Technical Abstract: Pecan scab (Fusicladium effusum) causes significant economic losses of pecan throughout the southeastern US. Disease assessment relies on visual rating of disease severity, which can be inaccurate, imprecise, with poor repeatability and reproducibility. Accurate, precise assessments are important for comparing treatments to manage disease. A total of 26 raters assessed 2 sets of 40 images of pecan fruit valves infected with pecan scab and estimated the area diseased. The first set of images (E1) was assessed without using standard area diagrams (SADs), while the second set (E2) was assessed using a SAD set comprised of 10 black and white images of known disease severity. Estimates of E1 and E2 were compared to the actual disease severity, determined by image analysis. Use of the SADs improved rater precision, accuracy and repeatability among raters. Linear regression showed that rater precision improved between the unaided and SAD-aided assessments (r2=0.44-0.94 and r2=0.84-0.95, respectively). Lin’s concordance correlation coefficient ('c) demonstrated improved agreement as a result of using the SAD set ('c=0.28-0.95 for E1 compared to 'c=0.68-0.96 for E2), and rater bias measured by location (µ) and scale (') shifts was reduced using SADs (E1, µ =-0.17-1.57; ' =1.01-1.53 and E2, µ =-0.08-0.84; ' =0.95-1.22, respectively). SADs improve rater estimates of pecan scab severity on fruit.