Onion Rust is caused by a fungus Puccinia allii. It is uncommon & rarely causes significant damage to onions in the UK, unless it is being grown next to an infected garlic crop.
SYMPTOMS
Small, reddish to dull orange, yellowy, oval-shaped spots and blotches (pustules) develop on the leaves. They produce reddish airborne spores in a powdery mass. These turn black later in the season. The pustules enlarge to form a girdle around the leaves or seed heads. All plant tissue above the girdle turns yellow and dies. If the infection is severe, the size and quality of the onion will be reduced.
PREVENTION

• Destroy volunteer onion, garlic and leek plants - that is any not planted in the garden that have grown on waste ground etc..
• Do not grow onions on land affected by rust in the previous season, because onion rust overwinters on garlic and volunteer Allium crops
• Grow varieties resistant to onion rust

TREATMENT

• Remove and burn diseased leaves
• Apply fungicides if a lot of pustules develop on the plants.� Follow the instructions on the label.