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Horticultural Plant
Disease
Chapter 3 Plant Pathogens
section 2 Plant Pathogenic Prokaryote
Prokaryote:
A kind of lower organism, no nucleus,
no nuclear membrane, and the genetic
substances disperse in the cytoplasm
forming a circular or elliptoid
Nucleolus region.
Including bacterium, actinomycete(放
线菌纲）, cyanobacterium（蓝细菌）,
mycoplasma（菌原体）etc.
1. Brief summery of plant pathogenic bacterium
The number and harm of bacterium disease is
inferior to fungus disease and virus disease, and
bacterium is situated on third pathogen.
Bacterium pertains to unicellular organism of
prokaryota , has cell wall, no nucleus.
General conformation of bacterium are globular,
bacilliform, helical, and bacteria are held together
as pairs, long chains or clusters; most of the plant
pathogenic bacteria are bacilliform, a few of the
bacteria are globular.
Most bacteria have flagella(鞭毛）；some bacteria have
polar flagella attached to end of the cell; others have many
flagella distributed over the cell surface.
Few of pathogenic bacteria produce spore(芽孢，endospore,
a resting body of bacterium in specific phase of life cycle).
Bacteria are non-obligate parasites, and can live on
substrate; bacteria with stronger parasitism can infect the
green leaf of plant, weaker parasitism bacteria only infect
fruit or storage organ of plant.
Thermal death point of pathogenic bacteria is in 48-53℃ for
10 minutes; killing the spore of bacteria need in 120 ℃ for
10-20 minutes.
Bacteria multiply by dividing, that is binary
fission( Each bacteria grows until it reaches a
certain size, then the cell becomes pinched in the
middle and separates into two equal parts or
daughter cells.). In suitable condition, bacteria can
divide every 20 minutes. Plant pathogenic bacteria
can grow well in 26-30℃, bacteria colony appear
after 28-48 hours.
Bacteria variance often occur, and the virulence
is easy to be weaken by cultivation. Inoculation
can make bacteria to acquire the virulence.
1.1 Shape and structure
1.1.1 Shape and size
 General conformation of bacterium are globular, bacilliform,
helical, and have great difference in size. Diameter of globular
bacteria is 0.5-1.3µm; size of bacilliform is 0.5-0.8µm×1-5µm,
helical bacteria are bigger with the size of 13-14µm×1-3µm.
 Most of bacteria are single, few of the bacteria are held
together as pairs, long chains or clusters.
 Most of the plant pathogenic bacteria are bacilliform, the
sizes are 0.5-0.8µm×1-3µm ; a few of the bacteria are globular.
1.1.2 General characters of bacterial spore（芽孢）
 Spore is a kind of endospore producing in
bacillus（芽孢杆菌属）.
 Spore is not the reproduction organ because
one bacterium only forms one spore and a spore
germinates to form one bacterium.
 Spore has strong resistance to abnormal
environment. Thermal death point of pathogenic
bacteria is 48-53℃ , killing bacteria spore need in
120 ℃ for 10-20 minutes.
1.1.3 Gram’s staining reaction
 Gram’s method involves the use of crystal violet(结晶
紫) and iodine (碘液)to determine if bacteria will absorb
these chemicals and turn dark (gram positive), most
plant pathogenic bacteria do not retain the crystal-violet
stain and are called gram negative.
 Gram’s staining reaction relate to other characters.
E.g. Gram positive bacteria can tolerate concentrated
saline solution, and can not be decomposed by
protease, but are sensitive to basic(碱性的) dyes and
detergent(去垢剂) ( e.g. soap).
 Having different reaction to antibiotics, penicillin(青霉
素) only can act on Gram positive bacteria, while
streptomycin(链霉素) can affect gram positive and
negative bacteria.
 Needing different nutrient substances, in general, gram
positive bacteria need complex nutrient substances.
 Cell wall of Gram positive bacteria is not slick( 光滑）
by electron microscope. It is obvious that gram’s staining
reaction reflected essential differences in constitution
and structure. So gram’s staining reaction is an
important trait in bacteria classification.
1.2 Reproduction, inheritance and variance of prokaryote
 Prokaryote multiply by dividing. Bacteria multiply
rapidly, in suitable condition, E.coli can divide each
20 minutes. Temperature also can influence growth
and reproduction of bacteria when other conditions
are in order. The suitable growth temperature is 2630 ℃ for plant pathogenic prokaryote; a few ones
grow well in high or low temperature, for example, P.
solanacearum(茄青枯菌) grow well in 35 ℃, while
Clavibacter michiganensis subsp. Sepedonicus(马铃
薯环腐病菌) grow well in 20-23 ℃.
 Genetic material of prokaryote is DNA which mainly
lies in nucleus region , but there is unattached
genetic material in bacterium cell, i.e. plasmid.
Nucleoplasm(核质) and plasmid compose the genetic
information library of prokaryote---genomics.
 Prokaryotes often appear variation, for instance,
the virulence of plant pathogenic bacteria is easy to
be weaken by cultivation, and the variation may be
owing to the selection to living condition. The similar
phenomenon appear in different host plant, In
different host plant which have diverse resistance, the
virulence of plant pathogenic bacteria can be
weakened or enhanced.
1.3 Physiology and biochemistry of bacteria
 Requiring simple nutrition, and can grow in
ordinary substrate.
 The suitable growth temperature is 26-30 ℃ for
plant pathogenic bacteria, and can be killed in 50
℃ for 10 minutes; In moderate and weak alkaline
condition, bacteria can grow well; most of the
pathogenic bacteria are aerobic, and few ones are
facultative anaerobe(厌氧生物）.
 Enzyme produced by bacteria can bring different
biochemical reaction to diverse substrate or
chemicals, this can be used to identify different
bacteria, e.g. biochemical reaction test.
Biochemical reaction test including following ones:
① glucide fermentation(糖类发酵)
② hydrolyzing amylum(淀粉)
③ liquefying gelatin(明胶)
④ utilization of lactose（乳糖） and protein on litmus-milk
（石蕊牛乳）substrate
⑤ testing metabolic product in peptone（蛋白胨） nutrient
solution, e.g.
hydrogen sulphide（硫化氢）, ammonia（胺）
and indole（吲哚）
⑥ reducing nitrate（还原硝酸盐）
⑦ decomposing grease（分解脂肪）
1.4 Cultivation characters of bacteria
 Forming different colony with various shapes
and colors on solid substrate.
 In general situation, the colors mostly are white
and yellow , and the shape mainly is circle, now
and then brown or irregular .
1.5 Main groups
The main groups of plant pathogenic prokaryotes
are bacteria and mycoplasma, the two clusters are
ascribe to Gracilicutes(薄壁菌门）、Firmicutes(厚
壁菌门）and Tenericutes(软壁菌门）.
1.2 Classification of plant
pathogenic bacteria
1.2.1 Classification basis
Morphological characters
Nourishing and living style
Cultivation properties
Physiological and biochemical properties
Pathogenicity
Symptom traits
Antigen conformation
Genetic character
1.2.2 Main groups of plant pathogenic bacteria
1.2.2.1 Agrobacterium(土壤杆菌属)
 Soil inhabiting fungus, short bacilliform, single
bacterium or holding together as pairs. Having 1-6 flagella
distributed over the cell surface. Aerobic, metabolism by
breathing. Gram staining reaction negative, no spore.
 The colony is circular, bulging and smooth; offwhite to
white, ropy(粘稠的） and pigment can not be produced
 Most of the bacteria have plasmid which separate from
bacteria chromosome. Plasmid control the pathogenicity
and drug resistance of bacteria.
1.2.2.2 Burkholderia(布克氏菌属)
 Separating from Psedomonas
 Forming smooth, moist, ridgy and
offwhite colony on combined substrate.
 Host range is wide, can harm more
than 30 families, 100 species. Typical
symptom is acute wilt, vascular bundle
of diseased stem turn into brown.
Spilling white bacteria ooze(菌脓) by
extruding.
 Soil inhabiting bacteria, can spread
by soil, water, contaminated stock and
diseased plant. infecting host plant
from wound.
 High temperature and humidity are
benefited to occurring disease.
1.2.2.3 Erwinia(欧文氏杆菌属)

short bacilliform, more than one
flagella around the bacteria cell surface.
 Offwhite colony, gram staining
reaction negative .causing rot, e.g.
Chinese cabbage soft rot.

E. Carotovora have wide host
range, and can infect more than 20
families including crucifers(十字花科),
gramineae(禾本科), solanaceae( 茄科)
etc. infecting host plant from wound or
by insect vector activity. In general, the
symptom appear during storage stage.
1.2.2.4 Pseudomonas(假单胞杆菌属)
 Short bacilliform or slightly flexual（弯
曲）. Having more than one flagella
attached to one end of bacteria cell.
 The colony is circular, bulging , smooth,
offwhite. Producing fluorescence on agar
substrate, some species can produce brown
pigment infusing into substrate.
 Gram staining reaction negative. Aerobic,
metabolism by breathing, no spore.
 Causing leaf spot, rot and wilt. E.g.
Cucumber angular leaf spot(黄瓜细菌性角斑
病), eggplant bacterial wilt(茄科青枯病),
Morus blight(桑疫病).
1.2.2.5 Xanthomonas(黄单胞菌属)

Short bacilliform, having one
flagellum attached to one end of
bacteria cell.
 Gram negative. Aerobic,
metabolism by breathing.
 The colony is circular, bulging ,
yellow. Producing non-watersoluble yellow pigment.
 Model species is Xanthomonas
Campestris(野油菜黄单胞菌）
 Causing cabbage black rot, rice
leaf blight, etc.
1.2.2.6 Clavibacter(棒形杆菌属)

Short bacilliform to irregular
bacilliform, no flagellum, no
endospore.
 Gram positive. Aerobic,
metabolism by breathing.
 The colony is circualr, smooth,
bulging, opaque（不透明）, offwhite
on agar substrate
 C.Michiganensis subsp.
Sepedonicum （马铃薯环腐亚种）
cause potato ring rot.
3. Plant mycoplasma
3.1 Common characters
 Spherical, no cell wall, having cell membrane
 Spiroplasma(螺原体) can be cultivated on
substrate, forming omelette-like(煎蛋形） colony
 Sensitive to tetracycline(四环素). Because of no
cell wall, mycoplasma is not sensitive to
penicillin and streptomycin
 Causing chlorosis, dwarf, clustering, leaflet.
 depending on insect vector to spread. E.g.
Leafhopper(叶蝉) and planthopper(飞虱).
3.2 Main group of plant
mycoplasma
3.2.1 Spiroplasma(螺原体)
 Helical, producing helical offshoot(分
支). Main host are dicotyledon(双子叶植
物） and hexapod(昆虫）.
 Causing Citrus stubborn (柑橘僵化病)，
maize corn stunt (玉米矮化病).
 Leafhopper can diffuse Spiroplasma
 Spiroplasma can winter in
Johnsongrass(假高粱) or leafhopper.
3.2.2 Phytoplasma(植原体属)
 Orbicular or elliptic. The size of
phytoplasma is 80-1000nm.
 to date, it can not be cultivated on
substrate.
 Spreading by hishimonoides sellatifrons
Ishihara(桑菱纹叶蝉).
 Causing Morus virus disease(桑萎缩病),
Jujube witches’ broom(枣疯病), paulownia
witches’ broom(泡桐丛枝病), Pear decline(梨
衰退病)and sweet potato witches’ broom(甘
薯丛枝病).