Development and validation of a diagrammatic scale for quantifying maize leaf spots caused by Diplodia macrospora

Abstract

The objective of this study was to develop and validate a diagrammatic scale to evaluate the severity of spots on maize leaves caused by the fungus Diplodia macrospora. Severity ranged between the minimal (0.5%) and maximal (55%) limits of disease severity, and intermediate severity levels were defined according to the "Weber-Fechner stimulus response law". The proposed scale describes six levels of severity based on how much of the leaf is affected: 0.5%, 3%, 8%, 23%, 36%, and 55%. Validation was carried out by eight evaluators, four inexperienced and four experienced. They estimated the severity of disease in 60 maize leaves, with and without the proposed diagrammatic scale. A relationship was shown by regression analysis between estimated and actual severity, with and without the use of the scale. When both inexperienced and experienced evaluators used the scale, they were able to estimate disease severity more accurately and precisely.

Diagrammatic scale for measuring severity of gray mould in thornless Castilla blackberry (Rubus glaucus Benth)

“…- 30%. With the scale five evaluators were outside the range of -10 to 10, two of them (15.38% of the total evaluators) with errors between 15 and 20%, similar results were found in previous studies (SANTOS et al, 2017;TROJAN et al, 2018;LORENZETTI et al, 2019). Therefore, eight evaluators can be considered good in the assessment of the disease when using the scale (LIMA et al, 2013;NUÑEZ et al, 2017).…”

“…Regarding the t-test for the values of the intercept (a), the null hypothesis was rejected in more cases when the scale was not used, with the scale accepted values increased in a 20%. R 2 values improved when the scale was used, the R 2 average showed an increase of 3.16% compared with the value obtained without the scale, in both cases the evaluation of severity showed good precision, the values of R 2 were ≥0.85, considered high for this type of estimate (LIMA et al, 2013;TROJAN et al, 2018;LORENZETTI et al, 2019).…”