Background: Plant production provides human and animal life with different requirements. The concern of workers in agriculture in general and those interested in plant diseases, in particular, has been focused on protection from all that is expected to have problems of production. As environmental conditions play a critical role in the treatment of diseases, the plant is prepared and rendered more susceptible to production, which is exposed and may result in the loss of the entire crop. Objectives: The main goal of this expert system is to get the appropriate diagnosis of Apple disease and the correct treatment. Methods: In this paper the design of the proposed Expert System which was produced to help Farmers, people interested in agriculture and agricultural engineers in diagnosing many of the apple diseases such as : Apple scab, Marssonina leaf blotch (pre mature leaf fall) , Black rot canker, Collar rot , Powdery mildew ,Sooty blotch and fly speck, Apple mosaic and other virus diseases , Alternaria leaf spot/blight , Core rot , Brown rot , White rot / root rot , Seedling blight . The proposed expert system presents an overview about apple diseases are given, the cause of diseases are outlined and the treatment of disease whenever possible is given out. CLIPS with Delphi were used for designing and implementing the proposed expert system. Results: The proposed apple diseases diagnosis expert system was evaluated by Farmer, Agricultural experts and teachers of the Gaza School of Agriculture and they were satisfied with its performance. Conclusions: The Proposed expert system is very useful for Farmers, and those interested in agriculture with apple disease and recent graduate students.