Ithaca,
NY CA (3/16/98)- A new variant of Phytopthora infestans,
the fungus that caused the Irish Potato Famine of the 1840s, now poses
a major threat to US agriculture, report researchers at Cornell University.
The fungus Phytophthora infestans causes a disease in potatoes
called late blight. Discoloration of the leaves accompanied by downy mildew
is sometimes the first sign. The tubers may also be infected in the field
or in storage, with discoloration of the skin, with a reddish brown dry
rot extending into the tuber. Later a slimy, foul smelling rot may destroy
the tuber.
Graphic: Late blight lesions on potato leaves
Now, more than 150 years after the blight destroyed Ireland's staple
food crop, causing more than a million deaths, along with major emigration,
new strains of the fungus are causing alarm on this side of the Atlantic.
Before 1992 the new late blight strain, US-8, was not found in North
America. Appearing first in New York and Maine in 1992 and 1993, the new
strain spread to 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.
"The impact of these new strains has a devastating effect on the growers.
Reduced yields, increased potato blight during storage and shipping, and
heightened fear of the new aggressive strains of late blight are just some
of the issues they have to deal with. Even trying to keep the fungus under
control causes significant economic hardship because of dramatically increased
fungicide costs. We hope that through the continued research of plant pathologists
worldwide, we'll find a strategy to effectively control late blight," said
William Fry, a Cornell University professor of plant pathology.
The US-8 strain is now the most widely distributed form of late blight
in the U.S. 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 noted.
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.
Two factors compound the problem posed by the fungus. First of all,
it is difficult to detect low levels of late blight in the field. Unless
it is detected early, the fungus does not respond to antifungal treatments.
The second problem involves the lifecycle of the fungus. The late blight
disease cycle of penetration, colonization, sporulation and dispersal can
occur in less than five days. 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.
"Plant pathologists are on the front line of this issue, warning growers
and consultants of the heightened risks posed by the new strains and investigating
improved management activities. Massive educational and international research
efforts are essential." says Fry.
The Irish Famine 150 years ago occurred when unsuspecting farmers planted
their crops and the perfect weather conditions for the potato blight. The
result was the first major late blight outbreak ever recorded.
"Throughout Europe the potato crops failed," says Gail Schumann, plant
pathologist at the University of Massachusetts and author of 'Plant Diseases:
Their Biology and Social Impact'. "The disaster was worst in Ireland because
of the nearly complete dependence of Irish peasants on the potato for their
food. The struggle to find a cure for this disaster actually led to the
birth of plant pathology as a science."
Fry's article, "Re-emergence," appears in Plant Disease (Vol. 81, Number
12, pages 1349-1357, December 1997.) Fry also co-authored an article, "Resurgence
of the Irish Potato Famine Fungus," in the journal BioScience (June 1997).
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