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Probably 'artillery fungus' (sometimes called 'shotgun mold'), a fungus that thrives on tree bark and usually shows up after long periods of humid weather on 'most' exterior surfaces, especially in spring. Look at the roof on your house, barn, etc. its probably there too. You cant stop the spores from drifting on the wind; but, I've found that putting a few bare (non anodized) scraps of aluminum sheeting seems to keep it in check .... on roofs, etc.
You can also lightly 'mist' surfaces with caustics such as detergents containing sodium silicate - a modern 'equivalent' to old fashioned 'whitewashing' - but such will quickly destroy any applied 'wax'.
Last edited by RichH; 08-01-2010 at 10:58 AM.