Is this powdery mildew or downy mildew?

The names suggest variations of one thing, and they are not even closely related organisms. Powdery mildew is a true fungus sitting on the leaf surface. Downy mildew is a water mould growing inside the leaf tissue, related to blight. Treating one as the other wastes the season, because the conditions they need are almost opposite.

Step by step

1. Find which surface the growth is on.Turn the leaf over. Powdery mildew is predominantly on the upper surface, sometimes on both. Downy mildew's visible growth is only ever on the underside, pushed out through the leaf pores.
2. Try to rub it off.Powdery mildew comes away on a finger, because the fungus is a mat on the outside of the leaf anchored only into the outermost cells. Downy mildew does not, because what you can see is the fruiting structure of something growing inside.
3. Look at the upper surface for angular patches.Downy mildew produces pale yellow or brown patches on the top of the leaf that are bounded by the veins into angular blocks, sitting directly above the fuzz below. That combination is diagnostic.
4. Think about the weather it appeared in.Powdery mildew arrives in warm, dry, still summer weather with humid nights, and is worst on plants dry at the roots. Downy mildew arrives after prolonged leaf wetness in cool damp conditions — a wet spring or a wet autumn.
5. Think about the plant.Powdery mildew: courgette and squash, roses, honeysuckle, asters, apples, phlox, acanthus, peas. Downy mildew: brassicas, onions, lettuce, spinach, impatiens, hebe, grapevine, and seedlings under glass.
6. For powdery mildew, water the roots properly.This is the single most effective measure and it surprises people, because the instinct is that a fungus means too much moisture. Powdery mildew exploits a plant under drought stress. Water thoroughly at the base, mulch, and keep the foliage dry.
7. For powdery mildew, then open the plant up.Thin congested growth, space plants further apart, and ventilate greenhouses freely. Remove the worst leaves. On a plant that has it every year, choose a resistant variety — modern courgette and rose breeding has made this the real answer.
8. For downy mildew, attack the leaf wetness.Water at the base in the morning so foliage dries fast, space plants widely, ventilate under glass aggressively, avoid overcrowded seed trays, and remove affected leaves promptly. There is no curative treatment available to gardeners.
9. Accept that downy mildew often ends the crop.Once it is established on lettuce, spinach, impatiens or a young brassica in wet weather it is very hard to stop. The honest plan is to harvest what you can, remove and bin the rest, avoid replanting the same crop in that spot for a few years, and buy resistant varieties next time.

Tips

• Impatiens downy mildew effectively removed busy lizzies from British bedding for several years. Resistant series now exist and are worth seeking out by name.
• A courgette plant with powdery mildew in late August has done its job. Keep it watered, keep picking, and do not spray it.
• Milk sprays for powdery mildew have some evidence behind them and are harmless, though not an approved product. They are preventive, not curative.
• Downy mildew on onions overwinters in the bulbs and in the soil, so do not save affected sets and rotate the bed.

Common mistakes

• Watering the leaves of a mildewed plant — It does nothing for powdery mildew and actively creates the leaf wetness downy mildew requires.
• Assuming powdery mildew means the plant is too wet — The opposite. Dry roots plus humid still air is the exact recipe, which is why it hits pots and dry walls first.
• Spraying downy mildew with a mildew product — Most garden fungicides labelled for mildew target powdery mildew, a true fungus. Downy mildew is a water mould and is not controlled by them.

Questions people ask