Diseases in crops cause serious problem in agriculture field and produce economic loss. Farmers identifying of leaf diseases is the crucial and one of the major early-stage concerns. Detection of disease spot accurately helps the farmers to spot the disease pant and take the remedial step to cure the disease. So there will not be any economic loss to the farmers. The aim of this paper is to let the farmers to detect the potato crop disease spot using colour thresholding method for different color models and comparing the result and concluding the efficient method for detecting the disease spots in the potato image using image processing tools.