Rust on leeks — symptoms and treatment

Severity: medium

What is Rust

Leek rust is caused by Puccinia porri, a fungus that produces the bright orange pustules every leek grower recognises. It is the most common disease of leeks in temperate climates — the mild, damp regions that suit leeks suit the fungus just as well. Severity is rated medium: rust rarely kills a leek and does not make it inedible, but severe infection reduces marketability, and a heavily spotted plant loses enough green leaf area to stop bulking up.

Symptoms

The disease is easy to identify and hard to miss. Small oval pustules erupt through the leaf surface in a vivid orange, usually running lengthwise with the leaf. Rub one and it smudges orange on your finger — a mass of spores breaking out of the leaf, and immediate confirmation. Pustules start scattered but can cover the leaf when conditions favour the fungus, and infected leaves yellow and die back from the tips. Later, pustules darken to black in autumn as the fungus switches to its overwintering stage.

Key signs:

• Early: scattered orange oval pustules on the leaves
• Early: orange spore dust that rubs off on fingers or clothing
• Advanced: dense pustules, yellowing and dieback of leaf tips
• Advanced: pustules darkening to black in autumn

Nothing else on a leek looks like this — the orange smudge test settles it.

Causes and conditions

Rust spores are produced in enormous numbers and carried on the wind, which is why the disease can appear in a garden with no leeks nearby and why it spreads so quickly once it starts. Spores need leaf wetness to germinate, so mild, damp weather, heavy dews, frequent rain and overhead watering all drive it. Anything that keeps the leaves wet or the air still helps the fungus: crowded plantings, weedy rows, low spots with poor air movement. Soft, lush growth from excessive nitrogen is markedly more susceptible. The fungus also carries over on infected leek debris and related alliums left on the plot, so volunteers and old crop remains keep the cycle running.

Treatment

Spacing and sanitation — cultural

Throughout season. Avoid excessive nitrogen. Ensure adequate spacing. Remove infected leaf tips. Destroy crop debris. Wide spacing lets leaves dry after rain and dew — the one condition the fungus cannot do without — and less nitrogen produces tougher leaves. Sprays cannot compensate for a crowded, over-fed, damp crop.

Tebuconazole — chemical

At first symptoms. Apply fungicide at first appearance of pustules. Repeat every 14 days if needed. Timing is the whole game: fungicides stop new infections but cannot erase pustules already there, so waiting until the crop is covered wastes the spray. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate — do not grow alliums in the same bed in consecutive seasons.
• Space plants generously so leaves dry quickly after rain and dew.
• Go easy on nitrogen; soft, lush leaves are the most susceptible.
• Water at the base rather than over the foliage.
• Clear and destroy leek debris and volunteer alliums at season's end.

Frequently asked questions

Can I still eat leeks with rust? Yes. Rust does not make leeks unsafe or affect the flavour — it is a cosmetic and vigour problem. Strip off the worst outer leaves and use the rest. After spraying, respect the 14-day pre-harvest interval.

Is it contagious to my onions and garlic? It spreads readily through leeks and related alliums, and wind-blown spores make a few metres of separation pointless. What helps is rotation and clearing the old crop debris that carries the fungus over.

When should I spray? At the first pustules you see — not before, and not once the crop is covered. If rust arrives late and the leeks are nearly ready, it is often not worth spraying.