Ahmad As'ad Najiy Pengarang Dian Asmarajati., M.Kom. Dosen Pembimbing 1 Saifu Rohman, M.Kom Dosen Pembimbing 2 Dian Asmarajati., M.Kom. Ketua Penguji Muslim Hidayat, M.Kom Penguji 2 Muhamad Fuat Asnawi, S.Kom., M.M. Penguji 1

Oyster mushrooms (Pleurotus ostreatus) are one of the most widely cultivatedmushrooms in Indonesia due to their nutritional and economic value. However,their cultivation often faces challenges, particularly from diseases that candecrease yield quality and cause economic losses. This research aims to develop anexpert system using the Certainty Factor method to assist farmers in diagnosingoyster mushroom diseases effectively and accurately.The expert system identifies diseases based on observed symptoms, such asdiscoloration, abnormal growth, and signs of infection. The Certainty Factormethod quantifies the confidence level of diagnoses based on the inputtedsymptoms. Data and diagnostic rules were collected through interviews withexperts and literature reviews.Testing results indicate that the system delivers highly accurate diagnoses andrelevant treatment recommendations, achieving an accuracy rate of 80.98%. Thisexpert system is expected to serve as a practical tool for oyster mushroom farmers,improving disease management and reducing losses, ultimately supporting betterproductivity and economic outcomes.Keywords: Expert System, Certainty factor, Oyster Mushrooms, Disease Diagnosis,Agriculture.