Plant Disease Epidemiology

Plant Disease Epidemiology

1. Basic definitions

   • Epidemic
     • is a disease that affects a large number of people within a community, population, or region.
   • Pandemic
     • is an epidemic that’s spread over multiple countries or continents
   • Outbreak
     • •is a greater-than-anticipated increase in the number of endemic cases. It can also be a single case in a new area. If it’s not quickly controlled, an outbreak can become an epidemic.
   • The study of the development of disease in plant populations

2. Epidemic?

   • A phenomenon when a pathogen spreads to and affects many individuals within a population over a relatively large area and within a relatively short time.

3. Elements of epidemic

   • host factor
     • Levels of genetic resistance or susceptibility of host
     • Degree of genetic uniformity of host in a particular field
     • Type of crops
       • Annual crops & foliar or fruit diseases - rapid
       • Perennial woody diseases- slow (years)
     • Age of host plants
       • Some plants are susceptible only during growth period & become resistant during mature period
   • pathogen factors
     • Levels of virulence
     • Quantity of inoculum near hosts
     • Type of reproduction of the pathogen
       • polycyclic
       • monocyclic
     • Ecology of the pathogen
       • depends on type of pathogen
     • Mode of spread of the pathogen
       • vector-bone
       • air-borne
       • soil-borne
   • time factors
     • Season of the year
     • Duration & frequency of favorable temp. & rains
     • Appearance of vectors, etc.
   • human factors
     • Site selection & preparation
     • Selection of propagative materials
     • Cultural practices
     • Disease control measures
   • environmental factors
     • moisture
       • rain
       • dew
       • high humidity
     • temperature
       • Affects disease cycles of pathogens

4. Patterns of epidemics

   • Disease -progress curve
     • monocyclic diseases
       • Only one cycle of development (one infection cycle) per crop cycle
       • Only primary inoculum
       • root diseases, wilt diseases
     • polycyclic diseases
       • Produce more than one infection cycle per crop cycle
       • leaf rust, leaf blight, leaf spot, mosaic
   • Disease -gradient or dispersal curve
     • The amount of disease is greater near the source of inoculum
     • The amount of disease decreases with increasing distance from the source

5. Comparison of Epidemic Models

   • Polycyclic Model
     • Fast epidemic
     • Wind-borne, vectors
     • Ex: rust, leaf blight, leaf spot, powdery mildew, mosaic
     • Control
       • Stop pathogen reproduction
       • spray fungicides
       • use resistant varieties
     • Ex: potato late blight, r = 0.3- 0.5/day
   • Monocyclic Model
     • Slow epidemic
     • Ex: Root diseases, foot rot, wilt diseases
     • Soil-borne
     • Ex: Verticillium wilt, r = 0.02/day
     • Control
       • Reduce initial inoculum
       • destroy diseased plants or plant debris

6. Measurement of Plant Disease and Yield Loss

   • Disease incidence
     • The number of plant units that are diseased in relation to the total number of units examined
     • Commonly used to measure the spread of a disease
   • Disease severity
     • The amount of plant tissue that is diseased
     • Measured using assessment scales or by determining the area under a disease progress curve (AUDPC)
   • Yield loss
     • The proportion of yield that the grower will not be able to harvest due to disease
     • Results in economic loss

7. New Tools in Epidemiology

   • Molecular tools
     • PCR, ELISA, DNA fingerprinting
     • rapid & accurate detection & identification of pathogens
   • Data management
     • GIS, GPS, remote sensing
     • assist in disease control strategies