Poster: Epidemiology: Analytical & Theoretical Plant Pathology

602-P

**Discussion on calculation of disease severity index values from scales with unequal intervals.**

K. CHIANG (1), H. Liu (1), C. Bock (2) (1) Division of Biometrics, Department of Agronomy, National Chung Hsing University, Taiwan; (2) USDA-ARS-SEFTNRL, U.S.A.

When estimating severity of disease, a disease interval (or category) scale comprises a number of categories of known numeric values – with plant disease this is generally the percent area with symptoms (e.g., the Horsfall-Barratt (H-B) scale). Studies in plant pathology and plant breeding often use quantitative interval scales. The severity is estimated as an interval scale value and is used to determine a disease severity index (DSI). However, very few studies have investigated the effects of different scales on accuracy of the DSI. Using an interval scale designed for pecan scab (*Fusicladium effusum*), we investigated performance of the DSI. We used interval scale values, the scale data converted to midpoint values of each category, and nearest percent estimates (NPEs). Results of the pecan scab data indicate that there is a consistent bias of the mean value when applying the interval scale values directly. The mean value of the DSI per cultivar is underestimated as compared with that of NPEs; results of the mid-point conversion are akin to those of the NPEs. Furthermore, based on the data used, the simulation studies were executed to explore the mean value of DSI for three different parameters. The results of these simulation studies are consistent with those of studies which used real data. Finally, a new disease index, which uses equal intervals with additional grades at low severities, was developed to minimize the effect of the scale on error.