This work deals with the development of web-based software to train the visual acuity of assessors with or without experience in the process of assessing the incidence and severity of plant disease lesions in certain crops. The proper training process for the evaluator requires standardized methods for quantifying the area damaged by the plant disease, seeking to minimize subjectivity in estimating the severity of plant diseases by training the evaluators' precision and accuracy. The severity variable is considered the best parameter to reference the damage caused by plant diseases to leaves or fruit and the diagrammatic scale is considered the best alternative to help assess plant disease lesions. The importance of adequate visual acuity training is directly reflected in efficient data collection that can determine the disease progress curve, serving as a parameter for the most appropriate management for effective disease control on the crop.