Lecture 3

Question 1

Parasite

Answer 1

an organism that lives on or in some other organism for its food requirements

Question 2

Pathogen

Answer 2

an organism that causes disease in another organism

Question 3

Pathogenicity

Answer 3

ability of a pathogen to cause disease

Question 4

whats the relationship between pathgoens and parasites

Answer 4

Every pathogen is a parasite but not every parasite is a pathogen

Question 5

What do we need for a disease to occur?

3 items and what is the image called

Answer 5

Disease triangle
- A virulent pathogen
- A susceptible host
- Conducive environment for pathogen infection and disease development

Question 6

diff between disease cycle vs life cycle

Answer 6

Disease cycle : series of events that lead to development of disease from beginning of infection to perpetuation to again beginning of infection.

Life cycle: series of events for a pathogen’s life from spore production to production of fruiting bodies and spores again

Question 7

Inoculation

Answer 7

Initial contact of a pathogen with a plant site where infection begins

Question 8

Inoculum

definition & Primary vs secondary

Answer 8

= The pathogen entity (spores or mycelium) that comes in contact with plant host

primary inoculum = very first infection in a season on healthy crop or plant

Secondary inoculum = results in secondary infections

inoculum produced from the primary inoculum is called secondary inoculum

Question 9

Propagule

Answer 9

single unit of inoculum

Ex. single spore of rust/sclerotium of a
fungus/single hypha

Question 10

Forms of inoculum

4 types & examples of them

Answer 10

1. Fungi : spores, fruiting bodies or fruiting body parts, ascospores, basidiospores, sclerotia, conidia, mycelium.
2. Bacteria, phytoplasmas : whole cells
3. Viruses/viroids: particles or nucleic acids
4. Nematodes: eggs, larvae, or adults

Question 11

Sources of inoculum

Answer 11

• Plant debri
• spropagules in the soil (fungi, bacteria, and nematodes)
• seed or plant parts, weeds, volunteer plants, alternate hosts
• Insect-vectors or other abiotic vectors (survival is not long and rare on abiotic
  vectors such as tools, machinery) for viruses

Question 12

How do inoculum travel

Passive & active

Answer 12

Mostly passive
- wind (e.g. rusts, mildews)
- water (many pathogens)
- insects (most virus diseases)
- soil (e.g. zoospores, nematodes, oospores),
- plant residue (e.g. most necrotrophs or hemi-biotrophs).

Active arrival
* examples of certain nematodes,zoospores, bacteria, or fungi that moves/grows towards plants in response to chemicals/signals released by plant roots.

Question 13

How do pathogens that need to penetrate into the plant attach to the surface

Answer 13

• The propagules of these pathogens have mucilaginous surfaces that have mixtures of water-soluble polysaccharides, glycoproteins, lipids and fibrillar materials, which upon coming in contact with moist surface become sticky and help pathogen propagules adhere to the surface

Question 14

How does Fungi Mildew attach themself to the plants? Whats the main difference from the main pathogen methods

Answer 14

• fungi (mildews), do not need free water/moisture on plant surface as the spores release cutinase enzyme from their spores which makes plant surface hydrophilic and helps stick the spores.
• mildew spores have good moisture content relative to other fungi

Question 15

How does pathoden and plants recognize eachother

Answer 15

• through biochemical signals
• pathogens can easily infect if the host fails to recognize or defend itself

Question 16

3 types of penetration used by pathogens

Answer 16

1. Direct penetration
2. penetration through natural openings
3. penetration through natural wounds

Question 17

Describe Direct penetration

Question 18

Describe penetrathough through natural openings

Question 19

Describe penetration through natural wounds

Question 20

When is it a good time for spores to germinate?

Answer 20

• no competition from other spores in the vicinity
• host surface and conditions (water
  availability etc) are ideal for the germination

Question 21

How does germination happen and what structure does it form?

Answer 21

• Stored food reserves of the spore (lipids, carbohydrates, polyols) are mobilized for rapid formation of cell wall and germ tube formation

Question 22

What is the Germ Tube

how does it differ from mycelium

Answer 22

• Formed during germination
• specialized structure that perceives host surface
• Differs from mycelium as it grows on the outside of the host’s surface. Dosent grow very far before forming appressorium

Question 23

What is the appressoria and what does it do to the plant?

Answer 23

• adhere tightly to the host surface and secrete extracellular enzymes, generate physical force to penetrate the plant cuticle

Question 24

How much pressure is inside the appressorium

Answer 24

40 times greater than the pressure of a typical car tire

Question 25

How does the appressorium create so much pressure to physically penetrate the plant surface

Answer 25

• Turgor pressure is result of enormous accumulation of ‘glycerol’ in the appressorium & due to high osmotic pressure drawing water into the call and pushing the thin hypha through the host cuticle

Question 26

What is the hypha

Answer 26

the appressorial penetration peg that gets pushed into the host cuticle

Question 27

Definition of Infection

what are the results of infections

Answer 27

• process by which pathogens get established with the host and start obtaining food and nutrition
• appearance of symptoms and sometimes both symptoms and signs

Question 28

What can halt and infection?

And what is it called

Answer 28

• adverse environmental or host developmental conditions
• quiescent infections

Question 29

What phase comes after infection

Answer 29

• Incubation / latent period

Question 30

How does Fungi, nematodes, bacteria, viruses, viroids, phytoplasmas invade the plant

Answer 30

• Fungi and nematodes : grow inter- and intra-cellularly.
• Bacteria: invades intercellularly.
• Viruses, viroids, phytoplasmas: invade intracellularly

Question 31

How does Fungi, nematodes, bacteria, viruses, viroids, phytoplasmas colonize and reproduce

Answer 31

• Fungi: mostly reproduce on plant surface
• bacteria : between cells and comes to the surface through wounds and cracks
• nematodes : lay eggs near root surface.
• Viruses, viroids, and phytoplasmas : reproduce inside cells

Question 32

Dissemination

Answer 32

= spread of the pathogen. (short thorugh nematodes, passivly though vectors)

Question 33

when does Overwintering happen, and what do the pathogens do?

Answer 33

= When the environment is not conducive and crop season is coming to an end, the pathogen pushes itself to start producing overwintering structures or ways to overwinter

• Fungi : eg. formation of resting spores, dormant mycelium, sclerotia, and in association with wild grasses or other hosts.
• Nematodes overwinter as eggs or even larvae.
• Bacteria : similar strategies as fungi but they do not survive well in soil except debris, seeds or weed plants.

Question 34

Monocyclic vs polycyclic diseases

their examples & inoculum type

Answer 34

Monocyclicdiseases
- pathogens complete only one cycle of disease in one year or one season
- smuts, root rots, vascular wilts
- primary inoculum is the only inoculum available
polycyclic diseases
- diseases can complete more than one generation of infection
- rusts, airborne pathogens, insect-borne viruses