PROVIDENCE — A strain of the fungus that caused the Irish potato famine of the 1840s is on the loose in Rhode Island and is damaging potato and tomato crops.

The state Department of Environmental Management’s Division of Agriculture Friday advised gardeners, farmers and commercial growers that the outbreak of late blight, scientific name Phytopthora infestans, should be countered with an application of fungicide. All tomato and potato crops — commercial and homegrown –– should be viewed as potentially suspect for the disease, and measures should be taken to monitor for and control the disease, the DEM said in a news release.

The DEM described the disease as “a serious threat to agriculture,” and said it will determine whether additional regulations or quarantine are necessary.

William Fry, a Cornell University professor of plant pathology, said in a posting on the university’s Web site that the new strain is causing “widespread crop devastation” throughout the United States and Canada. He said the fungus is doubly dangerous because it is aggressive and resistant to protective measures.

Fry, who published a paper on the subject with Stephen B. Goodwin, a U.S. Department of Agriculture researcher at Purdue University, in the journal Plant Disease, said the strain hit New York and Maine in 1992 and 1993 and is now found in Idaho, Texas, Colorado, Nebraska and South Dakota.

Accuweather.com said on its Web site that this outbreak is the “earliest and most widespread,” and that it has prompted the removal of tomato plants from stores in New York and New England. Accuweather.com said this season’s wet weather has helped spread the late blight spores.

The Web site said plants have been removed from Home Depot, Wal-Mart and Kmart stores, and said the disease has been found in all Atlantic coastal states except for Georgia. The site predicted that the price of tomatoes and other vegetables will rise as growers seek to cover the cost of treating the disease.

The DEM said that late blight’s most common early symptoms on tomato transplants are brown lesions on stems, with white fungal growth developing under moist conditions. Classic symptoms are large [at least nickel-sized] olive-green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid [early morning or after rain]. Sometimes the lesion border is yellow or has a water-soaked appearance. Leaf lesions begin as tiny, irregularly shaped brown spots. Brown to blackish lesions also develop on upper stems. Firm, brown spots develop on tomato fruit. If the lesion has a yellow border and is occurring on the bottom of the plant, it is likely due to infection of either early blight or Septoria leaf spot, two common diseases found in home gardens.

The University of Rhode Island’s Cooperative Extension advised commercial growers to apply fungicides, because if left untreated late blight “would mean certain death of their crops.”

The Cooperative Extension urged gardeners to act quickly to ensure that garden-grown tomato and potato plants don’t become a source of spores that could infect commercial farms, as late blight spores are easily dispersed by wind.

According to the Cooperative Extension, here are the steps home gardeners should take:

• Inspect tomato plants daily.

• If symptoms are already appearing on plants, remove them. Place them in a secured plastic bag and discard them in the trash or completely buried 2 feet underground so plants will decompose and not re-sprout. Do not compost plants, put them on a cull pile, or leave them outside.

• Begin spraying fungicide now — even before symptoms appear — and continue spraying regularly. Use a product that contains chlorothalonil. These products are only effective if used before the disease appears, and should be reapplied every five to seven days if wet weather persists. Chlorothalonil is a protectant fungicide, with no systemic movement in the plant, so thorough coverage is necessary.

• For organic farmers and gardeners, the options are very limited, since only copper fungicides can be used, and copper is not very effective on late blight. It is easily washed off by rain.

The Cooperative Extension said that most other vegetables, such as lettuce, eggplant, peppers, squash, carrots, green beans and broccoli, are not affected by the fungus.

Further information for commercial growers is available from Heather Faubert at the URI Cooperative Extension at (401) 874-2967. Home gardeners may contact the URI Master Gardener Hotline at (800) 448-1011.

tmorgan@projo.com