Basil downy mildew

Peronospora belbahrii

The commonest failure of the commonest herb: leaves yellow between the veins with a dirty gray fuzz beneath. Nothing cures it — grow a resistant cultivar next time.

Water mold No cure Airborne sporesSeed-borneRain splash Spreads to neighbors

What you'll see

Yellowing that spreads across the upper leaf but stops squarely at the veins, so the leaf looks blocked out rather than mottled.
A gray to almost purple-brown fuzz on the underside of the yellow patch, easiest to see in the morning.
Lower leaves first, then upward through the plant over one or two weeks.
Leaves browning, curling and dropping, leaving bare stems with a tuft at the top.
A plant that looked perfect when you bought it failing within a fortnight of coming indoors.

Why it happens

A specialist water mold of basil that appeared in North America in 2007 and is now everywhere basil is grown. It arrives on infected seed, on bought plants, and on spores carried long distances by wind — which is why a garden with no basil last year still gets it. It is not the same organism as the downy mildew on your lettuce and will not move between them.

Warm, humid nights and long leaf wetness — a July heatwave with thunderstorms, a crowded windowsill, or a greenhouse shut up overnight. It moves fastest above 20 °C with dew or misting.

Treating it

Strip and bin affected leaves at the first yellow patch, and take the whole plant out once a third of it is gone.
Harvest hard and early: pinch generously and freeze or make pesto rather than waiting for a plant that will not last.
Move surviving plants apart, into brighter air, and stop misting them altogether.
Water the soil only, in the morning, and never overhead.
Do not save seed from an affected plant; the mold travels on it.

Keeping it away

Grow a resistant cultivar. This is the whole answer, and it is a recent one — Prospera, Amazel, Rutgers Devotion and Obsession hold up where a Genovese collapses.
Space plants generously and keep the air moving; congestion in a windowsill pot is as bad as a crowded bed.
Sow your own from a reputable source rather than buying in plants that have come from a humid greenhouse.
Grow Thai and lemon basils alongside, which are markedly less susceptible than sweet basil.
Sow a second succession in midsummer so there is a young plant coming on when the first one goes.

Who gets it

Sweet basil above all; Thai, lemon and holy basil are affected but far more lightly.