Grease may not be the word, after all, as Joe Ponessa, emeritus professor of housing, indoor environment, and health at Rutgers Cooperative Extension, sees it.
Ponessa was responding to a recent column in which a reader reported what looked like "light grease stains on a portion of vinyl siding in the rear of the house."
The professor says: "If the 'grease stains' on siding are not slippery but in the form of small, hard black dots, they are most likely spores from artillery fungus."
The fungus, which grows in organic matter, forms vase-shaped structures that are capable of firing out spores six feet or more, and that are especially attracted to light-colored surfaces. They are notoriously difficult to remove, says Ponessa, who adds that he once tried a series of powerful organic solvents and that none worked.
Mulch may be the source of the problem, he says, and it has been suggested that bark mulch may be less problematic than wood mulch. The best prevention is to use gravel or black plastic as a mulch near buildings (or where cars are parked).
An Internet search brings up informative fact sheets from the Cornell and Penn State Cooperative Extensions, he says.
The latter lists numerous reader suggestions that might be helpful, and information from a professor there who has studied this issue (http://www.personal.psu.edu/ddd2/index.html.)
I wrote about artillery mulch a few years back, and it was then that I decided to use mulch only when it doesn't come into contact with my house.
From Cornell University's Plant Disease Diagnostic Clinic come these details:
"Artillery fungi spores do not normally structurally damage the houses, cars, and plants they cover."
"Removing the fungi is virtually impossible. Scrubbing and scraping with tools or washing with soap and water aids somewhat in removal; however, the use of tools or harsh chemicals may damage painted or otherwise colored surfaces."
"No fungicide treatment is recommended at this time."
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