Accurate assessments of disease severity are critical for several research areas in plant pathology. Several methods have been described to assess disease severity but visual evaluation remains as the main option despite any bias the researcher might have. Recently, a portable application called LeafDoctor was developed using six different foliar diseases, however this software has not been not tested in other crops and with different tissue organs. Black spot of maize (Phyllachora maydis), black sigatoka of banana (Mycosphaerella fijiensis) and frosty pod rot of cacao (Moniliophtora roreri) are the most devastating diseases that affect the main crops at Los Ríos, Ecuador. After sampling corn and banana leaves and cacao fruits using visual rating of symptoms and the LeafDoctor evaluation, linear regression data shows that the software can be used in black spot of maize but not in black sigatoka or frosty pod rot. Future refinements could be used in banana to increase its correlation values, however the time spent to increase their correlation values might be time restrictive for the researcher.