Cold is the most likely cause. Basil is frost-tender and can blacken from a single cold night well above freezing, no frost required — so check the weather first. Whole-plant blackening right after a cold night is cold injury: not treatable, only avoidable next time. Yellow patches on the leaf top with purplish-black fuzz underneath, showing up in warm, humid weather, is downy mildew instead — a different problem with no cure. A few isolated black spots only where a leaf was touched or knocked is neither of those, just bruising.

Ranked, most likely first

  1. Cold or chilling injury. Basil is genuinely sensitive even without a frost — chilling symptoms can start below about 12°C, and leaves visibly blacken once it drops below roughly 4-5°C. A plant transplanted too early, left out on a colder-than-expected night, or caught by a late-season cold snap can blacken within a day.
  2. Downy mildew. Yellow patches on the upper leaf, often in bands, paired with purplish-black fuzzy growth on the underside directly opposite. Unlike cold damage, it needs warmth and humidity to take hold and spread, and once it's there, there's no cure.
  3. Physical bruising. Basil marks easily wherever it's pressed or knocked — wind, harvesting by hand, brushing against another plant. This stays confined to the contact point rather than spreading like either problem above.

Why a temperate autumn buys basil more time

Basil goes in the ground on the same schedule regardless of zone — sow September to February, transplant November to February, both Egmont-sourced — so it's not the calendar that sets this zone apart. It's how long the season stays mild afterward: temperate's harvest window runs January to April, a full month longer than cool/mountain's January to March close. That extra month of milder autumn matters directly here, since basil is sourced as frost-tender with no cold tolerance of its own. Black leaves turning up mid-season in a temperate garden are comparatively more likely to be downy mildew — especially through a warm, humid summer stretch — than the cold-driven cause, simply because temperate basil has longer before autumn cold becomes the dominant risk.

Fixing it

  • Match the cause to the pattern first. Sudden, whole-plant blackening straight after a cold night means cold injury. Yellow top-side patches with fuzzy underside growth in warm, humid conditions means downy mildew. Isolated marks only where the plant was handled means bruising.
  • Cold injury has no fix for the damaged leaves, only prevention. Strip off the blackened growth so the plant can put its energy into fresh leaves, and don't transplant or move basil back outside until conditions are reliably warm. If a cold snap is forecast on an established plant, cover it overnight or bring pots inside.
  • Downy mildew means removing and destroying the plant — there's no cure. Bag it rather than composting, since spores travel easily. For the next planting, water at the base rather than over the leaves, give plants room for airflow, and pick a full-sun, well-drained spot so foliage dries quickly.
  • Bruising needs no treatment. Pinch or cut when harvesting rather than tearing, and the marks won't spread or return on new growth.

Cultivar notes

Koanga's Genovese carries a specific, useful number for the cold-injury risk above: transplant only once ground temperature reaches 15°C, not by the calendar alone — a genuinely practical check against exactly this problem. Kings Seeds NZ's Thai Siam Queen and Tulsi both carry the same warning in their own descriptions — don't sow or transplant too early, and give ongoing protection from cold through the growing season. Primo Genovese is separately noted for good general disease tolerance, though that's not specific to downy mildew.

Next season

Check ground temperature rather than the calendar before transplanting — 15°C, per Koanga's own Genovese guidance, is a sharper trigger than a fixed date. Toward the end of the harvest window, keep an eye on the forecast rather than assuming the season's mild run will hold, since a single unexpectedly cold night is enough to blacken an otherwise healthy plant.

More on this zone: see the full NZ Temperate Planting Calendar →.