Download Plant Diseases Caused by Bacteria and Mollicutes

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Plant Diseases Caused by
Bacteria and Mollicutes
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Important Characteristics
Bacteria
 Prokaryote
 Genetic material (DNA)
not bound by a
membrane (lack nucleus)
 Cytoplasm surrounded by
a cell membrane & a cell
wall
 Lack organelles such as
mitochondria and
chloroplasts
 Cells consist of cytoplasm
containing DNA & small
(70 S) ribosomes
Mollicutes
 Similar to bacteria
except lack of a cell wall
 In the past known as
mycoplasma-likeorganism (MLO)
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Important Characteristics (cont’)
Plant pathogenic bacteria
Plant pathogenic phytoplasmas
Source: Agrios (2005)
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Morphology of Bacteria
1.
2.
3.
4.
Coccus
Rod
Spiral
Filamentous
(branched
mycelium)
Source: JPT
1
2
3
Source: Agrios (2005)
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Plant Pathogenic Bacteria:
Morphology




Most species are rod shaped,
except Streptomyces
(filamentous)
1.0-5.0 (L) x 0.5-1.0 (W) µm
Cell walls of most species are
enveloped by a gummy
material
Most species have flagella:


polar flagella: one or many at
one end of the cell, e.g.,
Pseudomonas, Xanthomonas
peritrichous flagella: one or
many distributed all over, e.g.,
Agrobacterium, Erwinia
Agrobacterium
Pseudomonas
Erwinia
Xanthomonas
Source: Agrios (2005)
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Plant Pathogenic Bacteria:
Morphology
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Plant Pathogenic Bacteria:
Morphology (cont’)

Colony: different according
to species






Shape: circular, oval, irregular
Edge: smooth, wavy, angular
Elevation: flat, raised, wrinkled
Color: whitish, grayish, yellow
Some produce fluorescent
pigment under UV light
Example:

Colonies of Xanthomonas
producing xanthomonadin
Genus Xanthomonas produces
yellow pigment called
xanthomonadin
Source: Agrios (2005)
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Plant Pathogenic Bacteria:
Morphology (cont’)
Pigment production





Pseudomonas: fluorescent
Erwinia: zeaxanthine (cream), rubrifacin
Xanthomonas: xanthomonadins (yellow)
Clavibacter: carotenoids (yellow)
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Gram Stain




Developed by a Danish doctor, Hans Christian Gram
in 1884
Gram reaction:
Based on structure of bacterial cell wall

Divide bacteria into 2 groups:

1.
Gram positive: stained purple due to
peptidoglycan layer
2.
Gram negative: stained pink due to an
additional outer membrane (stain unable to
reach peptidoglycan layer
Gram stain procedure:
crystal violet + iodine + alcohol + safranin

Non-staining test: 3% KOH
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Plant Pathogenic Mollicutes:
Morphology
Divided into 2 groups:

1.
2.



Phytoplasmas
Spiroplasmas
Lack flagella and spores
Gram negative
Shape: spherical, ovoid, filamentous,
helical
Source: Agrios (2005)
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Plant Pathogenic Mollicutes:
Morphology (cont’)
1.
Phytoplasmas


2.
Spherical, ovoid,
filamentous
Diameter: 0.3-2 µm
Spiroplasmas



Phytoplasmas
Helical
Diameter: 0.12 µm
Length: 2-4 µm
Spiroplasmas
Source: Agrios (2005)
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Plant Pathogenic Bacteria and
Mollicutes: Ecology



Parasites (live in host plants)
Saprophytes (live in plant debris or soil)
Bacteria:

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Most species: facultative saprophytes, can be grown
artificially on nutrient media
Fastidious vascular bacteria: difficult to grow in culture
Nutrient intake: diffusion
E.g. of artificial nutrient medium: nutrient agar (NA), pH
~7.0
Mollicutes:


Phytoplasmas: cannot be grown on artificial nutrient media
& disease yet to be reproduced
Spiroplasmas: can be grown on artificial nutrient media &
disease can be reproduced
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Classification of Plant Pathogenic
Bacteria



≈ 1,600 species are known
≈ 100 species cause diseases in plants
References:


International Code of Nomenclature of
Bacteria (revised 1976)
Labotatory Guide for Identification of Plant
Pathogenic Bacteria (American
Phytopathological Society, 2001)
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Classification of Plant Pathogenic
Bacteria (cont’)

Classification based on:
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Gram stain
Morphology
Biochemical requirements
Classification not stable

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Pathovars (1980)
Subspecies
Biovars
Serovars
Phagovars
Example: Xanthomonas axonopodis (campestris)
pv citri (citrus canker bacterium)
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Classification of Plant Pathogenic
Bacteria (cont’)
Kingdom
Division
Prokaryote
Gracilicutes
Class
Attribute
Proteo- Gram
bacteria negative
bacteria
Family
Enterobacteriaceae
Pseudomonadaceae
Rhizobiaceae
Undefined
Firmicutes
FirmiGram
bacteria positive
bacteria
Thallo- Gram
bacteria positive
branching
bacteria
Genus
Erwinia
Pseudomonas
Ralstonia
Xanthomonas
Agrobacterium
Xylella
Bacillus
Clostridium
Streptomyces
Clavibacter
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Classification of Plant Pathogenic
Mollicutes
Kingdom
Division
Prokaryote
Tenericutes
Class
Mollicutes
Attribute
Family
Genus
Wall-free
prokaryotes
Spiroplasmataceae
Spiroplasma
Undefined
Phytoplasma
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Plant Pathogenic Bacteria and
Mollicutes: Reproduction


Asexual reproduction: binary fission (rodshaped bacteria & mollicutes)
Reproduce at an extremely rapid rate:
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divide every 20-50 min
1 bacterium could produce 106 progeny in < a day
Extremely large populations cause:


great chemical changes in their environment
development of bacterial diseases in plants
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Plant Pathogenic Bacteria and
Mollicutes: Reproduction (cont’)
Stationary phase
No. of cells
Death phase
Lag phase
Exponential phase
Time
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Infection Process
The process involved 3 steps:
Penetration
1.
2.
Invasion
3.
Colonization
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Infection Process (cont’)
Penetration
1.

Through stoma,
wound, hydathode,
nectarthode
Bacteria surrounding a stoma of host plant
Source: Agrios (2005)
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Infection Process (cont’)
Invasion
2.


Pathogens invade intra- and intercellular
tissues
Example: Vascular wilt bacterium invade
xylem tissues
Colonization
3.


Pathogens reproduce at a rapid rate
Progeny carried passively into new cells
through xylem & phloem
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History
1876 R. Koch: First bacterium discovered to infect
animals & humans, anthrax by Bacillus anthracis
1878 T.J. Burill: Fire blight bacterium discovered in
pear & apple trees
1901 E.F. Smith: Bacteria was proven to cause
plant diseases
1967 Y. Doi et al.: Discovery of Mycoplasma-likeorganisms (MLO) in Japan
1972 Windsor & Black: Studied epidemic of Pierce
disease of grapes caused by fastidious phloemor xylem-inhabiting bacteria
Davis: Discovery of spiroplasma
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Importance of Plant Pathogenic
Bacteria and Mollicutes

Cause tremendous economic losses


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Direct: lower yield & quality of plant
products
Indirect: increase costs - chemicals,
resistant varieties, management
Examples:


Soybean blight: losses of US$65 mil/year
Crown gall of fruits and nuts: losses of
US$23 mil/year
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Diseases Caused by Plant
Pathogenic Bacteria and Mollicutes
Bacteria
Citrus canker
1.
Bacterial leaf blight
2.
Bacterial wilt
3.
4.
Soft rot
5.
Crown gall
Citrus greening
6.
Mollicutes
1. Lethal yellowing
2. Citrus stubborn
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Diseases Caused by Plant
Pathogenic Bacteria
1.
Citrus Canker

Pathogen:
Xanthomonas
axonopodis
(campestris) pv citri



Host: Citrus
Distribution: Citrus
growing areas
Symptom: sunken
spots surrounded by
yellow halo on leaves,
branches & fruits
Source: Agrios (2005)
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Diseases Caused by Plant
Pathogenic Bacteria (cont’)
Bacterial Leaf Blight
2.

Pathogen:
Xanthomonas oryzae
pv. oryzae



Host: Rice
Distribution :
Worldwide
Symptoms: Necrosis on
sides of leaf blades
Source: Agrios (2005)
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Diseases Caused by Plant
Pathogenic Bacteria (cont’)
3.
(Southern) Bacterial
Wilt

Pathogen: Ralstonia
(Pseudomonas)
solanacearum



Hosts: Solanaceous
plants
Distribution: Places
with warm climates
Symptom: Sudden
wilt, brown
discoloration in
vascular tissues
Host: Tomato
Host: Potato
Brown discoloration
in xylem
Source: Agrios (2005)
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Diseases Caused by Plant
Pathogenic Bacteria (cont’)
4.
Soft Rot

Pathogen: Erwinia
caratovora



Hosts: Fruits,
vegetables,
ornamentals
Distribution: Worldwide
Symptom: watersoaked lesions,
disintegration of
tissues, foul odor,
infecting fleshy storage
tissues & aided by
insects
Host: Carrot
Host: Celery
Source: Agrios (2005)
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Diseases Caused by Plant
Pathogenic Bacteria (cont’)
5.
Crown Gall

Pathogen:
Agrobacterium
tumefaciens
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

Hosts: Woody &
herbaceous plants
Distribution: Worldwide
Symptoms:
Development of tumors
or galls of various sizes
& shapes at lower
stems & main roots
Source: Agrios (2005)
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Phloem-Inhabiting Fastidious
Bacteria
6.
Citrus Greening Disease

Pathogen: Candidatus
liberobacter asiaticus
(Asia), C. liberobacter
africanus (Africa)
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
Host: Citrus
Distribution: Asia & Africa
Symptom: Yellowing leaves
& stunted growth, lopsided
fruits & fail to ripe (green)
Dissemination: Vegetative
reproduction & psyllid
insects
Source: Agrios (2005)
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Diseases Caused by Plant
Pathogenic Mollicutes (cont’)
1.
Lethal Yellowing
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
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Pathogen: Phytoplasma
Host: Coconut
Distribution: USA,
Mexico, West Africa, etc.
Symptoms: Pre-mature
drop of coconuts,
blackened tips in
flowers, fronds turn
yellow, die & drop,
leaving only trunk
Phytoplasmas
passing through a
sieve-plate pore.
Source: Agrios (2005)
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Diseases Caused by Plant
Pathogenic Mollicutes (cont’)
2.
Citrus Stubborn
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Pathogen: Spiroplasma citri
Host: Citrus
Distribution: Hot & dry
areas
Symptoms: Stunted
growth, bunchy shoots,
mottled or chlorotic leaves,
very small & lopsided fruits
Dissemination: Budding &
grafting, leafhoppers
Source: Agrios (2005)
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Thank You
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Quiz
1.




What is the morphological difference
between bacteria and mollicutes?
What is the colour of the pigment that
Xanthomonas produces?
What is the name of the staining procedure
that is used for bacterial classification?
Name two types of Mollicutes
What is the causal agent of Citrus canker
(genus name) and Lethal yellowing of
palms?