Little white "flies"

I live in the Appalachian Mountains of NE Tennessee/ NW North Carolina/ SW Virginia. Each year my plants become infested with tiny little white "flies" (for lack of a better name). These flies tend to accumulate and reproduce on the underside of the leaves and turn the leaves black and kills them. They also cause the skin of the pumpkins to be black but I'm able to clean that off. I spray the leaves well but the flies are very difficult to kill and they invariably come back just as strong within 2-3 weeks. Anyone have an idea of what they are? 
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