THE EFFECT OF USING A HORSFALL-BARRATT SCALE ON PRECISION AND ACCURACY OF VISUAL ESTIMATION OF POTATO LATE BLIGHT SEVERITY IN THE FIELD

Assessments of potato late blight severity by eleven people using directly estimated percentage severity (EPS) of potato late blight caused by Phytophthora infestans were compared with the assessments of another group of ten people using a Horsfall-Barratt scale with wider intervals in the mid range of severity (25-75% infection). This scale is frequently used at the International Potato Center (CIP) to select for resistant germplasm. EPS readings had greater variability among assessors at mid range of severity than at extremes, as predicted by Horsfall and Barratt (1945). Transformation of EPS to CIP scale units or logits corrected uneven variances. Direct utilization of the CIP scale in the field, however, did not correct uneven variances. This may have been due in part to subconscious linearization of scale intervals, which are based on logarithms. Direct comparisons made by converting EPS to CIP scale levels demonstrated that precision and accuracy were greater among assessors using EPS.