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9.1 Disease Appraisal Scales

The evaluation of disease severity is necessary, especially for epidemiological experiments as well as for measuring the level of resistance. Several scales are proposed for different diseases but none of them is suitable for all the diseases. Use of specific scales are necessary for leaf blight fungi, powdery mildew (Fig. 9.1), rusts (Fig. 9.2), for some spike diseases (Fig. 9.3) and for hemilthosporium (Fig. 9.4).

Fig. 9.1
figure 1

Disease appraisal scale for powdery mildew. Source: Mehta (1993)

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Fig. 9.2
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Disease appraisal scale for rusts the modified Cobb scale (A) actual percentage occupied by rust uredinia; (B) rust severities of the modified Cobb scale. Source: after Peterson et al. (1948), Roelfs (1992)

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Fig. 9.3
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Disease appraisal scale for scab-the fusarium head blight (FHB). Source: James (1971a, b)

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Fig. 9.4
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Disease appraisal scale for Helminthosporium leaf blights

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Rogenski et al. (2012) suggested application of artificial neural networks to estimate infection by leaf spot in wheat crop. These authors proposed a system based in artificial neural networks in order to estimate the percentage of infections of diseases, assisting in decision making and facilitating the monitoring of potentially infected areas. However, the system needs to be validated under different wheat growing conditions.

In the following pages, some diagrammatic scales are presented to aid the reader in disease appraisal and its interpretation. Normally, disease ratings should be made 3–4 times during the crop cycle to plot the progress of the disease epidemic. For further details the reader may refer to specific publications in this respect (Chester 1950; Browder 1971; James 1971a, b; Roelfs 1992).