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1.3 Assessment of disease intensity
•Introduction and definitions
•Methods for assessing disease intensity
•Methods for analyzing disease records
•Methods for presenting disease records
•Concluding remarks
Why do we need (want) to assess disease intensity?or
What are the uses of disease intensity records
•For making decision concerning disease management
•For quantifying the effect of the pathogens on the host plant and on yield
•For determining the efficacy of various control measures (pesticides, genotype resistance, agronomic measures, etc.)
•For verifying the quality of agronomic products and seed
Methods to assess disease intensity
Disease incidence:
The proportion of infected host units, out of the total units sampled
4/10*100=40%
4/10*100=40%
Methods to assess disease intensity
Disease severity:
The proportion of infected part (or the infected area), in each of the sampled units
(60+10+30+1)/10*100=10.1%
60 10 30
1 0 0 0 0 0 0
Percentage scale (0-100%)
4 2 2 1 0
0 0 0 0 0
(4+2+2+1)/10*100=0.9
Index scale (0-4)
What is better: to assess disease incidence
or to assess disease severity?
•What is easier?
•What is less subjective to errors?
•What represents better the effect of the pathogen on the host?
Can disease incidence records be converted to disease severity ?
100Disease incidence (%)
0
Dis
ease
sev
erit
y (%
)
0
100
50
50
Disease incidence (%)0 100
Dis
ease
sev
erit
y (%
)
0
10
50
5
What scale to use in the assessments?
•Number of infected blossoms in a blossom-cluster
•Number of infected blossoms in a blossom-cluster, out of the total number of blossoms in that cluster
•Number of infected blossom-clusters in a tree
•Number of infected blossom-clusters in a tree, out of the total number of blossom-clusters in that tree
Sampling unit: a tree
Sampling unit: blossom-cluster
•The number of trees with infected blossom-clusters
•Number of trees with infected blossom-clusters out of the number of trees in the orchard
Sampling unit: an orchard
Which part of the plant should be assessed?
Flag leafLeaf -1
Leaf -3
Leaf -5
Leaf -2
Leaf -4
Septoria leaf blotch caused by Mycosphaerella graminicola
Which type of symptom should be assessed?
Fruit rot Fruit ghost spots
Grey mould caused by Botrytis cinerea
Leaf infectionStem infection
How to assess disease severity?
The Webber-Fechner low:
1. The eye can detect logarithm differences
1% 3%
40% 43%
How to assess disease severity?
The Webber-Fechner low:
2. Up to 50% severity - the eye detect the diseased, out of the healthy portionabove 50% severity - the eye detect the healthy, out of the diseased portion
3% 12%
88% 97%
Scales for assessment of disease severity
The Horsfall and Barratt scale
1) 0%
2) 1 - 2%
3) 2 - 6%
4) 6 - 12%
5) 12 - 25%
6) 25 - 50%
12) 100%
11) 98 - 99%
10) 94 - 98%
9) 88 - 94%
8) 75 - 88%
7) 50 - 75%
Pictorial scale (% diseased area)
0% 1% 2% 5% 10%
20% 50% 80% 90% 95%
Scales for assessment of disease severity
Descriptive verbal scale (%)
0% Late blight is not seen in the field
0.5% Up to one lesion per 100 m2
1% Up to 5 lesions per plant
5% One out of 10 leaflets is infected
25% Almost all leaflets are infected. The field still looks green, but with many infected foci.
50% Half of the foliage is infected. The field looks infected.
75% Most of the foliage is diseased. The field looks defoliated with some green points.
95% Leaves were defoliated. Some of the stems are still green.
Scales for assessment of disease severity
Descriptive indexes
Number of lesions per leaf
0 = no lesions
1 = 1-10 lesions
2 = 10-20 lesions
3 = 20-50 lesions
4 = >50 lesions
Index 0 4
0 = Healthy
1 = Low disease severity
2 = Moderate disease severity
3 = High disease severity
4 = Dead plant
Scales for assessment of disease severity
Descriptive indexes
Number of lesions per leaf
0 = no lesions
5 = 1-10 lesions
15 = 10-20 lesions
40 = 30-50 lesions
60 = 50-70 lesions
85 = 70-100 lesions
Scales for assessment of disease severity
Number of lesions per leaf
0 = no lesions
1 = 1-10 lesions
2 = 10-20 lesions
3 = 30-50 lesions
4 =50-70 lesions
5 = 70-100 lesions
Descriptive index - sporulation of Botrytis cinerea on tomato stem pieces
1 2 3 4 5
Scales for assessment of disease severity
Methods for analyzing disease records - Descriptive statistics
Sample: 5,100,2,3,6,5,5,5,6,4
Frequent record: the most frequent record in the sample 5.0
Mean: sum of the values divided by the number of records 14.1
Mid-range: mean of the two extreme values 51.0
Median: the value that split the distribution of the records 5.0
Nominal variableidentification of the records
Methods for analyzing disease records - Variable types
numbers on the shirts of football players
(1,2,7,12)
6
school grade
(grade A, B, C….F)
Methods for analyzing disease records - Variable types
Ordinal variable
identification of the records
the order of the records
Methods for analyzing disease records - Variable types
weight, length….20 kg
5 kg5 kg
5 kg5 kg
Continuous variable
identification of the records
the order of the records
the ratio between records
Methods for analyzing disease records
Statistics and variable types
Variable type
statisticsFrequent mid-range median meannumber
continuous
nominal
ordinal
Methods for analyzing disease records
Statistics and variable types
The outcome is that calculation of means and conducting analysis of variance (ANOVA) tests may be done
only
for disease records that are continuous variables
The Horsfall and Barratt scale
Type of variable:
ordinal
Most frequent no.
Mid-range
Median
Mean
1) 0%
2) 1 - 2%
3) 2 - 6%
4) 6 - 12%
5) 12 - 25%
6) 25 - 50%
12) 100%
11) 98 - 99%
10) 94 - 98%
9) 88 - 94%
8) 75 - 88%
7) 50 - 75%
Pictorial scale
0%
1%
2%
5%
10%
20% 50% 80% 90% 95%
Type of variable:
continuous
Most frequent no.
Mid-range
Median
Mean
Descriptive verbal scale
0% Late blight is not seen in the field
0.5% Few infected plants. Up to one lesion per 100 sq. meters
1% Up to 5 lesions per plant
5% One out of 10 leaflets is infected
25% Almost all leaflets are infected. The field still looks green, but with many infected foci.
50% Half of the foliage is infected. The field looks infected.
75% Most of the foliage is diseased. The field looks defoliated with some green points.
95% Leaves were defoliated. Some of the stems are still green.
Type of variable:
continuous
Most frequent no.
Mid-range
Median
Mean
Type of variable:
ordinal
Most frequent no.
Mid-range
Median
Mean
Number of lesions per leaf
0 = no lesions
1 = 1-10 lesions
2 = 10-20 lesions
3 = 20-50 lesions
4 = >50 lesions
Index 0 4
0 = Healthy
1 = Low disease severity
2 = Moderate disease severity
3 = High disease severity
4 = Dead plant
Type of variable:
ordinal
Most frequent no.
Mid-range
Median
Mean
Number of lesions per leaf
0 = no lesions
1 = 1-10 lesions
2 = 10-20 lesions
3 = 30-50 lesions
4 =50-70 lesions
5 = 70-100 lesions
Type of variable:
continuous
Most frequent no.
Mid-range
Median
Mean
Number of lesions per leaf
0 = no lesions
5 = 1-10 lesions
15 = 10-20 lesions
40 = 30-50 lesions
60 = 50-70 lesions
85 = 70-100 lesions
Descriptive index - sporulation of Botrytis cinerea on tomato step pieces
1 2 3 4 5Type of variable:
ordinal
Most frequent no.
Mid-range
Median
Mean
Descriptive index - sporulation of Botrytis cinerea on tomato stem pieces
0 1 2 3 4
Treatment 1: 4 replicates: 1,2,2,1
Mean index = 1.5
Treatment 2: 4 replicates: 3,3,4,4
Mean index = 3.5
0
5
10
15
20
0 1 2 3 4
Sporulation index
No.
of
spor
es (
x106 )
Mean number of spores: 1,500,000
Mean number of spores: 10,750,000
For disease records that are ordinal variables, non-parametric statistics should be used
Tests for comparing two treatments
Mann-WhitneyWilcoxon Single RankSpearman Rank Correlation CoefficientKolmogrov-SmirnovWald-Wolfowitz Runs
Tests for comparing three (or more) treatments
Krustal-WallisFriedman
Methods for presenting disease records
Treatment Disease severity (%)
Cultivar A 82.5 a
Cultivar B 64.3 b
Cultivar C 50.3 bc
Cultivar D 25.9 c
Cultivar E 4.1 d
Methods for presenting disease records
Time (days)
Dis
ease
sev
erit
y (%
)
=SE
Time (days)
Dis
ease
sev
erit
y (%
)
=LSD
LSD=Least Significant Difference
SE=standard error
There are also problems !
60% ?
Measurement of healthy leaf area
digital analyzers
video camera
measurement of photos
60%
Total leaf area = 125 cm2
healthy leaf area = 98 cm2
diseased leaf area = 27 cm2
Measurement of radiation
Dis
ease
sev
erit
y (%
)Reflected radiation (%)
reflected radiation
observed radiation
Measurement of radiation
Dis
ease
sev
erit
y (%
)
Reflected radiation (%)
radiometer
Concluding remarks
•Disease intensity may be quantified in terms of disease incidence, disease severity or infected area
•The plant organ to be sampled and the disease symptom to be assessed, should be determined according the objective of assessment
•There are several types of scales for disease assessment: descriptive, pictorial, verbal.
•Analysis of the data should be based on the type of variable used - nominal, ordinal or continuous. Calculation of means and conducting ANOVA tests are permitted only for continuous variables.
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