Avocado scab causes great economic losses in Mexico and the world. This study aimed to provide an automatic method of quantifying this symptom. The proposed method uses polygonal regions from the a-b plane of the CIE L*a*b* color format to identify a color (even with variations) and different avocado fruit shades. The polygons were used to determine the scab percentage in 70 Hass avocado fruit images. The scab percentage was estimated with a diagrammatic scale, and the regression equation y = 0.5848x, with an r 2 = 0.80, was found when comparing these results with scab percentages obtained from digitized images (observed). With the method presented in this study, the equation y = 0.9923x was found, with an r2 = 0.8672, so the proposed automatic method performed better.