North Carolina Insect Notes
North Carolina Agricultural Chemicals Manual
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For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service