plant diseases Flashcards
what are the 2 ways in which plant pathogens can be transmitted?
- direct
2. indirect transmission
Give 4 examples of direct transmission?
- Many pathogens present in the soil will infect the plant by entering the roots - especially if plant has been damaged by a burrowing animal or movement by storm
- Many fungi produce spores as a means of sexual or asexual reproduction. These spores may be carried by the wind - airborne transmission
- once a pathogen is inside the plant - it may infect all the vascular tissue - pathogens inside leaves are then distributed when the leaves are shed and carry the pathogens back to the soil where they can grown and infect other plants
- pathogens can also enter the fruit and seeds - this will then be distributed with the seeds - so many or all of the offspring are infected
give an example of indirect transmission?
indirect transmission often occurs as a result of an insect attack
spores or bacteria become attached to a burrowing insect e.g. beetle which attacks and infects the plant
when this occurs the pathogen is transmitted to an uninfected plant and the beetle is acting as a vector
what is ring rot caused by?
ring rot is caused by a bacterium
- ring of decay in the vascular tissue of a potato tuber or tomato accompanied by leaf wilting
what is tobacco mosaic virus caused by?
Tobacco mosaic virus is caused by tobacco mosaic virus
- mottling an discoloration of leaves
what is black sigatoka caused by?
black sigatoka is caused by a fungi
- causes leaf spots of banana plants reducing yield
what is blight caused by?
blight is caused by a protoctistan
blight affects both leaves and potato tubers
bight is often associated with the browning of leaves
what is dutch elm disease caused by?
dutch elm disease by a fungi
dutch elm disease is a fungal disease of elm trees that is spread by elm bark beetles
the elm bark beetles act as a vector
dutch elm disease is associated with leaves that start to wither and go yellow in summer, months before the normal autumnal leaf shedding
- this spreads progressively to the rest of the tree with further dieback of branches
why do plants get infected by pathogens?
plants manufacture sugars in photosynthesis and convert those sugars to a wide variety of compounds such as proteins and oils
therefore plants represent a rich source of nutrients for many organisms such as bacteria, fungi, protoctists, viruses, insects, vertebrates
what do plants not have?
Plants do not have an immune system comparable with animals but they have developed a wide range of structural, chemical and protein-based defences which can detect and prevent them from causing extensive damage
Give examples of the structural, chemical and protein-based defences which plants have to detect and prevent them from causing extensive damage:
- passive defences
- active defences
What are passive defences?
passive defences are present before infection
The role of passive defences is to prevent entry and spread of the pathogen
give 8 examples of passive defences?
cellulose cell wall lignin thickening of cell walls waxy cuticle bark stomatal closure tylose callose chemical defences
what is the role of the cellulose cell wall?
the cellulose cell wall not only acts as a PHYSICAL BARRIER but most plant cell walls contain a variety of CHEMICAL DEFENCES that can be activated when a pathogen is detected
what is the role of lignin thickening of cell walls?
Lignin thickening of cell walls provides a WATERPROOF and almost completely INDIGESTIBLE layer to prevent pathogens
what is the role of bark?
Most bark contains a variety of CHEMICAL DEFENCES that work against pathogenic organisms
what is the role of the waxy cuticle?
The waxy cuticle prevents water collecting on the cell surfaces. Since pathogens collect in water and need water to survive, the absence of water is a passive process
what is stomatal closure?
stomata are possible points of entry for pathogens
stomatal aperture is controlled by guard cells and when pathogenic organisms are detected the guard cells, the guard cells will close the stomata in that part of the plant
what is tylose?
tylose is a balloon like swelling that fills the xylem vessels
when a tylose is fully formed, it plugs the xylem vessel and therefore the xylem vessel can no longer carry water - this helps to prevent the spread of pathogens
What else do these balloon like tylose vessels contain?
tylose contain a high concentration of chemicals such as terpenes that are toxic to pathogens
what is a callose?
A callose is a large polysaccharide that is deposited in the sieve tube at the end of growing season
a callose is deposited around the sieve plates and blocks the flow in the sieve tube
it can help prevent the spread of the pathogen
what are chemical defences?
plant tissues also contain a variety of chemicals that have anti-pathogenic properties e.g. TERPENOIDS and PHENOLS
Why are most of a plant’s chemical defences not produced until the plant detects an infection?
Most of these chemicals are not produced until the plant detect an infection as the production of chemicals requires a lot of energy
what is an active defence?
when pathogens attack, plant cells can recognise pathogens due to specific proteins and glycolipids. Plants respond to this by increasing its physical defences and producing defensive chemicals
give 5 examples of active defences?
- thickened cellulose cell wall
- oxidative bursts
- deposition of callose between cell wall and cell membrane
- an increase in production of chemicals
5 necrosis .
describe the thickening of the cellulose cell wall?
the cell walls become thickened and strengthened further with additional cellulose to prevent the entry of pathogens
describe what oxidative bursts are?
certain plant cells produce highly reactive oxygen molecules capable of damaging the cells of invading organisms
describe the deposition of callose between cell wall and cell membrane?
this deposition of callose (large polysaccharide) between the cell wall and cell membrane happens in cells near the invading pathogen and helps strengthen the cell wall and block plasmodesmatas to prevent the spread of the pathogen
name the type of chemicals whose production is increase?
terpenoids phenols alkaloids defensins hydrolytic enzymes
What is the action of terpenoids?
Terpenoids are a range of essential oils
terpenoids have antibacterial and antifungal properties
terpenoids also create a scent e.g. methanols produced by mint plants
what is the action of phenols e.g. tannin
Tannins are found in bark and inhibit attack by insects
Tannins do this by binding to the digestive enzymes in insects and deactivating them
insects that ingest a high amount of tannins do not grow and will eventually die - this helps to prevent transmission by pathogens
what is the action of alkaloids?
alkaloids are nitrogen containing compounds such as caffeine and cocaine
Alkaloids have a bitter taste to prevent herbivores from feeding
if the plant can prevent grazing it will suffer less damage. Damage is what can allow pathogens to enter the plant
what is the action of defensins (defensive proteins)?
Defensins - these are proteins that appear to act upon molecules in the PLASMA MEMBRANE of pathogens, possibly inhibiting the action of ion transport channels
what is the action of hydrolytic enzymes?
Hydrolytic enzymes are found in spaces between cell walls
an example of hydrolytic enzymes is chitinases - chitinases break down chitin in fungal cell walls causing fungi to die
what is necrosis?
necrosis is deliberate cell suicide
Why does necrosis take place?
sometime a few cells are sacrificed to save the rest of the plant
- by killing cells surrounding the infection - the plant can limit the pathogen’s access to water and nutrients and therefore prevent further spread
what is a canker?
a physical symptom of disease sometimes seen in plants that have been infected with fungi or bacterial pathogens is a canker
What are cankers usually symptoms for?
Cankers are usually symptoms of injury associated with an open wound
describe a canker?
cankers are usually elongated, oval in shape and sunken into woody tissue
cankers are a result of necrosis in plants and help to prevent spread of disease in a plant
