METHODS TO ASSESS PECAN SCAB

Pecan scab (Fusicladium effusum [G. Winter]) is the most important disease of pecan (Carya illinoinensis Koch.) in the US. Measuring the severity of scab accurately and reliably and providing data amenable to analysis using parametric statistics is important where treatments are being compared to minimize the risk of Type II errors (failure to reject the null hypothesis, H0, when H0 is false). The Horsfall-Barratt (H-B) category scale and its derivatives are commonly used to assess disease.
Estimates using the H-B scale were compared to nearest percent estimate (NPEs) for rating scab severity on fruit valves.
The true severity was measured using image analysis.
Both inexperienced and experienced raters were included in the experiment.
Lin’s concordance correlation coefficient (ρ_c) showed that agreement was variable among raters using NPEs (ρ_c=0.57-0.96), and when estimates were made using the H-B scale (ρ_c=0.59-0.98). Neither experienced nor inexperienced raters were consistently better using either method.
However, a bootstrap analysis indicated that among experienced raters, precision (r) and agreement (ρ_c) were more often reduced when using the H-B scale compared to NPEs.
Inter-rater reliability using the H-B scale was never better than NPEs.
Regression analysis suggested that raters who were inherently fast in assessing disease with NPEs were often slower when using the H-B scale, but raters who were slow assessing with NPEs were faster using the H-B scale.
Thus, there is no advantage in accuracy or reliability, nor a reduction in time when raters use a category rating scale compared to a continuous ratio-type scale to assess pecan scab, and in some cases category-scale estimates are inferior.

I International Symposium on Pecans and Other Carya in Indigenous and Managed Systems

C.H. Bock , B.W. Wood, T.R. Gottwald

disease severity, Carya illinoinensis, pecan scab, integrated pest management

https://doi.org/10.17660/ActaHortic.2015.1070.24