The fungal disease – the black rust of wheat is caused by

The correct option is Puccinia graminis tritici.

Let us understand the reason step-by-step:

1. Black Rust of Wheat: Also known as stem rust, this is a devastating fungal disease affecting wheat crops globally. It primarily attacks the stems and leaves, forming dark reddish-brown to black pustules containing spores.

2. Causative Agent: The disease is caused by the obligate biotrophic fungus Puccinia graminis. Specifically, the forma specialis (special form) that infects wheat (Triticum species) is designated as Puccinia graminis tritici.

3. Analysing other options:
- Melampsora lini causes linseed rust.
- Claviceps purpurea causes ergot of rye.
- Albugo candida causes white rust in crucifers (like mustard).

Therefore, the black rust of wheat is caused by Puccinia graminis tritici.