Three separate things can blacken a basil leaf, and each leaves its own signature. Cold injury shows as the whole plant going dark all at once, right after a cold night — there's no undoing it, only avoiding it next time. A fungal infection announces itself with yellow patches on top of the leaf and a fuzzy, dark growth on the underside directly below, appearing once conditions turn warm and humid, and it doesn't clear up on its own. And a handful of dark marks confined to spots where a leaf was actually touched or knocked is simpler than either — just bruising.

Ranked, Most Likely First

  1. Downy mildew. Yellow patches on the upper leaf, often banded, with purplish-black fuzzy spore growth directly opposite on the underside. It needs warmth and humidity to develop and spread, has no cure once present, and — in this zone specifically — is the more probable explanation of the two disease-or-cold causes.
  2. Cold or chilling injury. Basil is frost-tender and genuinely sensitive even without an actual frost — chilling symptoms can start below about 12°C, with visible blackening once it drops below roughly 4–5°C. A plant transplanted too early or caught by an unexpected cold night can blacken within a day.
  3. Physical bruising. A basil leaf darkens easily anywhere it's pressed, knocked or brushed against something — wind, careless handling, a neighbouring plant. The mark stays exactly where the contact happened and never spreads outward the way the two causes above do.

Why This Zone Flips the Usual Order

Basil transplants September through February here — a full two months earlier than either other NZ zone — and harvests November through May, a month past temperate's own close. That's not just a warmer climate producing the same calendar; the planting window itself is genuinely different, reflecting the longest frost-free run of anywhere in the country this site covers. The practical result: cold injury is the least likely explanation for black leaves across almost this entire window, while downy mildew becomes comparatively the most likely cause of any NZ zone, since the warmth and humidity it needs stay present for longer here than anywhere else. Checking the underside of yellowing leaves is worth doing across more of the season here than in a colder garden, where that check matters mainly during a short summer stretch.

Fixing It

  • Match what you see to the pattern above before treating anything. Sudden whole-plant blackening after a cold night: cold injury. Yellow top-side patches with fuzzy underside growth in warm, humid conditions: downy mildew. Isolated marks only where handled: bruising.
  • Cold injury has no fix for leaves already damaged, only prevention going forward. Strip the blackened growth so the plant redirects energy into fresh leaves, and don't transplant or move basil back outdoors until conditions are genuinely settled.
  • Downy mildew means removing and destroying the plant. Bag it rather than composting, since spores travel easily. For future plantings, water at the base rather than over the foliage, space for airflow, and choose full sun with good drainage so leaves dry quickly.
  • Bruising needs no treatment at all. Pinch or cut when harvesting rather than tearing, and the marks stay put rather than spreading.

Cultivar Notes

Genovese (Koanga Institute) carries a genuinely practical number for timing: transplant once ground temperature reaches 15°C rather than by a fixed date. Thai Siam Queen (Kings Seeds NZ) is described as a tropical basil that "prefers warm nights" and shouldn't be sown too early. Tulsi (Kings Seeds NZ) calls for sowing only once frost risk has genuinely passed and notes that protection from cold matters throughout the growing period. Primo Genovese (Kings Seeds NZ) rounds out the list with a supplier note on good disease tolerance generally — real, but not written as a downy-mildew-specific claim, so it's not a substitute for the airflow and drainage habits above.

Planning for Next Season

Ground temperature, not the calendar, is the sharper trigger for transplanting — 15°C, per Genovese's own sourced guidance. Given how much of the season here favours downy mildew over cold as the real risk, make the underside check a routine part of inspecting yellowing leaves rather than something reserved for a hot summer stretch.

More on this zone: see the full NZ Sub-Tropical Planting Calendar →.