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.