Purple blotch disease is one of the most destructive diseases of onion in Egypt, which could cause 100% loss of onion seed production. The excessive use of chemical fungicide harms the environment and natural balance, leading to find another safe and effective alternative controlling onion purple blotch disease. Field experiments were conducted at Demo in El-Sultan Bahnas village, Fayoum County at El-Fayoum Governorate during the growing seasons 2017/2018 and 2018/2019 on onion for seed production. Plants were sprayed with four potassium salts; potassium silicate (K-silicate), potassium citrate (K-citrate), dipotassium phosphate (K2HPO4) and potassium carbonate (K2CO3). Two control treatments one with distilled water and the other with mancozeb 80%. K2HPO4 gave the best result in decreasing disease incidence or disease severity in leaves and flower stalks in respectively, followed by K2CO3 and K-silicate. While, the treatment of K2CO3 gave the highest inflorescences' fresh and dry weights followed by K2HPO4, then K-Citrate. The treatment of K2HPO4 generated the highest inflorescence and yield components compared to control treatment. K-silicate application exceeded the all other treatments and led to significant increases in free phenols and reduced sugars contents in onion leaves compared to control treatment. Examined Potassium salts in this study gave promised safe alternative control methods for onion purple blotch in onion seed production with reference to the chemical fungicide.