Powdery mildew

In short

Powdery mildew is a group of fungi that grow as a white, dusty coating on leaves, stems and flowers. Unlike most leaf fungi it does not need wet leaves: humid, still air around a plant that is short of water suits it best. Most species attack only a narrow group of related plants.

How it looks

A white to greyish, flour-like film that can be rubbed off, starting as small round patches and spreading until whole leaves look dusted. Depending on the species it sits on the upper leaf surface, the underside or both, and can also cover stems, buds and flowers. Late in the season tiny dark dots (the fungus's resting fruit bodies, chasmothecia) may appear in the white growth. On some hosts, such as wisteria and some rhododendrons, the white growth is sparse and the main sign is discoloured tissue.

Signs

White, powdery patches spread across leaves, stems, flowers and fruit. Affected leaves and shoots can become stunted or distorted (rose powdery mildew is a typical example), and on some hosts discoloured tissue is the main sign.

Life cycle

The fungus grows over the outside of the plant and feeds by pushing tiny feeding structures into surface cells. It produces masses of microscopic airborne spores that are carried to new leaves by air movement. Its spores hold unusually high water content, so they can infect in drier conditions than most fungal pathogens. It survives winter as dormant infections on green tissue or as dark resting bodies on fallen leaves, which release spores the following spring.

What favours it

High humidity without water on the leaves, still air (for example against walls or in crowded groups of plants), and plants suffering repeated, intermittent drought stress. Lush growth from excess nitrogen is more susceptible.

Prevention

• Give plants good light and space them so air can move between them.
• Keep watering consistent so plants are not repeatedly allowed to become drought-stressed; use a free-draining mix.
• Avoid heavy nitrogen feeding that produces soft, lush growth.
• Choose resistant cultivars where they exist.
• Check new plants for white patches before placing them near your collection.

Treatment

Start with the first step and move down only if the problem persists.

1. Remove badly affected leaves and prune out infected shoots promptly; bag them rather than shaking spores around.
2. Improve ventilation (a small fan helps indoors) and reduce crowding.
3. Products based on potassium bicarbonate, sulfur, horticultural oils such as neem, or Bacillus-based biofungicides are used against powdery mildew; follow the product label.
4. If a conventional fungicide is still needed, groups such as triazoles and chloronitriles are used against powdery mildew; some powdery mildews readily become resistant to fungicides. Follow the product label.

Pros and cons

Look-alikes

Downy mildew

Downy mildew makes coloured blotches on the upper leaf surface with fuzzy grey or purple growth underneath, and needs long periods of wet leaves; powdery mildew is a dry white dust that often sits on the upper surface.

Mealybugs

Mealybugs are individual, soft-bodied insects covered in white wax, usually clustered in leaf joints; powdery mildew is a flat film with no bodies.

Hard-water or spray residue

Mineral residue dries as spots where water droplets sat and does not spread or distort growth.

Affected plants

Very many plants (over 10,000 species), but each powdery mildew species usually attacks only a few related hosts. Common hosts include roses, cucurbits such as squash and melon, and many other ornamentals and crops.

Related

• Downy mildew
• Botrytis (grey mould)
• Low humidity damage
• Underwatering
• Integrated pest management (IPM)

Sources

1. Powdery mildews — RHS
2. Erysiphaceae (powdery mildew) — Wikipedia
3. Houseplant Diseases & Disorders — Clemson HGIC

Written in our own words from these sources. Check a product label before using any treatment.