Potato blight rots N.C. Christmas trees.

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He grows 300,000 Fraser firs on his 15 tree farms, but Mitch Poe frets about phytophthora, the fungus that caused the Irish potato famine in the 1840s, an ocean away from the slopes of Ashe County. "Wherever you go, it's the big buzz in the Christmas-tree industry. It's potentially devastating."

Phytophthora thrives in warm, moist soils. It kills plants by parching them, rotting their roots and preventing water absorption. It's been encouraged by warm winters in recent years and the rapid growth of the state's $100 million Christmas-tree industry.

The disease was virtually unknown in North Carolina until a decade ago -- roughly when Hurricane Hugo soaked western North Carolina and winter temperatures began to soar. Even now, Fraser firs in the state's higher, drier, cooler elevations -- above 5,000 feet -- are unaffected. But plant scientists say 7% -- roughly 400,000 -- of the state's 6 million trees already are, and the worst is yet to come.

The fungus needs more than mild weather to spread, and the industry inadvertently gives it a lift. The disease hitches rides in soil that clings to the roots of transplanted seedlings, tractor and truck tires and even the soles of thousands of lucrative, cut-your-own customers' footwear. "We don't have anything that'll kill it," says Poe, who has surrendered one...