Elm Disease Bacterium Identified
When 1,000 mature American elms mysteriously died in Illinois
during the last decade, scientists at first suspected that the elm yellows
(EY) plant pathogen was the culprit. The theory made sense because the
disease syndrome was so similar to that caused by the EY phytoplasma,
a cell-wall-less bacterium, which sickened North American elms during
the past several decades.
EY is unlike another tree malady, the fungal Dutch elm
disease. EY occurs in elms native to North America in a region extending
from eastern Massachusetts, New York, Pennsylvania, and New Jersey to
Michigan, Minnesota, Nebraska, and Mississippi. But historically, EY
had been absent from the northern third of Illinois.
Initial tests on the Illinois elms for EY phytoplasma
were conducted by a commercial diagnostic company and were inconclusive.
So what was causing the disease syndrome? To find out,
Agricultural Research Service
plant pathologist Ing-Ming Lee, with the Molecular Plant Pathology Laboratory
in Beltsville, Maryland, in collaboration with scientists from Illinois'
Morton Arboretum, tried another approach. They used a tool Lee developed
to systematically hunt for any phytoplasma that may be present in the
diseased Illinois elms.
Lee's test used polymerase chain reaction and DNA fingerprinting.
The phytoplasma detected in tissue scraped from the elms' bark was found
to be unrelated to the EY phytoplasma (taxonomic group 16SrV-A). Instead,
Lee identified the phytoplasma as representative of a new subgroup (16SrVI-C)
of clover proliferation phytoplasma (group 16SrVI). The carrier, or
vector, of this newly identified phytoplasma is most likely a leafhopper
that is different from the known EY vector.
"We think the Illinois elm disease is being carried
by a transient insect coming in from another state," says Lee.
"Tiny plant-feeding leafhoppers can migrate up to 1,000 miles on
wind power alone." The leafhopper deposits the pathogen while sucking
juices from the tree's phloem.
Dr. Lee will now concentrate on locating the leafhopper
vector, determining its species, and finding its origin. He will then
monitor the insect because, he says, "A rise in the population
of the vector signals a warning."By Rosalie Marion Bliss,
Agricultural Research Service Information Staff.
"Elm Disease Bacterium Identified" was published in the January 2003 issue of Agricultural Research magazine.