Plant Diseases Caused by Bacteria and Mollicutes 6 .pdf · Plant Diseases Caused by Bacteria and...
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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-like-organism (MLO)
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Plant pathogenic bacteria
Plant pathogenic phytoplasmas
Source: Agrios (2005)
Important Characteristics (cont’)
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1 2 3
Source: JPT
1. Coccus
2. Rod
3. Spiral
4. Filamentous (branched mycelium)
Morphology of Bacteria
Source: Agrios (2005)
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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 Erwinia
Pseudomonas Xanthomonas
Plant Pathogenic Bacteria: Morphology
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: Genus Xanthomonas produces
yellow pigment called xanthomonadin
Colonies of Xanthomonas producing 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. Phytoplasmas2. Spiroplasmas
Lack flagella and spores Gram negative Shape: spherical, ovoid, filamentous,
helical
Source: Agrios (2005)
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1. Phytoplasmas
Spherical, ovoid, filamentous
Diameter: 0.3-2 µm
2. Spiroplasmas
Helical
Diameter: 0.12 µm
Length: 2-4 µm
Plant Pathogenic Mollicutes: Morphology (cont’)
Phytoplasmas
Spiroplasmas
Source: Agrios (2005)
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Parasites (live in host plants) Saprophytes (live in plant debris or soil) Bacteria:
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
Plant Pathogenic Bacteria and Mollicutes: Ecology
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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 based on: Gram stain Morphology Biochemical requirements
Classification not stable Pathovars (1980) Subspecies Biovars Serovars Phagovars Example: Xanthomonas axonopodis (campestris)
pv citri (citrus canker bacterium)
Classification of Plant Pathogenic Bacteria (cont’)
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Kingdom Division Class Attribute Family Genus
Prokaryote Gracilicutes Proteo-bacteria
Gram negative bacteria
Enterobacte-riaceae
Pseudomona-daceae
Rhizobiaceae
Undefined
Erwinia
PseudomonasRalstoniaXanthomonas
Agrobacterium
Xylella
Firmicutes Firmi-bacteria
Thallo-bacteria
Gram positive bacteria
Gram
positive
branching
bacteria
BacillusClostridium
Streptomyces
Clavibacter
Classification of Plant Pathogenic Bacteria (cont’)
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Kingdom Division Class Attribute Family Genus
Prokaryote Tenericutes Mollicutes Wall-free prokaryotes
Spiroplas-mataceae
Undefined
Spiroplasma
Phytoplasma
Classification of Plant Pathogenic Mollicutes
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Asexual reproduction: binary fission (rod-shaped bacteria & mollicutes)
Reproduce at an extremely rapid rate:
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
Plant Pathogenic Bacteria and Mollicutes: Reproduction
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Stationary phase
No. of cells
Time
Lag phaseExponential phase
Death phase
Plant Pathogenic Bacteria and Mollicutes: Reproduction (cont’)
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The process involved 3 steps:
1. Penetration
2. Invasion
3. Colonization
Infection Process
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1. Penetration
Through stoma, wound, hydathode, nectarthode
Bacteria surrounding a stoma of host plant
Infection Process (cont’)
Source: Agrios (2005)
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2. Invasion
Pathogens invade intra- and intercellular tissues
Example: Vascular wilt bacterium invade xylem tissues
3. Colonization
Pathogens reproduce at a rapid rate
Progeny carried passively into new cells through xylem & phloem
Infection Process (cont’)
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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-like-organisms (MLO) in Japan
1972 Windsor & Black: Studied epidemic of Pierce disease of grapes caused by fastidious phloem-or xylem-inhabiting bacteria
Davis: Discovery of spiroplasma
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Importance of Plant Pathogenic Bacteria and Mollicutes
Cause tremendous economic losses
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
1. Citrus canker
2. Bacterial leaf blight
3. Bacterial wilt
4. Soft rot
5. Crown gall
6. Citrus greening
Mollicutes
1. Lethal yellowing
2. Citrus stubborn
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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
Diseases Caused by Plant Pathogenic Bacteria
Source: Agrios (2005)
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Diseases Caused by Plant Pathogenic Bacteria (cont’)
2. Bacterial Leaf Blight Pathogen:
Xanthomonas oryzae pv. oryzae
Host: Rice
Distribution : Worldwide
Symptoms: Necrosis on sides of leaf blades
Source: Agrios (2005)
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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: PotatoBrown discoloration in xylem
Diseases Caused by Plant Pathogenic Bacteria (cont’)
Source: Agrios (2005)
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4. Soft Rot Pathogen: Erwinia
caratovora Hosts: Fruits,
vegetables, ornamentals
Distribution: Worldwide Symptom: water-
soaked lesions, disintegration of tissues, foul odor, infecting fleshy storage tissues & aided by insects
Host: CeleryHost: Carrot
Diseases Caused by Plant Pathogenic Bacteria (cont’)
Source: Agrios (2005)
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Diseases Caused by Plant Pathogenic Bacteria (cont’)
5. Crown Gall Pathogen:
Agrobacterium tumefaciens
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)
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 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 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?
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