plant diseases

Question 1

what are the 2 ways in which plant pathogens can be transmitted?

Answer 1

1. direct

2. indirect transmission

Question 2

Give 4 examples of direct transmission?

Answer 2

1. 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
2. Many fungi produce spores as a means of sexual or asexual reproduction. These spores may be carried by the wind - airborne transmission
3. 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
4. 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

Question 3

give an example of indirect transmission?

Answer 3

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

Question 4

what is ring rot caused by?

Answer 4

ring rot is caused by a bacterium

• ring of decay in the vascular tissue of a potato tuber or tomato accompanied by leaf wilting

Question 5

what is tobacco mosaic virus caused by?

Answer 5

Tobacco mosaic virus is caused by tobacco mosaic virus

• mottling an discoloration of leaves

Question 6

what is black sigatoka caused by?

Answer 6

black sigatoka is caused by a fungi

- causes leaf spots of banana plants reducing yield

Question 7

what is blight caused by?

Answer 7

blight is caused by a protoctistan

blight affects both leaves and potato tubers
bight is often associated with the browning of leaves

Question 8

what is dutch elm disease caused by?

Answer 8

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

Question 9

why do plants get infected by pathogens?

Answer 9

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

Question 10

what do plants not have?

Answer 10

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

Question 11

Give examples of the structural, chemical and protein-based defences which plants have to detect and prevent them from causing extensive damage:

Answer 11

• passive defences

- active defences

Question 12

What are passive defences?

Answer 12

passive defences are present before infection

The role of passive defences is to prevent entry and spread of the pathogen

Question 13

give 8 examples of passive defences?

Answer 13

cellulose cell wall
lignin thickening of cell walls
waxy cuticle
bark 
stomatal closure
tylose
callose
chemical defences

Question 14

what is the role of the cellulose cell wall?

Answer 14

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

Question 15

what is the role of lignin thickening of cell walls?

Answer 15

Lignin thickening of cell walls provides a WATERPROOF and almost completely INDIGESTIBLE layer to prevent pathogens

Question 16

what is the role of bark?

Answer 16

Most bark contains a variety of CHEMICAL DEFENCES that work against pathogenic organisms

Question 17

what is the role of the waxy cuticle?

Answer 17

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

Question 18

what is stomatal closure?

Answer 18

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

Question 19

what is tylose?

Answer 19

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

Question 20

What else do these balloon like tylose vessels contain?

Answer 20

tylose contain a high concentration of chemicals such as terpenes that are toxic to pathogens

Question 21

what is a callose?

Answer 21

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

Question 22

what are chemical defences?

Answer 22

plant tissues also contain a variety of chemicals that have anti-pathogenic properties e.g. TERPENOIDS and PHENOLS

Question 23

Why are most of a plant’s chemical defences not produced until the plant detects an infection?

Answer 23

Most of these chemicals are not produced until the plant detect an infection as the production of chemicals requires a lot of energy

Question 24

what is an active defence?

Answer 24

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

Question 25

give 5 examples of active defences?

Answer 25

1. thickened cellulose cell wall
2. oxidative bursts
3. deposition of callose between cell wall and cell membrane
4. an increase in production of chemicals
   5 necrosis .

Question 26

describe the thickening of the cellulose cell wall?

Answer 26

the cell walls become thickened and strengthened further with additional cellulose to prevent the entry of pathogens

Question 27

describe what oxidative bursts are?

Answer 27

certain plant cells produce highly reactive oxygen molecules capable of damaging the cells of invading organisms

Question 28

describe the deposition of callose between cell wall and cell membrane?

Answer 28

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

Question 29

name the type of chemicals whose production is increase?

Answer 29

terpenoids
phenols
alkaloids
defensins
hydrolytic enzymes

Question 30

What is the action of terpenoids?

Answer 30

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

Question 31

what is the action of phenols e.g. tannin

Answer 31

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

Question 32

what is the action of alkaloids?

Answer 32

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

Question 33

what is the action of defensins (defensive proteins)?

Answer 33

Defensins - these are proteins that appear to act upon molecules in the PLASMA MEMBRANE of pathogens, possibly inhibiting the action of ion transport channels

Question 34

what is the action of hydrolytic enzymes?

Answer 34

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

Question 35

what is necrosis?

Answer 35

necrosis is deliberate cell suicide

Question 36

Why does necrosis take place?

Answer 36

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

Question 37

what is a canker?

Answer 37

a physical symptom of disease sometimes seen in plants that have been infected with fungi or bacterial pathogens is a canker

Question 38

What are cankers usually symptoms for?

Answer 38

Cankers are usually symptoms of injury associated with an open wound

Question 39

describe a canker?

Answer 39

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