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Protecting Prized Elms: Disease Bacterium Identified

By Rosalie Marion Bliss
January 9, 2003

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, or cell-wall-less bacterium. While the bacterium had sickened many North American elms over the past several decades, it had historically been absent from the northern third of Illinois.

Earlier tests on the Illinois elms for EY phytoplasma proved inconclusive. So, Agricultural Research Service plant pathologist Ing-Ming Lee, with the Molecular Plant Pathology Laboratory, Beltsville, Md., in collaboration with scientists from Illinois' Morton Arboretum, tried a different test method. They used a tool Lee developed to systematically hunt for any phytoplasma 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.

Lee is now investigating whether the Illinois elm disease is being carried by transient insects coming in from another state--perhaps tiny, plant-feeding leafhoppers that can migrate up to 1,000 miles on wind power alone. The leafhoppers deposit the pathogen while sucking juices from a tree's phloem. Lee is now focusing on locating the leafhopper, determining its species, and tracking its origin.

Read more about ARS' research on plant pathogens in the January issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.