The fungus responsible for the Irish Potato Famine of the 1840s is back, and could be more threatening than ever. More than 150 years after the famine that took an estimated 1 million lives, a newer virulent strain of the fungus is causing widespread crop devastation in the United States.

Agricultural scientists are finding the fungus, Phytopthora infestans, better known as late blight, difficult to control, says William Fry, a Cornell University professor of plant pathology. Fry believes the new strain of the fungus is a greater threat to potato and tomato crops in the U.S. and Canada than were previous strains because it is resistant to the most effective fungicide, and because the strain is more aggressive.

"Things are worse now than they were a few years ago," says Fry. He and Stephen B. Goodwin, a U.S. Department of Agriculture researcher at Purdue University, warn of the effects of the aggressive new strain of the fungus in an article in the journal Plant Disease.

Before 1992 the new late blight strain, US-8 was not present in the United States or in Canada. In 1992 and 1993, the new strain struck New York State and Maine. By 1995, it was to be found in all eastern states (except Virginia and South Carolina), the eastern Canadian provinces (except Newfoundland), along the entire Canadian border, and in California and Kansas. Last year, US-8 was also found in Idaho, Texas, Colorado, Nebraska and South Dakota.

Scientists say that US-8 is now the most widely distributed strain of late blight in the U.S., and, Fry says, it is also the most problematic because it is resistant to metalaxyl, a generic, commonly used (on late blight) fungicide. This resistance makes suppressing the fungal epidemic more difficult, he says.

US-8 is more aggressive than US-1, the strain responsible for the Irish Potato Famine, which is easily controlled by metalaxyl, according to Fry. The new strain is remarkably rapid and destructive, devastating apparently healthy potato fields within days.

In the article, the researchers explained a two-pronged problem: Why it is difficult to detect low levels of late blight in a field and how the fungus reproduces rapidly. The late blight disease cycle of penetration, colonization, sporulation and dispersal can occur in less than five days. The researchers note that each individual late blight lesion can produce as many as 300,000 sporangia a day. Some infected tubers may be destroyed before harvest, but with a multiplicity of virulent spores, harvested potatoes can easily become diseased in storage. Bacteria that cause soft-rot diseases often invade potato tubers infected with late blight, literally resulting in a "meltdown" of stored potatoes. Under severe infection, entire storages have to be discarded.

Scientists began noticing late blight's resistance to metalaxl as long ago as 1980, Fry says. The fungicide is the only way to salvage crops infected with late blight. Other available fungicides are only effective when applied before late blight strikes. Many agriculturists consider such treatment as ineffective against the disease.

In the article, the researchers note that the short-term response has been to use more fungicide. Other solutions, such as developing disease resistant varieties, they say, could be five to 15 years away from introduction.

The article, "Re-emergence," appears in Plant Disease (Vol. 81, Number 12, pages 1349-1357, December 1997.) Fry and Goodwin also co-authored an article, "Resurgence of the Irish Potato Famine Fungus," in the journal BioScience (June 1997).