Download DISEASE ASSESSMENT

DISEASE ASSESSMENT
Biological Monitoring Requirements
1. Meets objective = what you want to accomplish
2. Accurate and reproducible methods
3. Fast – consider cost and time required
***2 and 3 are often opposed (mutually exclusive)
Issues in disease measurement
Accuracy – goes down with size of sampling unit
Cost – goes up with the number of sampling units
There is some optimum number and size of sampling unit for a compromise with
accuracy and cost
Spatial variability
Aggregated
Random
Sampling plans
Methods to use
Measure incidence or severity?
Use area diagrams or scales?
Objectives for Disease Measurement
1. To relate epidemic development to climate
2. To assess cultivar susceptibility
3. To assess chemical efficacy
4. To relate pathogen development to physiological biochemical responses to infection
5. To relate disease to amount of yield loss (empirical)
6. To relate disease to crop growth (mechanistic)
7. To assess regional/national/global significance of disease
8. Determine if and when treatments are needed
Host Measurement
- Disease changes with time and other factors
- Course of diseases need to be tracked through season
- Diseases that begin later may have less severity
Disease measurements in different fields, regions,
and years must be taken at same growth stage of host
Need a way of describing host at time of assessment
- Growth stage diagrams
- Phenological degree-day models
Growth Stage Keys
These keys have diagrams illustrating the appearance of the plant as well as a brief written
description. They should also describe the amount of growth that has occurred at each
developmental stage.
This helps to make comparisons in situations when the time of year and length of growing season
may be different.
Descriptions actually refer to developmental stages not to “growth”.
Amount of growth between stages is not equal.
Determining stage is nondestructive so same plants can be observed repeatedly
Environmentally Driven Plant Development Models
Describe or predict phenological stage of host
Development rate changes with temperature
Use heat unit or degree-day concept
Plant development is a function of the number of degree-days accumulated between planting,
emergence, etc. and time of observation
Good for plants without distinctive growth stages
What part of plant to assess
Depends on objective and disease type
Youngest leaf
Oldest leaf
Selected leaves
All green leaves
Stems/awns/roots
Senescence?
Leaf Size?
When to assess
Depends on objective
Growth stage keys
Phenological timing
Days after planting or emergence
Disease sightings
Forecasting models
Quantifying Disease
Disease Intensity (x)
Can be Incidence or Severity
Disease Incidence = # of plant units infected as a % of all plant units
- Proportion of plant units diseased
+ or – , each unit is diseased or not
- Expressed as a %
e.g. 25% incidence of disease on leaves means
¼ of leaves show symptoms at any level
Disease Severity = area of plant tissue affected as % of total area
- Proportional area of plant unit diseased
- Expressed as a % of total tissue area
e.g. 25% severity of disease on leaves means
¼ of all leaf tissue shows symptoms
Disease Prevalence = percentage of fields attacked by a disease
- can be ambiguous, use cautiously at low levels of x
Disease Response = measure of “quality” of disease
- lesion may be necrotic, chlorotic, poorly sporulating
- keys should include response and severity
Measuring Disease Severity
Divide range between no disease and fully diseased into classes
Number of classes important
Too few = not enough resolution
Too many = takes too long
Best to have 10 or less, facilitates data entry
Disease Scales
= written and numerical description of classes
- diagrams or pictures are used to describe classes
Standard Diagrams (= standard area diagram)
= diagrams that show percentages of an area affected
- consist of a set of pictures illustrating classes
- observer tries to estimate the area symptoms would cover if all were collected together
Weber-Fechner Law = visual grading progresses logarithmically
As the area affected increases acuity of discrimination decreases.
Led to development of Horsfall-Barratt Rating scale
At < 50% disease, we see disease, at >50% we see healthy and subtract from total for disease.
Examples of Other Disease Indices
Peterson diagram for cereal rusts
= index based on area affected
Uredia can be different sizes
Flecks on leaves = open sporulating parts of leaves
Get two sets of numbers
1) actual % of leaf area covered (0-37%)
2) one transforming the area covered to relative disease (0-100)
100% disease is arbitrarily set at 37% of leaf area covered, defoliation occurs at about 40%
Second scale of relative disease is more commonly used.
= visual percentage of severity, not % of area
Field Keys = Written and numerical description of severity classes for rapid disease assessment
Disease Response Scales
Lesions reflect intensity of disease by their appearance, morphology or sporulation.
Response of plant to infection can be mild to severe = response classes are coded by letters,
numbers or codes.
Establishes a scale of response types or reaction types
Number of classes needs to be small as because response classification is difficult for average
observer.
***This type of scale measures characteristics of lesions, not % severity.
Useful for evaluating breeding material for resistance
Example: Rust Reaction Scale
I Immune = no attack
0 Resistant c – some chlorosis
h – hypersensitive reaction
n - necrosis
1 Few very small pustules (lesions) c, n, h
2 Intermediate size pustules
c, n
3 Normal pustule, but chlorosis also present
4 Fully compatible, normal sized pustule
Indirect Methods
Disease Incidence vs Disease Severity
It is easier to train people to count that to make proportional ratings.
Incidence on leaves (+ or -)
Can predict severity from incidence, which is easier to measure.
Then use severity values in relation to yield loss.
Only works at low levels of severity.
Otherwise, incidence approaches 100%
Good for scouting at low disease levels to determine if treatment is necessary.
Trapped spores vs incidence