Rust

Puccinia, Phragmidium, Melampsora and Uromyces spp.

Orange-brown powdery pustules on leaf undersides, weakening the plant over the season. Remove affected leaves, avoid overhead watering, and clear all debris in autumn.

Fungus Serious Airborne sporesRain splash Spreads to neighbors

What you'll see

Raised orange, rust-brown or yellow pustules on the leaf underside that leave a colored smudge on your finger.
Matching pale or yellow spots show through on the upper surface, directly above each pustule.
Later in the season the pustules darken to chocolate brown or black as the overwintering stage forms.
Leaves yellow and fall early, and heavily hit plants such as leeks and hollyhocks look thin and ragged by August.
On some hosts the pustules also appear on stems, petioles and even flower stalks.

Why it happens

A large group of highly specialized fungi, each tied to one host or to a pair of hosts it alternates between over the year. Wind-blown spores land on a wet leaf, germinate and grow inside, then erupt through the surface to release the next generation. Because each rust is host-specific, the rust on your mint will not move to your roses.

Warm days with cool, dewy nights suit it best, so it builds through late summer and autumn. Crowded planting, damp shade, overhead watering and soft nitrogen-rich growth all speed it up.

Treating it

Pick off and bin affected leaves promptly, and take off badly rusted stems at the base.
Water at soil level and early in the day so leaves are not wet overnight.
Thin and prune to open the plant, and lift congested clumps of mint or chives and replant a smaller piece.
Feed with a balanced fertilizer rather than a high-nitrogen one, and add potash to firm the growth.
On roses and leeks a sulfur or approved fungicide can help early in an outbreak, but hygiene does most of the work.

Keeping it away

Grow resistant cultivars, which are widely available for leek, antirrhinum, hollyhock and rose.
Rotate alliums so leek rust spores in the soil and debris are not carried straight into the next crop.
Rake up and bin every fallen leaf in autumn — the fungus overwinters on them and on old stems.
Cut mint, chives and comfrey to the ground after an attack and burn or bin the top growth.
Space plants well and avoid feeding heavily with nitrogen late in the season.

Who gets it

Leeks, garlic, roses, hollyhocks, antirrhinums, mint, plums and pears each have their own rust, along with many perennials.

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