Bacterial Plant Diseases Flashcards
Why are bacteria important?
Degrade organic matter, used in food production
Characteristics of bacteria
- single-celled
- prokaryotic
- various shapes
- Gram stain for ID
- many have flagella
- reproduce by binary fission
Gram +
- thick cell walls
- peptidoglycan retains stain
- stains purple
Gram -
- thin cell walls
- viscous
- stains red
Variation - transformation
cells picks up loose DNA
Variation - transduction
insertion of viral DNA
Variation - conjugation
transfer of plasmid
plasmid
unnecessary DNA
Three kinds of bacterial ecology
- host
- debris
- saprophytic in soil
Spread of bacteria
Long distance: die with prolonged exposure
- aerosols, rain, irrigation
- vectors: insects, birds, animals, man
Short distance: inches
- splattering
- swimming (wet areas)
Survival of bacteria
Living / dead plant tissue … 50% survive in / on seed.
Reduced metabolic state - do not produce endospores
Epiphytic - on host surface without causing infection
In soil, ooze, and vectors.
Entry of bacteria
Strictly passive! Through wounds, natural openings (stomata)
Colonize apoplast in xylem
Effects of bacteria
- Reduction of photosynthesis (necrosis)
- Increase phenol content -> plant compounds self-destruct
- Increase hormone production
- Produce toxins (to host and animals)
- Produce enzymes that break down plant compounds
Streptomyces
Not rod-shaped
Branched mycelium, pseudofilamentous, curled chain of conidia
Size, shape, and structure of bacterial colony changes with…
- age of culture
- composition media
- pH
- temperature
- staining method
Classification of bacteria
Difficult, variable, “controversial”
Based on morphological, physiological, and molecular traits
Genus
Taxonomic group above species
Species
Basic taxonomic unit
Group of strains that cluster due to shared traits
Pathovar
Strain of bacteria, or set of strains, with similar characteristics
Bacterial galls
Swollen areas in woody tissue, disorganized overgrowth
Olive knot disease
Pseudomonas syringae pv. savatanoi
Crown Gall causal organism
Agrobacterium tumefaciens
Crown Gall host
large range, especially on ornamentals
also popular crops like carrots
Crown Gall symptoms
Galls - small overgrowths at soil line on roots and stems… initially soft, but eventually enlarge, harden, and darken
Crown Gall signs
NONE
Crown Gall disease cycle
Monocyclic.
- OW in galls
- Enter root or stem, usually through wound
- Galls
Crown Gall favorable conditions
Moisture, temperatures suboptimal for plant growth
Crown Gall management
- inspect, reject infected shipments
- quarantine
- in soil, avoid susceptible hosts… grasses in rotation can help
- biocontrol … Agrobacterium radiobacter (no plasmid)
Bacterial spots and blights
Spots are most common, necrotic areas fall from leaf - leaves “shothole” damage.
Veins of foliage stop bacterial progress (angular appearance)
Angular leaf spot of cotton pathogen
Xanthamonas campestris pv. malvacearum
can also affect cereals, beans, stonefruits, and vegetables
Angular leaf spot of cotton geo
wherever cotton is grown
Angular leaf spot of cotton symptoms
- Small water-soaked spots on undersides, appear early in season
- Angular spots on leaves / bolls … necrosis stopped by veins
- Elongated black lesions on stems girdle and kill
Angular leaf spot of cotton signs
difficult to find
Angular leaf spot of cotton disease cycle
- OW in (colonization) / on seed (surface with biofilm)
- New seedlings are infected
- Symptoms
- Boll / seed development
- OW
Angular leaf spot of cotton favorable conditions
high temperatures, 85 - 95° F
Angular leaf spot of cotton losses
- death of plant
- reduced vigor
fewer bolls, less yield - soil infestation
Angular leaf spot of cotton management
- Resistance! so much available, disease hardly a problem
- Rotation (avoid cotton every year)
- Seed certification
Southern Bacterial Wilt pathogen
Ralstonia solanacearum races
one is considered a ‘bioterror threat’
Southern Bacterial Wilt hosts
Tomatoes, peppers, etc…
Southern Bacterial Wilt geo
tropics and warmer parts of the world
Southern Bacterial Wilt symptoms
- Wilting
- Early stage… during day with recovery at night
- Whole mature plants… wilt, quick death, leaves remain green
- any age can be infected
- infection apparent during fruit expansion
Southern Bacterial Wilt disease cycle
- In soil / water
- Infects through wounds in roots
- Colonizes plant, blocks vascular system
- Plant dies
- Repeat
Fire Blight pathogen
Erwinia amylovora
Fire Blight symptoms
- Twigs turn brownish black
- Hooked
- Leaves turn black
- Flowers are water-soaked, then shrivel… fall or hang on tree
Fire Blight geo
worldwide
Fire Blight host
Pome fruits: apple, pear, quince
Fire Blight management
Winter - cut twigs, branches, cankers, whole trees at least 10 cm below visible infection
Summer - cut blighted shoots (~30cm below)
- Disinfect tools
- Insect control
- Resistance
- Chemical (Bordeaux mixture)
Fastidious prokaryotes
Cause diseases similar to viruses, difficult to culture. Vectored.
“obligate parasites” - no saprophytic phases
Restricted to vascular system, classified by presence (Xylella) or absence (Mollicutes) of cell wall
RLO
Rickettsia-Like-Organism
Xylella fastidiosa
Xylem limited, insect vectored. Rod shaped, wide host range
Xylella fastidiosa host range
- Grape (Pierce’s Disease)
- Citurs (Citrus Variegated Chlorosis, CVC)
- Blueberry (Bacterial Leaf Scorch)
- Almond, peach, coffee, olive, pecan, plum…
Liberibacter spp.
Transport of organic compounds… associated with Citrus Greening Disease (aka Huanglongbing or yellow dragon disease)
Liberibacter spp. symptoms
- Leaf chlorosis
- Yellowing of tree in quadrants
- Immature fruits (greening)
- Small, lopsided fruit
- Bitter taste
Liberibacter spp. management
- No cure… control insect vectors!
- Scout for affected trees, remove (4x/year)
- Use pathogen free material
Candidatus Liberibacter asiaticus
LAS, suspected causal agent of HLB… non-culturable
Mollicutes
No cell wall
Two genera:
- Phytoplasma
- Spiroplasma
Phytoplasma sp.
- MLO (Mycoplasma-Like-Organism)
- Cell wall absent, “unit membrane”
- non spore forming
Spiroplasma citri
- No cell wall, helical shape
- Only known to parasitize plants
- May parasitize insects and grow saprophytically
