Appendix 5: Horsfall-Barratt Disease Rating System
An Explanation of Disease Rating Systems Used by Pest Management Scouts
There are many methods for estimating disease damage in agroecosystems. We have settled on the Horsfall-Barratt rating system. It is both precise and informative and takes a minimal amount of time. This system is based on some psychological studies done to determine the ability of the human eye to distinguish between different levels of disease. This psychologically based rating system is widely used among plant pathologists and other people across the country.
A list of the Horsfall-Barratt rating system follows. By reporting the actual results of the scouting survey, one can avoid words like "moderate," "severe," "light," and so on -- terms which are too vague and many times misleading.
The Horsfal-Barraatt rating system:
First establish a rating of:
- 1 = 0% Disease
- 2 = 1-3% Disease
- 3 = 3-6% Disease
- 4 = 6-12% Disease
- 5 = 12-25% Disease
- 6 = 25-50% Disease
Then work backwards based on the amount of disease-free tissue you have.
Rate 12 if there is no disease free tissue at all. (Everything is completely covered with disease.)
Rate 11 if 1-3% of the tissue is left free of disease.
Rate 10 if 3-6% of the tissue is left free of disease.
Rate 9 if 6-13% of the tissue is left free of disease.
Rate 8 if 13-25% of the tissue is left free of disease.
Rate 7 if 25-50% of the tissue is left free of disease.