Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 30, 2012
Publication Date: June 1, 2013
Citation: Bock, C.H., Van Den Bosch, F., Parnell, S., Wood, B.W., Gottwald, T.R. 2013. The effect of Horsfall-Barratt category size on the accuracy and reliability of estimates of pecan scab severity. Plant Disease. 97:797-806. Interpretive Summary: Pecan scab (Fusicladium effusum) is the most destructive pecan disease. The ability to properly quantify scab severity in orchards is key to the development of improved control strategies The Horsfall-Barratt and the ‘nearest percent estimates (NPE)’ rating scales were evaluated over the natural range of actual disease. It was found that the NPE scale results in more precise and accurate estimates of disease severity. This work enables researchers and orchard managers to better assess the actual amount of disease in orchards.
Technical Abstract: Pecan scab (Fusicladium effusum) is a destructive pecan disease Disease assessments may be made using interval-scale based methods or estimates of severity to the nearest percent area diseased. To explore the effects of rating method [‘Horsfall-Barratt (H-B) scale estimates’ vs. ‘nearest percent estimates’ (NPEs)] on the accuracy and reliability of severity estimates over different actual pecan scab severity ranges on fruit valves, raters assessed two cohorts of images with actual area (0-6, 6+-25% and 25+-75%) diseased. Mean estimate of disease within each actual disease severity range varied substantially. Means estimated by NPE within each actual disease severity range were not necessarily good predictors of the H-B scale estimate at <25% severity. H-B estimates by raters most often placed severity in the wrong category compared to actual disease. Measures of bias, accuracy, precision and agreement using Lin’s Concordance Correlation depended on the range of actual severity, with improvements increasing with actual disease severity category (from 0-6 to 25+-75%), but the improvement was unaffected by the H-B assessments. Bootstrap analysis indicates that NPEs provided either equally good or more accurate and precise estimate of disease compared to the H-B scale at severities of 25+-75%. Inter-rater reliability using NPEs was greater at 25+-75% actual disease severity compared to using the H-B scale. Using NPEs compared to the H-B scale will more often result in more precise and accurate estimates of pecan scab severity, particularly when estimating actual disease severities in the range 25+-75%.