Downy mildew

What you are looking at

Pale yellow blotches appear on the upper surface of a leaf. Their edges are straight where they meet a vein, so the patch looks angular rather than round, and it stops dead at the vein instead of running across it.

Turn the leaf over. Directly under each yellow patch there is a fine bloom, off white to grey or purple depending on the species, and it will not rub off the way a dust would.

On lettuce the outer leaves go first and then collapse and rot at the base. On onions the leaves grey over, fold halfway up and die from the tip. On basil the yellowing runs between the veins with a dark grey fuzz beneath, which is what sends most people to look this up.

The opposite of powdery mildew in almost every way

The two names sound like variants of one thing and they behave as opposites.

Powdery mildew grows on the surface of the leaf, shows as white dust on the top, rubs off on a finger, and is at its worst in warm dry weather where the plant is short of water at the root.

Downy mildew grows inside the leaf and sends its spore-bearing structures out through the pores on the underside. It appears yellow from above, will not rub off, and needs cool wet conditions with the leaf staying wet for hours.

Getting the two the right way round changes what you do. Watering a dry plant helps against powdery mildew and makes downy mildew worse.

Downy mildews are also not fungi. They are oomycetes, water moulds, the same group as potato blight, which is why products aimed at true fungi are weak against them. No amateur fungicide in Britain treats these anyway, so the question rarely arises in practice.

One name, many organisms

Downy mildew is a family rather than a species, and each member is tied to its own host. The one on your lettuce cannot move to your onions and the one on your basil cannot touch the grapevine.

That matters for rotation. Moving lettuce to a new bed is worth doing, and it does nothing about the onions there. It also means resistance bred into a lettuce cultivar protects lettuce and nothing else.

Two of these have changed British gardening within living memory. Basil downy mildew arrived in the late 2000s and made outdoor basil unreliable across the country. Impatiens downy mildew swept through busy lizzies from 2011 and took what had been the best-selling bedding plant in Britain off the shelves for years, until resistant series were bred.

What to do about it

Water at the base, in the morning, and keep the foliage dry.

Space plants further apart than the packet suggests and thin the seedlings early.

Ventilate anything under cover, even on cool days.

Pick off marked leaves as soon as they appear and take them off the plot.

Pull out collapsing plants rather than nursing them.

Choose resistant cultivars, and read the resistance list in the catalogue rather than the headline.

Rotate lettuce and spinach around the plot, and clear the old crop out instead of digging it in.

What it gets mistaken for

Powdery mildew coats the upper surface white and comes off on a finger.

Magnesium deficiency yellows between the veins on older leaves, with soft graded edges, and there is nothing on the underside. It greens up after a feed.

Grey mould grows a loose fuzz that puffs spores when disturbed and usually starts from a dead flower or a wound.

Rust breaks the surface into raised pustules that shed coloured powder onto a finger.

Potato and tomato blight, which is the closest relative on this list, produces dark brown blotches on the leaf that spread across veins without regard to them, often with a pale halo, and it moves through a plant in days rather than weeks.