Horsfall–Barratt scale

The Horsfall-Barratt scale is a system of assessing plant diseases where each plant is assigned a numerical value according to the percentage of leaf area showing disease symptoms. The Horsfall-Barratt scale was designed to compensate for human error in estimating the amount of disease present. The scale is as follows:

Index	Percent Affected
1	0
2	0 to 3
3	3 to 6
4	6 to 12
5	12 to 25
6	25 to 50
7	50 to 75
8	75 to 87
9	87 to 94
10	94 to 97
11	97 to 100
12	100

The scale is often used in modified form with index values from 0-11 or with different numbers of index points.

References

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