1. 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

2. 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

3. Methods to assess disease intensity Disease incidence: The proportion of infected host units, out of the total units sampled 4/10*100=40%

4. 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% 60103010 0 0 Percentage scale (0-100%)

5. 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?

6. Can disease incidence records be converted to disease severity ? 100 Disease incidence (%) 0 Disease severity (%) 0 100 50 Disease incidence (%) 0 100 Disease severity (%) 0 10 50 5

7. 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

8. 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

9. Which part of the plant should be assessed? Flag leaf Leaf -1 Leaf -3 Leaf -5 Leaf -2 Leaf -4 Septoria leaf blotch caused by Mycosphaerella graminicola

10. Which type of symptom should be assessed? Fruit rot Fruit ghost spots Grey mould caused by Botrytis cinerea Leaf infection Stem infection

11. How to assess disease severity? The Webber-Fechner low: 1. The eye can detect logarithm differences 40% 43%

12. How to assess disease severity? The Webber-Fechner low: 2. Up to 50% severity - the eye detect the diseased, out of the healthy portion above 50% severity - the eye detect the healthy, out of the diseased portion

13. 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%

14. Pictorial scale (% diseased area) 0% 1% 2% 5% 10% 20% 50% 80% 90% 95% Scales for assessment of disease severity

15. Descriptive verbal scale (%) 0%Late blight is not seen in the field 0.5%Up to one lesion per 100 m 2 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

16. 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

17. 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

18. Descriptive index - sporulation of Botrytis cinerea on tomato stem pieces 12345 Scales for assessment of disease severity

19. 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

20. Nominal variable identification of the records Methods for analyzing disease records - Variable types numbers on the shirts of football players (1,2,7,12) 6

21. 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

22. Methods for analyzing disease records - Variable types weight, length…. 20 kg 5 kg Continuous variable identification of the records the order of the records the ratio between records

23. Methods for analyzing disease records Statistics and variable types Variable type statistics Frequent mid-range median mean number continuous nominal ordinal

24. 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

25. 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%

26. Pictorial scale 0%0% 1%1% 2%2% 5%5% 10% 20% 50% 80% 90% 95% Type of variable: continuous Most frequent no. Mid-range Median Mean

27. 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

28. 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

29. 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

30. 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

31. Descriptive index - sporulation of Botrytis cinerea on tomato step pieces 12345 Type of variable: ordinal Most frequent no. Mid-range Median Mean

32. Descriptive index - sporulation of Botrytis cinerea on tomato stem pieces 01234 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 01234 Sporulation index No. of spores (x10 6 ) Mean number of spores: 1,500,000 Mean number of spores: 10,750,000

33. For disease records that are ordinal variables, non-parametric statistics should be used Tests for comparing two treatments Mann-Whitney Wilcoxon Single Rank Spearman Rank Correlation Coefficient Kolmogrov-Smirnov Wald-Wolfowitz Runs Tests for comparing three (or more) treatments Krustal-Wallis Friedman

34. Methods for presenting disease records Treatment Disease severity (%) Cultivar A82.5 a Cultivar B64.3 b Cultivar C50.3 bc Cultivar D25.9 c Cultivar E 4.1 d

35. Methods for presenting disease records Time (days) Disease severity (%) =SE SE=standard error

36. There are also problems ! 60% ?

37. Measurement of healthy leaf area digital analyzers video camera measurement of photos 60% Total leaf area = 125 cm 2 healthy leaf area = 98 cm 2 diseased leaf area = 27 cm 2

38. Measurement of radiation Disease severity (%) Reflected radiation (%) reflected radiation observed radiation

39. Measurement of radiation Disease severity (%) Reflected radiation (%) radiometer

40. 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.