Rust

What rust actually is

Rust is a group of fungal diseases that show up as small raised spots — called pustules — on the leaves of a plant. They are usually a vivid orange, though they can fade to brown or black as the season wears on, and if you brush them with a finger they leave a rusty, powdery smudge. That smear of orange dust is where the name comes from, and it is the giveaway that tells rust apart from other leaf spots.

There are many different rust fungi, and most are fussy about their host. The rust that attacks your leeks and garlic is a different species from the one on broad beans, mint or roses, so it rarely jumps from one crop to another. On a UK veg plot, the kind you are most likely to meet is leek rust, which also troubles garlic, onions and chives.

What triggers it

Rust thrives in the same conditions as most fungal troubles: damp, mild and crowded. Spores spread on the wind and in splashing rain, then settle and germinate on leaves that stay wet for long spells. A soggy UK summer is ideal for it.

The things that make rust worse are mostly within your control:

• Overcrowding. Plants packed tightly together stay damp because air can't move between the leaves to dry them. Generous spacing is your best defence.
• Too much nitrogen. Soft, sappy, over-fed leaves are more easily infected, so go easy on high-nitrogen feeds with alliums.
• Wet, sheltered, humid corners where foliage never quite dries out.

Does it ruin the crop?

Here is the reassuring part: rust is almost always a leaf disease, not a problem with the part you eat. A leek with rusty leaves still produces a perfectly good white stem below, and rusty garlic foliage usually sits above sound bulbs. A mild attack late in the season can be safely ignored — you can simply trim off the worst leaves and eat the rest as normal. Only a heavy, early infection that browns most of the foliage will noticeably check growth and shrink your harvest.

To keep it in check, space alliums well, water at the base rather than over the leaves, and clear away badly affected foliage rather than leaving it to spread spores. Don't compost infected leaves on a cool heap, as the spores can survive; bin or burn them instead. Practising sensible crop rotation stops rust building up in one spot year after year, and growing in an open, breezy position helps leaves dry quickly. Like powdery mildew, rust is really a damp-weather nuisance you manage rather than a disease you can cure, so prevention through airflow and spacing is the whole game. For more on healthy plants, see our guide to growing leeks.