Erysiphe cruciferarum, a causative agent of powdery mildew disease, has emerged as a serious threat in Brassica juncea and its closely related species. To date, no resistant cultivars have been identified in Brassica species against powdery mildew. Here, we used histopathological, biochemical, and molecular approaches to elucidate the powdery mildew disease progression and host responses in three Brassica cenospecies, namely B. juncea, Camelina sativa, and Sinapis alba. Based on the results of disease progression, S. alba was found to be extremely resistant to powdery mildew infection, whereas B. juncea and C. sativa were highly vulnerable. In addition, the disease spread rate to uninfected parts was comparatively higher in B. juncea and C. sativa. Histopathological results revealed more pathogen-induced cell death in B. juncea and C. sativa compared to S. alba. We also examined the role of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in B. juncea, C. sativa, and S. alba after powdery mildew infection. Based on our findings, the enzyme activity of SOD, POD, and CAT was relatively higher in S. alba then that of B. juncea and C. sativa after powdery mildew infection. Furthermore, we evaluated the expression levels of salicylic acid (SA) signature genes, including pathogenesis-related protein viz., PR1, PR2, and PR5 in B. juncea, C. sativa, and S. alba after E. cruciferarum infection. Based on our findings, the expression levels of SA marker genes PR1, PR2, and PR5 increased in all three species after infection. However, the fold change was relatively higher in S. alba than in B. juncea and C. sativa. In future, further studies are required to identify the potential candidates in S. alba that are involved in powdery mildew disease resistance.