ANNAPOLIS, AUG. 11 -- Maryland's oyster harvest this year will be the worst ever as a result of the unexpected spread far up the Chesapeake Bay of a deadly parasite, state officials said today.

"It means probably an oyster disaster this next season," which starts in September, said state Sen. Gerald Winegrad (D-Annapolis).

Up to 80 percent of the state's oysters will likely die this year from the disease MSX, or multinucleate sphere unknown, according to Winegrad, who learned of state test results during a boat tour on the bay with state Department of Natural Resources officials.

MSX is a parasite that infects an oyster's intestines and digestive system, killing it within five weeks, said George Krantz, a state scientist at the Oxford Biological Laboratory.

Krantz said tests of the state's 700 oyster beds found that only about 10 percent are free of MSX.

Krantz said he was shocked by a study the last two weeks during which he and others tested almost every oyster bed in the state.

"The thing that surprised me was how rapidly it spread up the bay," Krantz said. "They're dying like flies."

Biologists thought the disease would not spread as far as it has because it usually thrives in saltier water than that in the upper bay.

The dry weather of the last two years may have spurred the recurrence of MSX, Krantz said. Without rainfall, upper bay waters are not diluted and have higher salinity.

The latest development is particularly disheartening because watermen counted on the good season of 1985 to produce a large crop of mature oysters this year.

Last year, for the first time in 50 years, the bay yielded fewer than 1 million bushels of oysters.

The federal government gave $1.43 million in emergency funds and the state legislature appropriated about $2 million to be used this fall to rejuvenate the population.

The state also is experimenting with an MSX-resistant breed that scientists have developed in the last 20 years. It is not completely resistent to MSX and not at all to a fungus disease called dermo.