Rust

What you are looking at

Turn the leaf over. Rust announces itself on the underside as raised pustules that have broken through the surface from within, powdery to the touch, and coloured somewhere between bright orange and dark brown depending on the species and the time of year. Rub one and the powder comes off on your finger.

On the upper surface, directly above each pustule, there is a pale yellow spot where the tissue has been killed from below. Someone looking at a leek or a rose from above sees only the yellowing and diagnoses a nutrient problem.

Pelargoniums do something distinctive. The pustules form in concentric rings on the leaf, which is unlike anything else that marks a leaf and is worth recognising on sight.

Every rust has one host, or two, and no more

There are several thousand rust fungi and each is restricted to a narrow range of plants. Leek rust lives on leeks, onions, garlic and chives. Rose rust lives on roses. Mint rust lives on mint. None of them will cross to a plant outside that group, however close together they are growing.

That is not a technicality. It means an infected mint bed poses no threat to the leeks beside it, and it means the only source of infection for your roses is roses, which narrows the problem to the plants and debris in front of you.

Some rusts do need two unrelated hosts to complete their cycle, alternating between them across the year. That biology explains a few strange local patterns in fruit rusts, and for a gardener the practical version stays the same. Deal with the host in your garden.

Why it needs a wet leaf

A rust spore lands, and to germinate it needs several hours of liquid water on the leaf surface at a moderate temperature. Give it that and it pushes a thread into a stomatal pore and it is inside the plant. Deny it and nothing happens.

So the outbreaks come after warm wet spells, in dense plantings that hold dew past mid-morning, in gardens where a sprinkler runs in the evening, and against walls and fences where the air does not move.

That is the opposite of what powdery mildew requires, and the two are constantly lumped together as fungal diseases to be treated the same way. Powdery mildew wants humid air and dry leaves. Rust wants the leaves wet. The one measure that serves both is watering the soil rather than the foliage, in the morning rather than at dusk.

What the damage amounts to

Rust is a biotroph. It needs the plant alive and takes nutrients from living cells rather than killing tissue outright, which is why an infected leaf carries on working for weeks before it yellows and drops.

The cost is cumulative. Leaves lost through summer mean a smaller plant, poorer flowering and a weaker crown going into winter. A leek crop can lose most of its leaf area in a bad August and still produce usable stems, and a hollyhock left untreated for three seasons becomes a stick with rusty rags on it.

Where the fungus gets into the stem, as it does on roses and brambles, the shoot distorts and swells and produces spores from the wood. That one is worth cutting out promptly.

What to do about it

Change the watering first. Soil rather than leaves, morning rather than evening, and no sprinklers over susceptible crops. Then open the planting up, since a leek row or a rose against a fence that stays wet until noon is the condition the disease is asking for.

Pick off affected leaves as soon as you see them and put them in the household waste. Each pustule is producing spores continuously, so the first few leaves in June are worth more than a bin bag full in September.

Clear everything in autumn. Fallen rose leaves, dead leek foliage, hollyhock stems. The late-season pustules that look almost black are the overwintering stage, and they survive on that debris to start again in spring.

Rotate alliums on three years. Grow hollyhocks as biennials from seed. Start a rusted mint bed again from a clean plant somewhere else in the garden, because the fungus is in the rhizomes and picking leaves will never reach it.

Why there are no products listed here

Nothing can be introduced to eat a fungus living inside leaf tissue, so the biological tier is empty.

The last tier is empty because amateur fungicide approvals in Great Britain have narrowed sharply, several of the actives older books recommend for rust can no longer be used, and the ones that remain are protectant rather than curative. They do nothing to an infection already established, which is the state a plant is in by the time anyone notices rust on it. Spacing, watering and hygiene do more, and they work on next year as well as this one.

What it gets mistaken for

Leaf spot diseases produce flat marks in the leaf tissue rather than raised pustules on the underside, and nothing rubs off on a finger.

Red spider mite yellows and dulls a leaf, and the damage is a fine even stippling across the upper surface with no pustules beneath and often fine webbing at the shoot tips.

A magnesium or nitrogen shortage yellows leaves from the bottom of the plant upwards, exactly as heavy rust does. Turn a leaf over before concluding anything. Pustules mean rust, and a clean underside means it is worth looking at feeding and watering instead.

Orange powder on a lawn in late summer is grass rust, a different member of the same group. It follows drought and poor feeding, and it goes when the grass grows again.