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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)


Plant pathogenic bacteria

Plant pathogenic phytoplasmas

Source: Agrios (2005)

Important Characteristics (cont’)


1 2 3

Source: JPT

1. Coccus

2. Rod

3. Spiral

4. Filamentous (branched mycelium)

Morphology of Bacteria



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



Plant Pathogenic Bacteria: Morphology


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



Plant Pathogenic Bacteria: Morphology (cont’)

Pigment production

Pseudomonas: fluorescent

Erwinia: zeaxanthine (cream), rubrifacin

Xanthomonas: xanthomonadins (yellow)

Clavibacter: carotenoids (yellow)


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


Plant Pathogenic Mollicutes: Morphology

Divided into 2 groups:1. Phytoplasmas2. Spiroplasmas

Lack flagella and spores Gram negative Shape: spherical, ovoid, filamentous,

helical



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



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


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)


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’)


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’)


Kingdom Division Class Attribute Family Genus

Prokaryote Tenericutes Mollicutes Wall-free prokaryotes

Spiroplas-mataceae

Undefined

Spiroplasma

Phytoplasma

Classification of Plant Pathogenic Mollicutes


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


Stationary phase

No. of cells

Time

Lag phaseExponential phase

Death phase

Plant Pathogenic Bacteria and Mollicutes: Reproduction (cont’)


The process involved 3 steps:

1. Penetration

2. Invasion

3. Colonization

Infection Process


1. Penetration

Through stoma, wound, hydathode, nectarthode

Bacteria surrounding a stoma of host plant

Infection Process (cont’)



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’)


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


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


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


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



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



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




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





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



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



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.



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



Thank You


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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