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New Finding on Bacterium Linked to Disease of Orange, GrapesBy Rosalie Marion Bliss
July 31, 2003
WASHINGTON, July 31--A type of bacterium that scientists previously thought was spread only by insect vectors has now been found to be transmitted by seeds to seedlings, according to Agricultural Research Service scientists and cooperators in Brazil.
For 60 years, experts have believed that the bacterium, named Xylella fastidiosa, is spread only by insects such as the glassy-winged sharpshooter, or through contaminated bud wood. Xylella bacteria have various strains which are known to cause several diseases, such as citrus variegated chlorosis (CVC) in Brazilian oranges, coffee leaf scorch in South America and Pierce's disease of grapevines in North and Central America--leading to substantial yield reductions.
ARS plant pathologist John Hartung and colleagues reported this month that X. fastidiosa can infect and colonize orange fruit tissues and seeds, and that the bacterium transmits from infected seeds to seedlings. The finding is reported as the first known demonstration that Xylella can infect plants via seed.
Hartung, with the ARS Fruit Laboratory in Beltsville, Md., and collaborators from the University of Sao Paulo, Brazil, reported the findings in the August issue of Phytopathology. ARS is the U.S. Department of Agriculture's chief in-house scientific research agency.
"The implication that X. fastidiosa could propagate itself within other plant species through seed, from generation to generation, is a change in paradigm," said Edward B. Knipling, acting ARS administrator.
The idea of this significant new route of transmission casts new light on another finding reported last year. In greenhouse experiments, Hartung and collaborators in Brazil unexpectedly found that the strain of X. fastidiosa that causes disease in Brazilian sweet orange also causes symptoms of Pierce's disease in grapes. "We knew that the strain in orange was genetically different from the one in grapes, so we had assumed that it would not be able to also cause disease in grapes," said Hartung. The CVC strain thus could become a threat to U.S. orange and grape growers.