on Bacterium Linked to Disease of Orange, Grapes
By Rosalie Marion
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.