Diagrammatic scale for assessment of soybean rust severity

TLDR

A diagrammatic scale was developed to assess soybean rust severity. The scale was constructed from leaflets representing severity levels defined by Weber‑Fechner law and validated by eight raters who estimated rust severity on 44 leaflets with and without the scale. The scale defined severity levels of 0.6, 2, 7, 18, 42, and 78.5 % and, in validation, raters using the scale achieved higher accuracy and precision, with inexperienced raters showing the greatest improvement, while all raters except one tended to overestimate severity without the scale.

Abstract

A diagrammatic scale to assess soybean (Glycine max) rust severity, caused by the fungus Phakopsora pachyrhizi, was developed in this study. Leaflets showing different severity levels were collected for determination of the minimum and maximum severity limits; intermediate levels were determined according to "Weber-Fechner's stimulus-response law". The proposed scale showed the levels of 0.6; 2; 7; 18; 42, and 78.5%. Scale validation was performed by eight raters (four inexperienced and four experienced), who estimated the severity of 44 soybean leaflets showing rust symptoms, with and without the use of the scale. Except for rater number eight, all showed a tendency to overestimate severity without the aid of the diagrammatic scale. With the scale, the raters obtained better accuracy and precision levels, although the tendency to overestimate was maintained. Experienced raters were more accurate and precise than inexperienced raters, and assessment improvements with the use of the scale were more significant for inexperienced raters.