While landscapers are replacing some of the trees, they cannot replace large mature ones, meaning that some homeowners have lost some of their biggest and oldest trees.

“I’m very concerned,” said Amy Frankmann, executive director of the Michigan Nursery and Landscape Association, who has heard from many members and who says the disaster could threaten the livelihoods of landscapers whose insurance will not cover the cost. “Absolutely. One member is looking at having to replace a thousand trees.”

Mark Utendorf, owner of Emerald Lawn Care in Arlington, Heights, Ill., has seen dozens of customers’ trees turn brown. “It’s unfortunate, because the product works exceedingly well on turf,” he said.

“It kills creeping Charlie, and that’s something that’s very hard to kill,” Mr. Utendorf said, referring to a type of ivy that has been known to take over lawns.

He noted that the product had been viewed as part of a more environmentally safe lawn industry and a game changer. “I hope people will give DuPont a chance to make this product work,” Mr. Utendorf said, adding that he was still using it, though very carefully and not where there were conifers.

Imprelis went through about 400 trials, including tests on conifers, and performed without problems, according to experts at DuPont and at the E.P.A. The agency reviewed the herbicide for 23 months before granting its conditional approval, meaning that all of the safety data was not yet in but the agency judged Imprelis to be a good product.

Even if the product is eventually proved to be a tree killer, it is considered unlikely that the E.P.A. will ban it, experts said. The agency would probably work with DuPont to change the herbicide’s labeling or to mandate larger buffer zones, they added.