How much do standard area diagrams improve accuracy of visual estimates of plant disease severity? A systematic review and meta-analysis

Authors: DEL PONTE, EMERSON - Universidade Federal De Viçosa; CAZON, LUIS - Universidade Federal De Viçosa; ALVES, KAIQUE - Universidade Federal De Viçosa; PETHYBRIDGE, SARAH - Cornell College - Iowa; Bock, Clive

Submitted to: Tropical Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2021
Publication Date: 1/11/2022
Citation: Del Ponte, E.M., Cazon, L.I., Alves, K.S., Pethybridge, S.J., Bock, C.H. 2022. How much do standard area diagrams improve accuracy of visual estimates of plant disease severity? A systematic review and meta-analysis. Tropical Plant Pathology. https://doi.org/10.1007/s40858-021-00479-5.
DOI: https://doi.org/10.1007/s40858-021-00479-5

Interpretive Summary: Plant disease severity is commonly estimated visually without or with the aid of standard area diagram sets (SADs). It is generally believed that the use of SADs leads to less biased (more accurate) and thus more precise estimates, but the degree of improvement has not been characterized in a systematic manner. We built on a previous work using meta-analysis to analyze data from 72 SAD studies that had linear regression data (intercept = constant bias; slope = systematic bias, precision = Pearson's correlation coefficient, r) for each rater comparing estimates without and with SADs. The results showed an overall gain of 0.07 (r increased from 0.88 to 0.95) in precision. There was a reduction of 2.65 units in the intercept - from 3.41 to 0.76, indicating a reduction in the constant bias. Slope was least affected - the analysis determined that slope was reduced slightly from 1.09 to 0.966, indicating marginally less systematic bias when using SADs. Lesion size, number and coalescence affected accuracy of estimates. By quantitatively exploring how characteristics of the pathosystem and how SADs affect precision and constant and systematic biases, we affirm the value of SADs for reducing bias and imprecision of visual assessments. We have also identified situations where SADs have greater or lesser effects as an assessment aid.

Technical Abstract: Plant disease severity is commonly estimated visually without or with the aid of standard area diagram sets (SADs). It is generally believed that the use of SADs leads to less biased (more accurate) and thus more precise estimates, but the degree of improvement has not been characterized in a systematic manner. We built on a previous review (Del Ponte et al. 2017) and screened a total of 153 SAD studies published from 1990 to 2021. A systematic review resulted in a selection of 72 of the studies that reported three linear regression statistics for individual raters, which are indicative of the two components of bias (intercept = constant bias; slope = systematic bias) and precision (Pearson's correlation coefficient, r), to perform a meta-analysis of these accuracy components. The meta-analytic model determined an overall gain of 0.07 (r increased from 0.88 to 0.95) in precision. Globally, there was a reduction of 2.65 units in the intercept - from 3.41 to 0.76, indicating a reduction in the constant bias. Slope was least affected - the analysis determined that slope was reduced slightly from 1.09 to 0.966, indicating marginally less systematic bias when using SADs. A multiple correspondence analysis suggested an association of less accurate, unaided estimates with diseases that produce numerous lesions and that for which maximum severities of 50% are rarely attained. On the other hand, more accurate estimates were observed with diseases that cause only a few lesions and those diseases where the lesions coalesce and occupy more than 50% of the specimen surface, which was most pronounced for specimen types other than leaves. By quantitatively exploring how characteristics of the pathosystem and how SADs affect precision and constant and systematic biases, we affirm the value of SADs for reducing bias and imprecision of visual assessments. We have also identified situations where SADs have greater or lesser effects as an assessment aid.