Many of the diseases in corn today can make farmers confused in determining or choosing the type of treatment in accordance with the disease of corn. This is what makes farmers hard to get results because they can not perform proper diagnosis, so that the level of productivity decreases. Computer-based expert systems can be used to help solve problems in terms of assisting each farmer in determining treatment options. The method used in diagnosing the disease in corn plants is Bayes Method, where each alternative provided will be done ranking to obtain the best results. The resulting results are sorting of disease data on corn plants that serve as a tool in decision-making for farmers. The built system is expected to help farmers in choosing appropriate treatment and appropriate diseases in corn.