Explanation:

White rust or white blister disease of crucifers is caused by the oomycete *Albugo candida* (D). This pathogen produces characteristic white, blister-like pustules on leaves and stems. *Alternaria brassicae* (A) causes Alternaria leaf spot. *Peronospora brassicae* (B) causes downy mildew. *Puccinia brassicae* (C) is not a recognized pathogen for rust in crucifers; rusts are typically caused by *Puccinia* species on other host plants. Thus, (D) is the correct causative agent.