A Virulent Strain Of The Fungus That Caused The Irish Potato Famine Is Devastating Crops In North America, Cornell Scientist Says

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

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

Before 1992 the new late blight strain, US-8 was not present in the UnitedStates or in Canada. In 1992 and 1993, the new strain struck New YorkState and Maine. By 1995, it was to be found in all eastern states (exceptVirginia and South Carolina), the eastern Canadian provinces (exceptNewfoundland), along the entire Canadian border, and in California andKansas. Last year, US-8 was also found in Idaho, Texas, Colorado, Nebraskaand South Dakota.

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

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

In the article, the researchers explained a two-pronged problem: Why it isdifficult to detect low levels of late blight in a field and how the fungusreproduces 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 asmany as 300,000 sporangia a day. Some infected tubers may be destroyedbefore harvest, but with a multiplicity of virulent spores, harvestedpotatoes can easily become diseased in storage. Bacteria that causesoft-rot diseases often invade potato tubers infected with late blight,literally resulting in a "meltdown" of stored potatoes. Under severeinfection, entire storages have to be discarded.

Scientists began noticing late blight's resistance to metalaxl as long agoas 1980, Fry says. The fungicide is the only way to salvage crops infectedwith late blight. Other available fungicides are only effective whenapplied before late blight strikes. Many agriculturists consider suchtreatment as ineffective against the disease.

In the article, the researchers note that the short-term response has beento use more fungicide. Other solutions, such as developing diseaseresistant varieties, they say, could be five to 15 years away fromintroduction.

The article, "Re-emergence," appears in "Plant Disease" (Vol. 81, Number12, pages 1349-1357, December 1997.) Fry and Goodwin also co-authored anarticle, "Resurgence of the Irish Potato Famine Fungus," in the journal"BioScience" (June 1997).