The use of chemicals as a form of prevention, protection, and control to combat diseases that affect plants is a feasible solution involving control measures. However, due to the emergence of microorganisms resistant to these products, new tests need to be performed in the laboratory, requiring the effort of the specialist in phytopathology and in order to investigate these microorganisms. One of the most common methods used includes laboratory tests, periodically verifying the growth of the fungal colony with measurements using ruler or caliper, but this method is imprecise. The study was developed using images containing Petri dishes with growing fungus colonies, comparing the manual method performed by the specialist in Phytopathology and by images processing. For this, processes were developed involving region segmentation by color analysis and unnecessary information removal to measure the size of colony. It was possible to verify the application and potential of the method developed for creating mobile applications. Therefore, this work presents a method for measurement of the fungal colony on Petri dishes from isolates of the fungus Corynespora cassiicola, using techniques involving digital image processing, with the purpose of assisting the phytopathologist professional, providing other means besides the visual analysis.