A prominent South Florida horticulturist believes he has identified - and devised a cure for - the bacterium that threatens to wipe out the palm tree.

James K. Dunaway and his microbiologist partner, Dana Mead, said they consistently have seen a particular bacterial strain, Pseudomonas aeruginosa (PSA), in palm trees that have died from a disease known as "Lethal Yellowing."

Dunaway estimated that two-thirds of the world's palm trees and 90 percent of all palms in South Florida since 1971 have been affected by this disease.

Mead and Dunaway have been experimenting with a chemical compound that, when sprayed on afflicted trees, has cured 99 percent of them, they say.

Scientists from the University of Florida Agricultural Research Center doubt their findings, but the Dade County Department of Recreation and Parks is allowing Dunaway and Mead, at their own expense, to spray their formula on 200 sick trees in a park on Virginia Key a few miles from Richard Nixon's former winter White House in Key Biscayne.

"The question is, how long will the good effect last? Will it be temporary, or will it actually sustain the trees?" asked Richard Jones, a spokesman for the county.

"But it's worth a try. The disease is just as great as ever. We're continually removing dead trees from our parks," Jones said.

Mead said about half the species of palm trees in the state - 50 - have been affected by the epidemic, and every year another two or three species succumb.

"The palm epidemic in South Florida is probably the worst disease in the history of horticulture," Dunaway said. "There have been diseases in soybeans, wheat, and other crops, but a disease doesn't usually affect more than one or two species."

Florida just doesn't look the same without as many palms. It looks like a lunar landscape down here," Mead said. "Dade County has been denuded of its palms."

Only skeletons and stumps remain on some streets that until recently were lined with palms. Elsewhere, still-living palms show drooping branches and brown leaves. Coconuts are hard to find.

The time between onset of the disease and death of the tree can range from six months to six years.

The university's Institute for Food and Agricultural Studies in Fort Lauderdale has maintained that a microorganism called mycoplasma has been the primary cause of Lethal Yellowing. However Dr. Henry Donselman said a second disease might exist that the Dunaway-Mead formula cures.

"There might be two diseases," Dunaway said. "But we don't care. Our formula works on all the species of palms we tried."

In addition, Mead said they have checked for mycoplasma colonies in diseased palms and have never found any.

The university spent millions of dollars, according to an official, trying to eradicate the palm blight by injecting tetracycline, an antibiotic, into thousands of trees. But four years later, in 1977, the university concluded that the disease was not controllable and scaled down the campaign, said Lewis Watson who works with both the university research center and Broward County.

"We found that all we were doing was delaying the inevitable," he said. CAPTION: Picture, Miami trees cut after being blighted by a disease called Lethal Yellowing.