David M. Francis, The Ohio State University
The Horsfall–Barratt scale (Table 1) is used for the visual assessment of plant disease on a semi-quantitative scale. The scale is designed to compensate for human error in interpretation of the percentage of foliage infected. Small differences are easier to discriminate at the extremes of the scale, so the range of values for low infection and high infection percentages are narrow. In the middle of the scale, where it is more difficult to assess small differences, the range is larger.
Development of this lesson was supported in part by the National Institute of Food and Agriculture (NIFA) Solanaceae Coordinated Agricultural Project, agreement 2009-85606-05673, administered by Michigan State University. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the United States Department of Agriculture.