Brassica juncea, belonging to the Brassicaceae, is usually consumed as a raw, fresh vegetable in the diet of Vietnamese people during the winter and spring seasons. In February 2014, powdery mildew was first observed on 80 to 100% of the area under B. juncea cultivation in Ha Noi and Lang Son. Within a week, symptoms appeared as irregular white patches on both sides of the leaves, and entire plants were affected. Economic loss was unavoidable as farmers were unable to safely spray chemicals in time to suppress the disease. Based on morphological characteristics, sequence analysis of the rDNA internal transcribed spacer region and a pathogenicity test, the causal agent was identified as Erysiphe cruciferarum. This fungus is also known to infect numerous host species belonging to various genera of the Capparidaceae, Fumariaceae, Papaveraceae, Resedaceae, and Brassicaceae and is almost circumglobal in range, including Africa, North and South America, Asia, Australia, Europe, New Zealand. This is thought to be the first report of E. cruciferarum on B. juncea in Vietnam.