Downy mildew

Peronospora, Bremia, Plasmopara and Hyaloperonospora spp.

also called False mildew

Yellow patches on the upper leaf with gray-purple mold beneath, in cool wet weather. Improve air circulation, water at the base, and remove infected leaves promptly.

Water mold Serious Rain splashAirborne sporesSoil-borne Spreads to neighbors

What you'll see

Pale yellow or brown patches on the upper leaf are sharply squared off by the leaf veins.
Turn the leaf over and the matching patch carries a fine gray, mauve or off-white down.
Lower and older leaves go first, then the damage works upward and the patches turn papery brown.
Onions and leeks show whole leaf tips dying back; lettuce hearts rot on from the outer leaves.
Unlike powdery mildew, the growth is on the underside and cannot be rubbed away.

Why it happens

Not a true fungus at all but an oomycete, a water mold more closely related to algae — which is why fungicides aimed at powdery mildew do little against it. It grows inside the leaf and pushes its spore-bearing threads out through the breathing pores on the underside. Spores arrive on wind and rain splash, and resting spores can sit in the soil or in debris for years.

It needs prolonged leaf wetness and cool to mild temperatures — spring and autumn, misty mornings, a wet fortnight in summer, or a crowded seedling tray. Overhead watering late in the day gives it exactly the overnight dampness it wants.

Treating it

Remove and bin affected leaves as soon as you see them, before the undersides start shedding spores.
Water the soil, not the plant, and do it in the morning so foliage dries by nightfall.
Thin the row or the seedlings hard to get air moving between plants.
Ventilate greenhouses and cloches freely, even on cool days, to break the overnight humidity.
Pull badly affected lettuce, spinach or basil entirely — they rarely make a decent crop once it is through the heart.

Keeping it away

Grow resistant cultivars, especially of lettuce, spinach, basil and impatiens, and change them between seasons.
Rotate susceptible crops so the resting spores in the soil are not fed each year.
Sow thinly and space generously; congestion matters more than almost anything else.
Clear all crop debris at the end of the season rather than leaving it to overwinter on the ground.
Buy plants and seed from clean stock — several downy mildews travel on seed.

Who gets it

Lettuce, onions and leeks, spinach, brassicas, basil, grapes and impatiens are the usual sufferers, each with its own specialized species.