About 150 horse owners gathered in a Poolesville barn last night and pledged about $12,000 toward finding a cure for the mystery disease that has been killing their animals for the past five summers.

They also won the promise of $4,000 from Montgomery County to fund veterinary field work.

But reports from a group of scientists were bleak: The incidence of acute equine diarrhea syndrome--known as Potomac Fever--is up sharply and the cause remains a mystery.

So far this summer, 80 cases have been reported, 25 of them fatal. In 1982, the first year records were kept, there were 113 cases and 28 deaths. With the exception of one case in Carroll County, Md., all the cases have been in Montgomery, mostly along the Potomac River.

At the meeting on the estate of Austin Kiplinger, editor and chairman of the board of the Kiplinger Report, horse owners formed a steering committee to act as clearinghouse for information.

A representative of Montgomery County Executive Charles Gilchrist, noting Maryland is the country's sixth largest horse-breeding state, with 6,000 horses in Montgomery County, said $4,000 has been allotted to continue research. The fund will pay for two students to assist Dr. Ralph Knowles, state assistant veterinarian, in data and specimen collection and other pathological detective work.

Rep. Beverly Byron (D-Md.) told the gathering that Agriculture Department officials are concerned because the illness has reached epidemic proportions. An expert from the National Animal Disease Laboratory in Ames, Iowa, will be flown in Monday.

The symptoms of the disease, all too familiar to many in the crowd, were described by local veterinarian Chet Anderson. First, Anderson said, the horse stops eating and its temperature soars to 103 or 104 degrees 99.5 is normal for a horse . The horse "looks depressed" and experiences acute diarrhea.

Then it goes into shock and dies.

The hint of a possible breakthrough came from Dr. Stanley Joseph of the Maryland Department of Agriculture, who said one theory is that mosquitoes may be transmitting the disease.

Other scientists cautioned that the disease probably has a combination of causes. Fred Troutt of the Virginia-Maryland Regional College of Veterinary Medicine said, "my guess is that if the cause is found early on it will be a lucky fluke."

Pledges from the horse owners and representatives of organizations generally ranged from $100 to $1,000, with the largest--$5,000--from Herman Greenberg, a race horse owner and Washington area developer.

Cindy Magazian, a Potomac horse owner, called the pledges "a nice start." Her 11-year-old thoroughbred, Golden Native, recently died of Potomac Fever.

"You just hope your horse helps in an autopsy toward a cure," she said.