The species causing white rust of cruciferae has a botanical name, it is

The correct answer is Albugo candida.

White rust (also known as white blister) is a disease affecting cruciferous plants (members of the family Brassicaceae, such as mustard, cabbage, and radish). This disease is characterized by the appearance of white, shiny, pustule-like blisters on the leaves, stems, and floral parts of infected plants.

The causative agent of this disease is Albugo candida, an oomycete pathogen. Although historically classified as a fungus, it is biologically distinct and belongs to the family Albuginaceae within the oomycetes.

To understand why this is the correct choice, let us briefly look at the other options provided:
1. Pythium debaryanum: This oomycete is well-known for causing damping-off disease in seedlings, not white rust.
2. Peronospora parasitica: This organism is responsible for downy mildew in crucifers, which presents differently from white rust.
3. Puccinia graminis: This is a basidiomycete fungus that causes black stem rust of wheat and other cereal crops, which is unrelated to crucifers.

Thus, Albugo candida is the specific botanical name of the organism that causes white rust of cruciferae.