Black Spot
TreatableFungal disease · spreads to other plants · also called rose black spot
Black spot is the defining disease of roses in humid climates, and the reason so many rose bushes are bare-legged by August. It needs leaves to stay wet for about seven hours to infect, which is why overhead watering in the evening is the single worst thing you can do to a rose.
What it looks like
Round black spots up to half an inch across with feathery, fringed edges rather than clean ones. The leaf around each spot turns bright yellow, and leaves drop from the bottom of the bush upward.
How to confirm it
- Fringed, feathery edges to each black spot — not a clean circle.
- Vivid yellowing of the leaf around and between the spots.
- Defoliates from the bottom upward through the season.
- Spots do not rub off, unlike sooty mould.
What to do
- 1Pick off and bin affected leaves, including fallen ones under the bush.
- 2Water at the base early in the day so foliage never stays wet overnight.
- 3Mulch to stop spores splashing up from the soil.
- 4Fungicides protect new growth but will not clear existing spots; start early in the season if you use them.
What it gets confused with
One observation settles each of these.
Sooty mould wipes off with a damp cloth and sits on top of sticky honeydew. Black spot is in the leaf tissue and does not wipe away.
Rose downy mildew produces purplish angular blotches and drops leaves very suddenly, usually in cool wet spells.
Preventing it next time
- Choose resistant varieties — this matters more than anything else you can do.
- Prune for an open centre so air moves through the bush.
- Clear all fallen rose leaves in autumn; the fungus overwinters on them.
Plants affected
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.