CONCORD, N.H. (AP) — Tomato plants have been removed from stores in half a dozen states as a destructive and infectious plant disease makes its earliest and most widespread appearance ever in the eastern United States.

Late blight — the same disease that caused the Irish Potato Famine in the 1840s — occurs sporadically in the Northeast, but this year’s outbreak is more severe for two reasons: infected plants have been widely distributed by big-box retail stores and rainy weather has hastened the spores’ airborne spread.

The disease, which is not harmful to humans, is extremely contagious and experts say it most likely spread on garden center shelves to plants not involved in the initial infection. It also can spread once plants reach their final destination, putting tomato and potato plants in both home gardens and commercial fields at risk.

Meg McGrath, professor of plant pathology at Cornell University, calls late blight “worse than the Bubonic Plague for plants.”

“People need to realize this is probably one of the worst diseases we have in the vegetable world,” she said. “It’s certain death for a tomato plant.”

Tomato plants have been removed from Home Depot, Wal-Mart, Lowe’s and Kmart stores in all six New England states, plus New York. Late blight also has been identified in all other East Coast states except Georgia, as well as Alabama, West Virginia and Ohio, McGrath said.

It is too early in the season to know whether infected plants will taint large crops or negatively affect commercial growers. But if that happens, growers could be forced to raise prices to cover costs associated with combating the disease.

Agriculture officials in the various states still are trying to determine where the outbreak started. One major grower, Alabama-based Bonnie Plants, supplies most of the tomato plants to big-box stores, but it is unclear whether the plants were infected before or after leaving the supplier’s multiple greenhouses.

“There’s no way in the world you can pin this on one plant company, but we just happen to be the biggest,” said Dennis Thomas, the company’s general manager.

The company has regularly inspected greenhouses in 38 states, including Maine, New Hampshire and New York. Its most recent inspections — in New Jersey and Pennsylvania — found no evidence of disease.

“We’ve not been written up one time for any late blight disease that was confirmed,” Thomas said, noting that Bonnie Plants sprays seedlings before shipping them to stores, but that doesn’t happen after the plants arrive. He said the company was proactive in removing plants once the outbreak occurred.

In the meantime, plant experts are warning gardeners to be on the lookout for the disease and to take quick action if it crops up. The first sign is often brown spots on plant stems, followed by nickel-sized olive-green or brown spots on the tops of leaves and fuzzy white fungal growth underneath. Tomato fruit will show firm, brown spots.

Spraying with fungicides can control late blight if begun before symptoms appear, but many plant experts recommend removing and destroying the plants instead to prevent spores from traveling.

Donald Flannery, executive director of the Maine Potato Board, said the state’s potato farmers are concerned, but not in crisis mode.

“It’s pretty easy to make our growers aware of it, that’s the simple part. But what we’ve started to do is really reach out to home gardeners throughout Maine to ask them to be very diligent about checking their tomato plants or potato plants,” he said.