Pecan Scab Severity—Effects of Assessment Methods

Abstract

Pecan scab is caused by the fungus Fusicladium effusum, and is the most destructive disease of pecan in the United States. Accurate and reliable disease assessments are needed to ensure that data provide a measure of actual disease intensity. The Horsfall-Barratt (H-B) category scale and its derivatives are commonly used to assess disease. Estimates using the H-B scale were compared with nearest percent estimate (NPEs) for rating disease severity of pecan scab on valves of fruit. Both inexperienced and experienced raters were included in the experiment. Lin's concordance correlation showed that agreement using NPEs was variable (ρc = 0.57 to 0.96), whereas estimates of disease severity using the H-B scale had similar agreement among most raters (ρc = 0.59 to 0.98). Converted values of NPEs to the H-B midpoints (NPEH-B) also provided a similar range (ρc = 0.61 to 0.96). Neither experienced nor inexperienced raters were consistently better using any of the three methods. Bootstrap analysis indicated that, among experienced raters, precision (r) and agreement (ρc) were often reduced when using the H-B scale compared with NPEs. There was no consistent effect of converting NPEs to NPEH-B midpoint values compared with actual H-B values. Inter-rater reliability using the H-B scale was never better than NPEs. Bootstrap analysis indicated no difference in the length of time needed to assess disease but regression analysis suggested that raters who were inherently fast in assessing disease with NPEs were often slower when using the H-B scale; conversely, raters who were slow assessing with NPEs were often faster when using the H-B scale. Thus, there appears to be no advantage in accuracy or reliability or reduction in time when inexperienced or experienced raters used a category rating scale to assess pecan scab.