An unusual blue mold, nurtured by the long, cool spring and early summer temperatures has infected tobacco leaves along the East Coast, causing millions of dollars in damage to crops in Maryland, Virginia and North Carolina.

"What's good for humans is not so good for tobacco,." Furney Todd, a professor of plant patholog at North Carolina State University, said yesterday from the burley country of north-western North Carolina, apparently the hardest hit state.

"We've had blue mold outbreaks in 1954 and 1963, but this is far the worst epidemic we've ever had of field mold," Todd said. He estimated crop damage at more than $20 million and losses in some countries at 50 to 60 percent.

Damage to southern Maryland's $37 million tobacco crop, most of which is exported to Switzerland, could reach $2 million, according to Claude G. Mickee, head of the University of Maryland's experimental tobacco farm near Upper Marlboro.

"I think you can find some [mold] on every field in southern Maryland," Mickee said. "There are some right severe cases."

James L. Jones, Mckee's Virginia counterpart at the Blackstone Southern Piedmont Center, said the loss in his state "would probably be in the million [of dollars.]."

"We had it reported in most every county growing tobacco in the state," said Jones. Virginia's 1978 harvest amounted to 178 million pounds, compared to 30 million pounds in Maryland.

The harmful fungus travels in spores, carried by northerly winds. It usually arrives earlier in the tabacco growing seasons to attack the plants while they are still in their seedbeds. This is a normal problem for which adequate controls exist, tobacco experts said.

The cool, damp weather this year delayed the arrival of the spores, which settled in the plants after they had been moved into the field. The result was a bluish mold on the underside on the plants' lower leaves, causing spotting and yellowing of the tops of the leaves.

The disease has gradually worked its way north along the East Coast since May, but appears to have been halted by the hot temperatures of the last two weeks, the tobacco specialists agreed.

Mickee said the fungus caused the loss of a leaf or two from stalks which generally contains 18 or 20 leaves. "In some cases, it appears to be killing real young plants," he said.

"The tobacco plant has a way of compensating," said Jones. "The leaves left will probably be larger. The weight might be better in what is left. "It's very difficult to tell."

In North Carolina, the disease destroyed most of the bottom four leaves, according to Todd. "It looks like the top and middle parts of the stalks wll be good," he said of the flue-curved variety grown in the southern part of the state.

The air-cured burley variety grown in the cool mountainous sections suffered more damage, he said. "We've never had such an outbreak before," he said. "We weren't expecting it."

In South Carolina, where the disease hit in May, it was a "blessing in disguise," according to Ben U. Kitrell, extension tobacco specialist at Clemson University.

"The growers took the [affected] bottom leaves off," he said. "We'd been advocating that anyway because of the low demand for that part of the stalk." The growers than took advantage of a federal program that allows them to plant 10 percent more acreage than normal to compensate for the loss of leaves.