Gray mould, caused by the fungus Botrytis cinerea Pers., is a disease that largely affects the crops of Rubus glaucus Benth. (Castilla blackberry) in Colombia. In spite of the economic losses that it causes in the production of Castilla blackberry at national level, a standardized method to quantify the disease severity caused by this fungus in the fruits has not yet been reported. In the present work, a diagrammatic scale was prepared to assess gray mould severity in thornless fruits of R. glaucus. The proposed scale showed the levels of 5, 10, 20, 40, 60, 80, and 100%. Photographic images were obtained for each of these levels, which were processed using the ImageJ software. The scale was validated by 13 evaluators who assessed affected fruits with and without the scale. The precision and accuracy of each evaluator was determined by simple linear regression between actual and estimated severity. The evaluators showed better precision, accuracy, and reproducibility in the assessments performed with the scale. The proposed scale is appropriate to estimate the severity of gray mould in R. glaucus fruits.