Plant Disease - Bacteria and Fungi Flashcards
What is a plant disease?
Any disturbance brought about by a pathogen or environmental factor which interferes with manufacture, translocation or utilisation of food, mineral nutrients and water in such a way that the affected plant changes in appearance and/or yields less than a healthy plant of the same variety
When was the potato late blight and what caused it?
1845-47
Phytophthora infestans
What are the losses due to disease?
Loss of plants Reduction in yield Reduction in quality Post-harvest loss Costs of management
7 different types of pathogens
- Viruses and viroids
- Bacteria, Mollicutes and mycoplasmas
- Protists
- Protozoa
- Fungi
- Nematodes
- Parasitic plants
What is a pathogen?
An organism capable of causing disease
What is virulence?
The degree of disease caused on a particular host
What is a pathovar?
(pv.)
In bacteria; a subspecies / group of strains that infect a particular host
Forma specialis
(f.sp.)
In fungi; a group of races that infect a particular genus or species of plant
What are Koch’s 4 Postulates?
- The microoorganism must be found in abundance in all organisms suffering from the disease, but it must not be found in healthy organisms
- The microorganism must be isolated from a diseased organism and grown in pure culture
- The cultured microorganism should cause disease when introduced to a healthy organism
- The microorganism must be reisolated from the inoculated, diseased experimental host and be identified as being identical to the original specific causative agent
What did Koch prove?
That Anthrax and TB were caused by a bacterial infection
What are the problems with Koch’s postulates?
- Difficult for obligate parasites (restricted to a particular function or mode of life e.g. intracellular pathogens)
- Complex for viruses/viroids but transmission can show it is not a pathogen
- Difficult if host physiology/environment matters for the infection to occur
What is “Normal microbial flora”?
- Microbes that can benefit the plant
- Bacteria, yeasts and filamentous fungi
- Occupy suitable environmental niches and perhaps reduce pathogen attachment
- Promoted by secretion of sugars/amino acids
- Phyloplane (leaf surface) and rhizosphere (roots)
- Also endophytes/mycorrhiza
Disease: damping off
Poor germination - infection of the seed or seedling, especially in waterlogged soil
Causes of damping off
Pythium sp.
Rhizoctonia
Disease: wilt
Infection of roots or vascular tissue
Causes of wilt
Fusarium/Verticillium sp. Armillaria mellea (Honey fungus)
Disease: stem base
Causes
Collapse of plant Erwinia atroseptica (Blackleg of potato)
Disease: canker
Causes
Corky lesions on stems/roots/bark Itersonilia perplexans (parsnips) Stereum purpureum (plums)
Disease: leaf spots
Causes
Necrotic lesions on leaves Diplocarpon rosae (Rose blackspot) Mycosphaerella graminicola (Septoria on wheat)
Disease: leaf and glume blotch
Causes
Poor seed quality
Stagonospora nodorium
Cause of seed infection
Claviceps purpurea
Disease: Downey mildew
Cause
(Oomycete) hyphae inside leaf, spores made outside
Perenospora parasitica
Disease: powdery mildew
Cause
Fungus outside except feeding structures
Blumeria graminis
Disease: rusts
Cause
Pustules of spores erupt through leaf or stem cuticle
Puccinia sp.
Disease: smuts and bunts
Causes
Black masses of spores in seeds, flowers and galls
Ustilago sp.
Tilletia caries
Disease: galls/distorted tissue
Causes
Nematodes
Agrobacteria (crown gall)
Gibberella fujikuroi
Witches brooms
Cause of grey mould
Botrytis cinerea
What plant diseases do viral agents cause?
Yellowing
Flower breaking
Mosaics
Stunting
Environmental Factors which can cause plant diseases
- Temperature and light
- Water availability (flood/drought)
- Nutrient availability
- Pollution (and pesticides/herbicides)
- Soil type (pH and mineral content)
What nutrients do plants need?
Potassium Phosphorus Nitrogen Magnesium Manganese Iron Boron
What does Mg deficiency cause?
Chlorosis
What does Mn deficiency cause?
Grey flicking of leaves
If plants are supplied with the metal ions, the new plant cells will be healthy, but the grey cells will not recover
What does frost damage do to potatoes?
Scorch on foliage
Necrosis of tubers
What can uneven temperature do to tomatoes?
Cause blossom end rot (necrotic lesions on bottom of tomato)
What can hail do to wheat?
Cause seed to be threshed out of ear/seed head
What are the three components of the disease triangle?
Environment
Pathogen
Host
Where can a pathogen invade a plant?
Via: stomates, hydathodes, wounds (require free water) Via vectors (insects, nematodes)
Structure of plant cuticle?
- Leaf cuticle: waxy later
- Sub-cuticular layers: celluloses, hemicelluloses, pectates, proteins etc.
- Plant cell walls
- Dissolves extracellular enzymes
What is an appressorium?
A specialised cell typical to many fungal plant pathogens that is used to infect host plants. Uses turgor pressure to punch through plant cuticle
What are some diseases bacterial infections can cause?
- Necrotic lesions on leaves
- Soft rot of tissues
- Fire blight
- Scabs on fruit/tubers
- Galls
- Increased frost damage - ice nucleation
Features of most bacteria which infect plants?
Gram-negative
Rod-shaped
Motile (flagellate)
What plant causes wildfire in tobacco and how?
- Pseudomonas syringe pv. tabaci
- Tabtoxin produced, causing chloroform halos around the lesion beyond the spread of bacteria
- Plant cells leak nutrients and water into apoplast, bacteria use these for growth
- Pectic enzymes degrade the mid-lamella of walls, separating cells, giving soft tissue
- Water-soaked lesions, rapidly turning necrotic
What are the three factors in the disease triangle?
Host, environment and pathogen
How do pathogens pass the plant cuticle?
Via stomates, hydathodes or wounds
Requires free water or a vector (Nematodes or insects)
What are examples of diseases caused by bacterial pathogens?
- Necrotic lesions on leaves
- Soft rot of tissues
- Fire blight
- Scabs in fruit/tubers
- Galls
- Increased frost damage - ice nucleation
Features of most bacterial plant pathogens and exceptions
-Gram-negative
-Rod shaped
-Motile (flagellates)
Exceptions: Streptomycetes and Corynebacterium
Wildfire in tobacco
- Caused by Pseudomonas syringe pv. tabaci
- Produces tabtoxin, causing chlorotic halos around the lesion beyond the spread of the bacteria
- Plant cell’s leak nutrients and water into the apoplast, bacteria use these for growth
- Pectic enzymes degrade the mid-lamella of walls, separating cells, giving soft tissue
- Water-soaked lesions, rapidly turning necrotic
- Bacteria overwinter in soil, debris and on tobacco seeds
What causes Fireblight in fruits such as apples in orchards?
Erwinia amylovora
Fireblight in apples or pears
- Insect transmission to nectaries, also wounds, stomates, lenticels
- Rapid colonisation of vessels -spread through tissue
- Little if any CWDE or toxin production
- Canker on bark, oozing bacteria when humid
- Dry, shrivelled fruit stay on tree
- Produces amylovoran - an extracellular polysaccharide. Non-producers show reduced virulence
- Trees look as scorched as if by fire damage
What is Quorum sensing?
Method used by bacteria to communicate and coordinate their behaviour
Can be use to monitor size of bacterial population and coordinate efforts to infect
What is a common signal molecule used by bacteria to communicate?
N-sculpt Homoserine Lactone (AHL)
What do bacteria do if only a small concentration of AHL is being produced?
They exhibit ‘individual behaviours’
What happens when there are sufficient levels of AHL present that they cross a threshold concentration?
- It means there is a large enough population of bacteria for them to exhibit ‘group behaviour’
- Gene expression begins in all bacteria, to produce a toxin and affect the plant to the highest degree
Are signal molecules in Quorum sensing the same across species?
No, very limited cross-talk between species
Bacterial soft rot example
Erwinia carotovora (aka Pectobacterium carotovorum)
- Enter via wounds
- Secrete massive amounts of CWDEs
- Tissue collapse
- Infection of nearby material
- Problem in storage
- Control by good hygiene
What can tissue distortions due to hormone production cause?
Tumors, galls, cankers and “crazy roots”
What does Agrobacterium tumefaciens do?
- Natural genetic engineer
- Transfers T-DNA into plant genome
- Causes synthesis of opines (food for bacterium - strain specific)
- Plant cell’s produce cytokines and auxin (triggers cells to reproduce and get bigger)
- Hyperplasia and hypertrophy (galls)
- Bacteria multiply within the gall
How does agrobacterium find plants to infect?
Wounded plant cells give out chemical signals which the bacteria detects and uses flagella to swim towards
What is T-DNA?
Transfer DNA
Transferred DNA of the tumour-inducing (Ti) plasmid of some bacterial species
Many genes including Opine and Cytokinine genes only expressed once in the plant cell
What types of eukaryotes can cause plant diseases?
Protists and protozoans
Examples of protists and protozoans
Myxomycetes
Plasmodiaphoromycetes
Oomycetes
(Fungus-like organisms, often motile)
Example of amoebal slime mould
Physarum sp.
Feautures of Physarum sp.
Relatively uncommon
Amoebal mass
Not serious but unsightly
Appear in warm wet weather
How do amoebal cells infect plants?
- Motile cell
- Enters plant
- Becomes MEGACELL
What is a megacell and what happens to it during sporulation?
Multinucleate mass of cytoplasm but with one cell membrane
When sporulating it goes through ‘free cell formation’ meaning the megacell forms a membrane and wall around each nucleus, generating lots of small separate cells from within the one megacell
Examples of amoebal (plasmodial) infections
Clubroot of brassicas
Hook-root of watercress
Difference between resting spores and motile zoospores
Resting spores germinate if near roots
Motile zoospores search out roots
What is hypertrophy?
Cell enlargement
What is hyperplasia?
Cell division
Symptoms of clubroot in brassicas
Hypertrophy Hyperplasia Produce large swellings on roots Plant continues to grow weakly Plants susceptible to drought
How long do the resting spores of Plasmodiophora brassicae last?
Over 10 years
What is the solution to preventing spread of P. brassicae
Don’t grow brassicas
Treat fields with lime
