What are the spots on my rose leaves?

Telling black spot, rust and powdery mildew apart from the marks they make and where they appear, and the cultural changes that actually reduce all three.

Difficulty: beginner
Time: 20 min
Read: 5 min

Short answer

Dark blotches with ragged fringed edges and yellowing around them, starting on the lowest leaves, is black spot. Bright orange pustules on the leaf underside is rust. A white felty coating on the youngest leaves and buds is powdery mildew. Black spot and rust are wet-weather diseases spread from fallen leaves; mildew is a dry-root problem. Clear the fallen leaves, water the roots deeply, open the plant up, and replace bad performers with resistant varieties.

Rose diseases are worth identifying rather than treating generically, because the three common ones are driven by opposite conditions and the useful responses differ. All three are almost impossible to eradicate in a British summer, and the realistic aim is a plant healthy and well-fed enough to grow through the damage.

Telling black spot, rust and powdery mildew apart on a rose leaf

Step by step

Look at the shape of the mark, not just the colour.Black spot produces irregular dark blotches with feathery, fringed edges, usually 5 to 15mm across, on the upper leaf surface, with a yellow halo spreading around them. Clean-edged circular spots are more likely to be a minor leaf spot or physical damage and matter far less.
Turn the leaf over.Rust shows as small bright orange pustules on the underside, which rub off onto a finger, with matching pale yellow flecks visible from above. Late in the season those orange pustules turn black — that is the overwintering stage.
Check where on the plant it started.Black spot begins on the lowest leaves and works upward, because the spores are splashed up from infected fallen leaves lying on the soil. Mildew starts at the top, on the youngest growth, buds and shoot tips.
Assess the leaf texture for mildew.Powdery mildew is a white or grey felt on the upper surface of new leaves, often with the leaf edges curling and puckering. Unlike the other two it does not need wet weather; it thrives in warm days with cool nights and, critically, on plants with dry roots.
Deal with mildew by watering, not spraying.Roses against walls, in dry borders and in pots get mildew because the roots are dry. Water deeply at the base — a full can or two soaking in slowly, not a sprinkle — and mulch thickly in spring. This alone often ends the problem.
Clear and bin every affected fallen leaf.Both black spot and rust overwinter on fallen leaves and, in the case of black spot, on small lesions on the stems. Rake up under the plant through the season and again thoroughly in autumn. Bin them or burn them; do not compost them, because a garden heap does not get hot enough reliably to kill the spores.
Prune to open the centre of the plant.Air moving through a bush dries the leaves faster after rain, and a leaf that dries quickly is much harder to infect. Remove crossing and inward-pointing stems when you prune.
Feed and mulch so the plant outgrows the damage.A rose that loses half its leaves to black spot but is well fed and watered will push out new ones. A starved, dry rose defoliates and weakens year on year. A rose fertiliser in spring and again after the first flush, plus a compost mulch, is the practical answer.
Water at the base, never over the foliage.Overhead watering keeps leaves wet and splashes spores upward. Water the soil, in the morning if possible.
Replace repeat offenders with resistant varieties.Resistance varies hugely between varieties. A rose that defoliates completely every July is telling you something no amount of care will fix, and swapping it for a variety bred for resistance is the single most effective step available.

Tips

Stripping every affected leaf off a badly infected plant in midsummer does more harm than good — you remove the plant's ability to feed itself. Pick off the worst, clear what falls, and feed.
Black spot arrives in warm wet weather and is worst in a British July. A bad year is a bad year; judge a variety over several seasons.
Mulching in early spring, before the spores start splashing, physically blocks the ones lying on the soil surface from reaching the leaves.
Roses in pots almost always get mildew if they are allowed to dry out even once a week. They need watering far more often than people expect.

Common mistakes

Composting infected rose leaves — Most garden compost heaps do not reach or hold a high enough temperature to kill fungal spores, so the disease returns with the compost.
Treating mildew as a wet-weather disease and watering less — It is the opposite. Powdery mildew on roses is strongly associated with dry roots, and reducing water makes it substantially worse.
Reaching for a spray as the first response — Fungicide approvals for garden use change frequently, sprays need repeating through the season to have any effect, and none of them fix the underlying conditions. The cultural measures work better and last.

If it doesn't work

Lower leaves yellowing and dropping from midsummer, with fringed dark blotches

Cause: Black spot, splashed up from infected debris — Fix: Clear and bin all fallen leaves, mulch, feed, water at the base, and prune to open the plant. Replace the variety if it happens every year.

Orange dust on your fingers from the leaf undersides

Cause: Rust — Fix: Remove and bin affected leaves, clear all litter in autumn — the black late-season stage is how it survives the winter — and improve air movement.

White felt on new shoots and flower buds, especially against a wall

Cause: Powdery mildew, driven by dry roots — Fix: Soak the root area deeply and repeatedly, mulch heavily, and keep it watered through dry spells.

Leaves eaten to a skeleton, not spotted

Cause: Rose sawfly larvae rather than a disease — Fix: Pick them off by hand. This is damage, not infection, and needs no fungicide.

Sticky leaves with black sooty deposit

Cause: Aphid honeydew with sooty mould growing on it — Fix: Control the aphids; the mould washes off once the honeydew stops.

Questions people ask

Will black spot kill my rose?

Rarely on its own. It weakens the plant by removing leaves it needs to feed itself, so a rose defoliated every year for several years declines and flowers less. A fed, mulched, well-watered rose usually copes indefinitely.

Should I strip all the affected leaves off?

Take off the worst, and be thorough about clearing the ones that fall. Removing every marked leaf from a badly infected plant in July leaves it with nothing to photosynthesise with and does more damage than the fungus.

Does the milk or bicarbonate remedy work?

There is some evidence for bicarbonate sprays suppressing powdery mildew on some plants, and much less for milk. Neither addresses the dry roots that cause rose mildew in the first place, so they are at best a supplement to watering properly.

Is it worth buying disease-resistant varieties?

It is the single most effective thing you can do. Modern breeding has produced roses that stay clothed in leaves through a wet British summer with no intervention at all.