Biology Reference
In-Depth Information
Bacteria
The fact that bacteria can cause plant diseases was
discovered almost simultaneoualy in four different
countries, with the United States claiming first
honors. In 1878 Professor T. J. Burrill of the
University of Illinois advanced the theory that fire
blight of apple and pear was due to the bacteria that
he found constantly associated with blighted tissues.
In 1879, the French scientist Prillieux published
a paper on bacteria as the cause of rose-red disease
of wheat; in 1880 the Italian Comes recognized
bacteria as pathogenic to plants; in 1882 Burrill
named his fire-blight organism Micrococcus
amylovorus ; and in 1883Walker inHolland reported
the bacterial nature of yellows disease of hyacinth.
Itremained,however,forErwinF.Smith,ofthe
U.S. Department of Agriculture, to do most of the
pioneer work in this field andtoconvincetheworld
that bacteria were to blame for so many diseases. He
spent a lifetime in the process, starting with peach
yellows, and going on to a study of crown gall and its
relation to human cancer. In 1905 the first volume of
his monumental work Bacteria in Relation to Plant
Diseases was published.
There are about 80 species of bacteria which
cause plant disease and many of them consisting
of numerous pathovars. Bacterial diseases fall
into three categories: (1) a wilting, as in cucum-
ber wilt, due to invasion of the vascular system,
or water-conducting vessels; (2) necrotic blights,
rots, and leaf spots, where the parenchyma tissue
is killed, as in fire blight, delphinium black spot,
soft rot of iris and other plants with rhizomes or
fleshy roots; (3) an overgrowth or hyperplasia, as
in crown gall or hairy root.
Pathogenic bacteria apparently cannot enter
plants directly through unbroken cuticle but get in
through insect or other wounds, through stomata,
through hydathodes, possibly through lenticels,
and often through flower nectaries. They can sur-
vive for some months in an inactive state in plant
tissue, as in holdover cankers of fire blight, and
perhaps years in the soil, although claims for
extreme longevity of the crown-gall organism in
soil are discounted.
Most of these plant disease bacteria have had
their genus names changed several times since
they were first described, and some species have
been combined. Classification of bacteria will
probably change further in future years. Where
genus and/or species names have been changed,
the old name is given in parentheses. The genera
and species used in this text agree with those given
in Ninth Edition of Bergey's Manual of Determi-
native Bacteriology (1994) and recent articles
in the J. Systematic Bacteriology. Walter H.
Burkholder, of Cornell University, who revised
the portions of the Manual dealing with plant
pathogens, followed in the footsteps of Erwin F.
Smith by spending his life with bacterial diseases
of plants, as did Charlotte Elliott of the U.S.
Department of Agriculture, from whose Manual
of Bacterial Plant Pathogens much information on
disease symptoms have been taken.
Two kinds of prokaryotes (organisms that lack
atruenucleus)causediseaseinplants.Bacteriahave
a cell membrane, a rigid cell wall, and often one or
more flagella. The mollicutes, or phytoplasmas lack
a cell wall and have only a single-unit membrane.
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