This document provides an overview of plant pathogenic bacteria, including their characteristics, reproduction, classification, and the diseases they cause in plants. It describes that bacteria are unicellular prokaryotes that lack organelles and can have different shapes. It also discusses the different types of flagella and how bacteria reproduce through binary fission or sporulation. The document outlines several important genera of plant pathogenic bacteria such as Xanthomonas, Pseudomonas, Erwinia, and Ralstonia. It concludes by describing common bacterial diseases in plants including leaf spots, blights, wilts, soft rots, and scabs, and provides examples for each type of disease.
2. INTRODUCTION
• Bacterium is a unicellular, microscopic, prokaryotic (lacking true
nucleus) organism, which lacks chlorophyll and multiplies by fission.
• It has rigid cell wall and its nucleus materials are not separated from
the cytoplasm by a nuclear membrane.
• T. J. Burril was the first to describe a bacterial disease in plants
(1878 – 1884)
• (Fire blight of apple and pear caused by Erwinia amylovora)
3. Types of bacteria
• Coccus- spherical shape, may be arranged singly (micrococcus), in
pairs (diplococcus), tetrads, chains (streptococcus), in box-like cubical
pockets (sarcinate) or irregular grape like clusters (Staphylococcus)
• Bacillus- Straight rod or cylindrical, occur mostly singly (microbacilli) or in
pairs (diplobacilli) in chains (streptobacilli) or in parallel palisade like
arrangement
• Spirillum- Cork screw shaped or helically curved rod.
• Most of the plant pathogenic bacteria are rod shaped except
Streptomyces (filamentous)
4.
5. Flagella – Hair like helical structures that protrude through cell
wall and are used for used for locomotion.
• Atrichous (without flagella) – Xylella sp.
• Monotrichous (single flagella at one end) - Xanthomonas sp.
• Cephalotrichous (several flagella at one end) - Pseudomonas
fluorescens
• Amphitrichous (at least one flagellum at each pole) – Pseudomonas
spp.
• Lophotrichous (two or more flagella at both the poles of the
bacterium)- Most of the Spirillum types
• Peritrichous (protruding from all sides)- Pectobacterium, Erwinia
6.
7. Reproduction (Both asexual and sexual)
• Rod shaped PP bacteria reproduce asexually by binary fission.
In addition, bacterial cells also produce spores either within
the cell (endospore) or external to the cell (exospore)
• Binary fission: The single cell of the bacterium split
transversely into two equal cells.
Sexual reproduction
• Conjugation
• Transformation
• Transduction
8.
9. Phylogenetic classification of bacteria
• Earlier classification was based on morphology, metabolic reactions, nutrient
utilization, pigment production, etc.
• Recent classification is based on cell wall composition, fatty acid and protein
profiling, comparison of DNA and RNA composition and sequences.
• Molecular characterization of 16S ribosomal RNA distinguishes bacteria from
one another.
• There are 24 phyla within the domain Bacteria and the plant pathogens are
found in 3 phyla.
• Phylum: Firmicutes
• Phylum: Actinobacteria (Gram positive bacteria)
• Phylum: Proteobacteria (Gram negative bacteria)
10. SOME IMPORTANT GENUS OF PLANT
PATHOGENIC BACTERIA
Xanthomonas
• Rod shaped, motile by polar flagellum, Gram
negative, causes blights, leaf spots, produce
yellow colonies on agar media, slow growing
Agrobacterium (Rhizobium)
• Rod shaped, motile by one to four peritrichous
flagella, colonies are smooth, non-pigmented,
Gram negative, soil inhabitants causes crown
gall
11. Pseudomonas
• Straight or curved rods, motile by one or many
polar flagella, Gram negative, common
inhabitant of soil, produce yellowish-green
diffusable fluorescent pigments (fluorescent
pseudomonads) others do not produce pigments
(non-fluorescent pseudomonads)
Ralstonia
• Rod shaped, motile, Gram negative, cause
vascular wilts
12. Erwinia
• Straight rods, motile (by several to many
peritrichous flagella), Gram negative,
facultative anaerobes, soil invader - two
groups.
• * amylovora - blight or wilt diseases (do
not produce pectic enzymes and cause
necrosis).
• *carotovora- soft rots, (have strong
pectolytic activity).
13. Clavibacter (Corynebacterium)
• Straight to slightly curved rods,
generally non-motile but some species
are motile by one to two polar flagella,
Gram positive, soil invader,
causes wilts
14. Streptomyces
• Slender branched hyphae without cross
walls. At maturity aerial mycelium forms
chains of three to many spores. Gram-
positive, soil inhabitants and
many produce antibiotics, causes scab
disease.
15. Symptoms of bacterial diseases
Leaf spot
• Bacteria invade leaves through stomata and necrosis of tissue
around the sub-stomatal spaces takes place.
• Dead tissue in the leaf lamina looks either water-soaked or brown.
• Spots may be circular, irregular or angular and are sometimes
encircled with a yellow halo.
• Spots restricted by veins and veinlets are called angular leaf spots.
• Angular leaf spot of cotton Xanthomonas axonopodis pv.
Malvacearum
• Bacterial leaf spot of tomato - X. axonopodis pv. Vesicatoria
• Angular leaf spot of cucumber – Pseudomonas syringae pv.
Lacrymans
16. Blights
• Rapid and extensive necrosis of tissues of affected
plant parts viz., leaves, petiole, stem and twigs
resulting in scorched appearance.
• Browning or blackening of veins and veinlets is
called vein blight or black vein in cotton.
• Bacteria blight (cotton) Xanthomonas axonopodis
pv. malvacearum
• Extensive blighting of leaves with wavy inner
margin is observed in
• Bacterial leaf blight (rice) Xanthomonas oryzae pv.
oryzae
17. Leaf streak
• Fine translucent streaks found on the veins, which later enlarge lengthwise
and advanced laterally over larger veins and turn brown.
• In severe cases, the leaves may dry up.
• Bacterial zone can be seen on the surface of the lesions / streaks.
• Bacterial leaf streak of rice Xanthomonas oryzae pv. oryzicola
18. Soft rot
• Mostly found in fleshy plant parts like fruits,
bulbs, tubers, succulent stem etc.
• Mode of entry through wounds or bruises. After
infection bacteria produce enzymes and
disintegrate and dissolute the middle lamella
leading to softening of plant tissues.
• Dirty brown liquid oozes out from the affected
plant part.
• Well defined demarcation between the healthy
and decayed tissues.
• e.g. Soft rot of vegetables -
Pectobacterium carotovorum
subsp. carotovorum
19. Canker
• Corky outgrowths occur on leaves, twigs, fruits and
other plant parts above ground level.
• Restricted outgrowths are the result of reaction of
the host tissues to the pathogenic bacteria.
• Deep seated localized reactions are restricted to
parenchymatous tissues of the host plants.
• e.g., Citrus canker caused by Xanthomonas
axonopodis pv. citri
• Bacterial canker in mango – X. campestris pv.
mangiferae indicae
• Bacterial canker in tomato –
Clavibacter michiganense subsp.
michiganense
20. Wilts
• Yellowing, drooping, wilting and death of aerial & ground plant parts
• Caused by plugging of water conducting tissues or vascular bundles
by bacteria and by toxins produced by the bacteria in the plant
(destroys the cell wall of the xylem vessels).
• Wilted plants show brown to black discolouration in vascular bundles
and this may be due to oxidation of phenolics to quinones by phenol
oxidase enzyme secreted by bacterial cells.
• e.g., Bacterial wilt of cucurbits - Erwinia tracheiphila
• Bacterial wilt of tomato - Ralstonia solanacearum
• Brown rot / Bacterial wilt of potato - Ralstonia solanacearum
• Bacterial wilt of tomato – Ralstonia solanacearum
21. Tumours and galls
• Bacteria enter through bruises and wounds,
produce globose, elongated or irregular large
sized outgrowth on affected plant parts.
• Pathogen triggers a chain of reactions to
produce Indole Acetic Acid, which cause
hyperplasia and hypertrophy of host cells.
•
• E.g. Crown gall of apple/ rose caused by
Rhizobium radiobacter
(= Agrobacterium tumefaciens)
22. Scab
• Affected tissues become rough, corky,
roughened or crust-like or ulcer-like, slightly
raised with rusty surface and pitted owing
to abnormal proliferation of tissues in the
epidermis.
• Unlike canker, corky outgrowth is formed
only at epidermal level and not deep
seated. Gives scabby appearance.
• Fruits, tubers, foliage and stems are mostly
affected.
• e.g. Common scab of potatoes caused by
Streptomyces scabies
