Mint: dark or black spots on leaves

What it is likely to be on mint

1. Rust

   Candidate 1common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Mint rust

   Checked 2026-08-13

   Raised pustules on the underside of leaves that release orange, brown or dark powder when rubbed, with pale spots visible on the upper surface above them. Rust fungi are highly host-specific — leek rust lives on alliums and rose rust on roses — so the rust on one plant is not a threat to an unrelated one beside it.

   Points towards it

   • · rust erupts as separate raised pustules rather than a spreading film
   • · the lower older leaves carry it first and worst
   • · it overwinters in the roots of established outdoor clumps

   Argues against it

   • · a white film across the upper surface is a mildew, not a rust — this rules it out entirely.

   Check this next

   Turn a spotted leaf over and rub the underside. Mint rust leaves bright orange powder on your finger from raised pustules, and the top of the leaf may show only a dull yellow speck — it is the underside that identifies it, and nothing else on mint is orange.

   What to do now

   Dig up and dispose of affected plants rather than composting them, because the fungus survives in the rootstock and will come back every year in the same clump. Start again with fresh plants in a different spot, and do not move runners from an infected clump to a clean one.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Mint rust — control

   Checked 2026-08-13

2. Powdery mildew

   Candidate 2common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Powdery mildew

   Checked 2026-08-13

   A white, floury coating on the upper leaf surface, most familiar on courgettes and cucumbers late in the season. Unlike most fungal diseases it does not need the leaves to be wet — it needs the roots to be dry and the air to be humid, which is why it arrives in a warm dry August.

   Points towards it

   • · a white or grey dusty film sits on the surface of the leaf
   • · dry roots with humid air around the leaves is the classic combination

   Argues against it

   • · growth beneath the leaf rather than a film on top is a downy mildew — this rules it out entirely.

   Check this next

   Wipe the mark with a fingertip. Powdery mildew is a surface film and comes away leaving green leaf beneath it, which no rust pustule will do — and the affected leaves stay flat rather than developing raised bumps.

   What to do now

   Cut the clump to the ground and water it well at the roots. Mint regrows clean within a few weeks, and the usual underlying cause is a plant dry at the root in still humid air, so a bigger pot or a damper position prevents the next round.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Mint — care

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on mint — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general dark or black spots on leaves page lists causes across all plants, and the Mint profile lists the problems it is prone to.