Basil: dark or black spots on leaves

What it is likely to be on basil

1. Downy mildew

   Candidate 1common on this plant

   ·source

   Sourced guidancehigh confidence

   • University of Minnesota Extension — University of Minnesota: Basil downy mildew

   Checked 2026-08-13

   Pale yellow patches on the upper leaf surface with a greyish or purplish furry growth directly beneath them, the patches often bounded by the leaf veins into angular blocks. It is routinely confused with powdery mildew and behaves in the opposite way: downy mildew needs wet conditions, powdery mildew thrives in dry ones.

   Points towards it

   • · the yellowing is bounded by the leaf veins and the growth sits on the underside beneath it
   • · still humid air with crowded plants is what the disease needs

   Argues against it

   • · a white surface film that rubs off is a powdery mildew, not this — this rules it out entirely.

   Check this next

   Turn an affected leaf over and look at the underside in good light. Basil downy mildew grows a grey-purple fuzz there, directly beneath the yellow patch on top — nothing else on basil does that, and the top surface alone will not tell you.

   What to do now

   Remove affected leaves, space the plants further apart and improve the airflow around them. The disease spreads on damp still air, so watering the compost rather than the foliage and not crowding pots together does more than anything applied to the leaves.

   ·source

   Sourced guidancehigh confidence

   • University of Minnesota Extension — University of Minnesota: Basil downy mildew — management

   Checked 2026-08-13

2. Cold damage

   Candidate 2very common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Basil — hardiness

   Checked 2026-08-13

   Blackened, translucent or limp tissue appearing suddenly after a cold night or a spell beside a draught. Tender plants show it well above freezing — basil blackens around 10 °C — so 'there has been no frost' does not rule it out.

   Points towards it

   • · the marks appear within a day or two of the plant being chilled
   • · cold blackens the leaf edges and the thinnest tissue first

   Argues against it

   • · growth on the underside of the leaf is a fungus rather than cold — this rules it out entirely.

   Check this next

   Check whether the marks are on the oldest leaves or the ones that were most exposed. Cold damage follows the draught — the side facing a window or an open door goes first — and the underside of the leaf stays completely clean.

   What to do now

   Move the plant away from the cold surface or draught that caused it and keep it above 12 °C. The blackened leaves will not recover and can be picked off, but a plant that is otherwise firm will carry on growing from the tips once it is warm.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Basil — growing

   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 basil — 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 Basil profile lists the problems it is prone to.