When their plants and trees began withering in early 1990, landscape plant- growers throughout Florida used a time-test ed remedy - Du Pont's fungicide Benlate.

Unknowingly, they were killing their crops with kindness.Many varieties of both ornamental and vegetable plants have shown what a Manatee County nursery's lawsuit against Du Pont terms "inexplicable behavior" - symptoms that growers have discovered are tied to their use of an ''improved" formulation of Benlate.

Benlate, sold by Wilmington, Del.-based E.I. du Pont de Nemours & Co., kills fungi that attack plants in hot, wet conditions. It has been a favorite fungicide of ornamental and vegetable growers for nearly a quarter-century.

At first, no one suspected Benlate of causing the stunted growth, leaf abnormalities and other plant illnesses.

"We couldn't figure out what was going wrong, so what everybody did was

put more (Benlate) on," said Ray Pritchett, co-owner of Dickinson's Wholesale Nursery in Palmetto, Fla.

But, as Du Pont has admitted, the Benlate was contaminated. With what, no one still seems to know. Or, if they do, they will not say.

"It's like an AIDS virus to the plants," said Mr. Pritchett. "It's not like you spray and tomorrow it's dead. But three weeks later, 70 out of 150 plants might start to do weird things."

(Du Pont, in releasing its 1991 earnings, said it took a $343 million charge for the Benlate re call.)

While many local ornamental growers experienced Benlate damage, only a few vegetable and citrus growers have complained about their crops, said Phyllis Gilreath, extension agent with the Manatee County Agriculture Extension Service.

No connection has been made between the Benlate contaminants that may have been used on vegetables and any health problems in people.

Contaminated batches of Benlate have not been sold commercially since March, Du Pont says.

"We don't know what it is," said Morris Bailey, Du Pont's Benlate team leader. "We haven't been able to reproduce the symptoms."

Benlate wrought havoc with farmers' livelihoods nationwide.

Growers in 40 states have claimed compensation from Du Pont, with 70 percent of the claims coming from Florida, Mr. Bailey said.

The chemical has also hurt nursery suppliers - the businesses that sold Benlate to growers - but Du Pont is reimbursing them for credit ing customers for the recalled product.

Since taking the product off the market, Du Pont has worked to set tle claims before they become law suits, assigning Atlanta-based catastrophic claims adjuster Crawford and Co. to contact growers who have complaints about the product.

Du Pont has acted responsibly by paying some growers for damages, providing interim assistance money and arranging for contaminated plant material to be shredded and burned, said Earl Wells, executive vice president of the central Florida-based Florida Nurserymen and Growers Association, but he added that many growers were not satisfied with what they considered only partial settlements.