L15 - Epidemiology (1) Flashcards
(15 cards)
Describe what percentage of plant disease is caused by each broad pathogen class
Virus - 47%
Fungus and Oomycete - 30%
Bacterium - 16%
Others (E.g. Nematode) - 7%
How can plant disease be studied generally if there is a large diversity of pathogens causing it?
- Concentrate on epidemiological features common to all
- E.g. infection, dispersal, cycles
Give the three necessary conditions for a large epidemic
Sketch the “epidemic triangle” and describe the key points
- Large no. of susceptible hosts
- Suitably virulent pathogen at high enough density
- Good environmental conditions for pathogen growth, infection + dispersal
(See sketch on pg 2)
Further expand on the host factors, pathogen factors and environmental factors that influence the epidemic triangle
Host factors:
- Resistance or susceptibility (age dependent)
- Degree of genetic uniformity
- Spacial extent (e.g. close together)
Pathogen factors:
- Virulence level
- Amount + quality of inoculum
- Life cycle + timing of reproduction
- Mode of spread
Environmental factors:
- Temperature and moisture
- Other microorganisms
- Human activity effect
Name the three general types of cycle we can study epidemiology through
1) Life cycle (point of view of pathogen)
2) Epidemiological cycle (view of host-pathogen interaction)
3) Infection cycle (point of view of host)
Sketch a reduced life cycle of Phytopthora infestans infecting potato (don’t worry about sexual reproduction)
Then sketch an epidemiological cycle of Phytopthora infestans
Then sketch an infection cycle (general) and define the terms used
See sketches on pg 2/3
Infection cycle:
- Incubation period: from infection to symptoms
- Latent period: from infection to infectiousness
Give the two general categories through of how disease is spread and describe the subcategories of each
Active dispersal:
- Mycelial growth of fungi
- “swimming” oomycetes, bacteria + nematodes
Passive dispersal: (generally inefficiency counteracted by lots of propagules)
Air:
- e.g. spores, carried large distance if canopy escaped
Water:
- Small, soil-borne organisms move w/ soil water flow
- Splash dispersal between leaves w/ rain
- Rain + wind = effective
Vectors:
- Mostly for viruses but not exclusive
- Humans often vector!
What are the four classes of strategy that can be used to control plant disease?
- Cultural control
- Biological control
- Chemical control
- Genetic control
Define cultural control and list 4 examples
Give the general disadvantages
- Human manipulation of growing systems to the disadvantage of plant pathogens
- Improve sanitation
- Increase inter-row distance
- Mixtures and multilines
- Crop rotation, delaying of planting or baiting
Can be wasteful, difficult + costly
Define biological control and list 4 example mechanisms
Give the general disadvantages
- Using natural enemies of pathogens to eradicate or control pathogen populations
- Competition
- Hyperparasitism and predation e.g. Rhizoctonia solani hyphae “strangled” Trichoderma viride
- Antibiosis
- Induced resistance
- Variable success, often only used under controlled conditions, e.g. glasshouse. Not in field
Define chemical control
List 4 general disadvantages with a specific example for one of them
Using chemicals to control disease:
- inorganic + organic chemicals used
- insecticides also sprayed to kill vectors
Disadvantages:
- Costly
- Harmful to environment, workers + host plants
- Disliked by consumers
- Pathogens can evolve resistance, especially sexual pathogens (quickly spreads due to huge selective advantage, e.g. Strobilurin resistance in wheat powdery mildew)
Define genetic control
List 4 general disadvantages with a specific example for one of them
- Use of resistant host varieties, created via selective breeding or GM
- Expensive to develop
- Resistance often overcome quickly especially w/ sexual pathogen or single R gene
- Widespread objection + legislation
Why do we model plant disease epidemics?
What is a compartmental model?
- To predict future progress of given epidemic
- Determine successful controls
Compartmental model:
- Divides plant population into classes
- Biological processes become transitions between compartments
Describe the most basic SI model
Give the differential equation that describes the rate of infection w.r.t. the no. of plants already infected
Sketch the graph of this equation as well as the curve of no. of infected plants against time
Two compartments:
- (S)usceptible
- (I)nfected
- Transition (infection) at rate beta
dI/dt = beta x I (N - I) - derive
(See graphs at end of L1)
What are the key outcomes of the basic SI model
- If no. of infected plants at t=0 > 0, all plants eventually become infected
- Unrealistic as we know not all plants always become infected
- Need to alter this model (next lecture)
