Incoming Spores

Do you have tiny, inexplicable, black spots on your house or car? There is a good chance that you have been "shelled" by a fungus living in your mulch, often referred to as the "Artillery Fungus." This fascinating organism is commonly found in the forest where it helps to decompose fallen trees. However, it can also grow in many kinds of landscape mulch, where it can create a nuisance by "firing" tiny black spore masses that make hard- to-remove, dark spots on house siding and car paint. The fungus produces structures on the surfaces of woody particles in mulch that forcibly eject spore masses (gleba). Oddly enough, the tiny cannons tend to fire their projectiles preferentially toward light, so light colored paint on houses or cars makes a good target. Although the fruiting structures generally only fire gleba a few feet, air currents can carry them as high as the second story of a house.

Dr. Donald Davis, Dr. Larry Kuhns, plant pathologist and horticulturist, respectively, from Penn State University have done extensive studies on the Artillery Fungus (Sphaerobolus). Dr. Davis has published an outstanding summary of the types of questions he commonly fields at this time of year, when the fungus gets cranked up in landscape mulches. See: http://www.personal.psu.edu/faculty/d/d/ddd2/artillery_fungus.html. The most common questions are focused on how to prevent the fungus from developing and how to remove the spots. In a field study of 27 different mulches, Drs. Davis and Kunhns found that Sphaerobolus can grow to some extent in just about any organic mulch. It tends to be most prevalent during cool periods when the mulch stays moist for an extended time. It is often worst on the north sides of buildings. Since it is a wood decomposer, it tends to grow better in pure wood mulches than in bark. Cypress, cedar and pine straw were among the mulches supporting the least growth of the fungus. However, as they decompose, they become more conducive for growth of the Sphaerobolus. Regardless of the type of mulch used, it appears that topping off with a fresh layer of mulch each season is one way to significantly reduce spotting problems caused by the fungus. There are currently no fungicides labeled for control of Artillery Fungus (AF) in mulch and Dr. Davis notes that fungicides may be counterproductive, since they may eliminate other organisms that may serve to inhibit AF. Interestingly, they found that incorporating spent mushroom compost into mulch can significantly suppress AF development.

The general consensus as to removal of the spots caused by AF is that, once the spots have been on the surface for any period of time, it is virtually impossible to completely remove them without leaving behind colored dots. In the above referenced article, Dr. Davis lists some home remedies related to him over the years by homeowners. He stresses, however, that he can not recommend any of these and that they should be tested on a small scale before use on a large area. Apparently, there is considerable debate among lawyers and insurance adjusters as to where to lay the blame when AF strikes. Lawsuits against Mother Nature are rarely successful.

Chris Starbuck
573-882-9630