Grapefruit mold problem

My grapefruit tree has a bit of a problem; it appears to have an attack of sooty mold (http://aggie-horticulture.tamu.edu/Citrus/l2315.htm#sootymold ). On fairly close inspection, there don't appear to be any insects on the tree that would excrete the honeydew the mold feeds on.
So, two questions; If it IS sooty mold, what else might be causing it? If it ISN'T sooty mold, then what on earth is it???
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A friend of my son's gave me a dozen from her tree in St. Petersburg, FL. They all looked scuzzy grey on the skin. It washed right off and didn't affect the taste one bit.
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Betsy
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