Plant disease

Powdery mildew

Pathogen: Fungi of the order Erysiphales

Symptoms

A white, powdery coating on leaves, shoots and buds. The leaves turn yellow, become deformed and drop prematurely. The coating develops on both sides of the leaf blade, is often clearly visible from above, and can easily be rubbed off with a finger — it is the superficial mycelium of an ectoparasite that draws nutrients from the epidermal cells by means of feeding structures (haustoria). At first single, round, white spots appear, which in time merge and cover the entire leaf surface and the young growth. Young leaves and the current season's shoots are the most susceptible; heavy infection causes them to pucker, stiffen and stop growing, and on buds it leads to deformation or death of the flowers. On the fruit of some species (gooseberry, apple, cucurbits) the coating leaves russeting or a corky skin. Towards the end of the season, small, dark fruiting bodies (chasmothecia, formerly called cleistothecia) — the pathogen's overwintering structures — appear within the coating. The disease affects many plants, including roses, oak, privet, lilac, cucurbits, cereals, grapevine, apple, gooseberry, phlox and coneflowers, with each of these hosts being infected by a specialised species of fungus.

Treatment

Remove infected parts, spray with sulphur- or triazole-based fungicides, and in milder cases use preparations based on plant extracts. Infected shoots and leaves are best removed as they appear and destroyed (not composted) to limit the source of spores. Sulphur acts by contact and preventively, but it should not be applied in hot weather (above about 28–30°C — risk of scorching) or on sulphur-sensitive species (some cultivars of gooseberry, currant and cucurbits). In cases of heavy infection, systemic fungicides are used — triazoles (sterol biosynthesis inhibitors, the DMI group) or strobilurins (the QoI group) — remembering to alternate different groups (rotation according to FRAC), because the pathogen quickly develops resistance to a single mechanism. As a supplement, at low intensity, preparations based on potassium bicarbonate, plant oils and plant extracts work well. Treatments are repeated every 7–14 days, ensuring thorough coverage of the whole plant; spraying is most effective at the first symptoms, because advanced infection is hard to reverse.

Prevention

Ensure good air circulation, avoid wetting the leaves when watering, and keep nitrogen feeding moderate. Plants should be spaced to allow airflow, placed in sunny positions, and crowns and shrubs thinned regularly, avoiding congestion and shaded, damp corners. An excess of nitrogen encourages lush, soft growth that is particularly susceptible — it is better to feed in a balanced way, with potassium included. Choosing resistant or tolerant cultivars is very important (available in roses, cucumbers, squash and cereals, among others). Water at the base and in the morning, so that the leaves have time to dry off. In autumn, fallen leaves and infected debris, on which the fungus's fruiting bodies overwinter, must be removed — sanitation limits infection in the following season. In chronically infected species, the preventive use of sulphur or potassium bicarbonate before symptoms appear is helpful.

How to tell it apart from look-alikes

Feature	Powdery mildew	Downy mildew
Location and colour of the coating	Coating on BOTH sides of the leaf, often clearly visible from above; white, powdery, easily rubbed off with a finger	Coating on the UNDERSIDE of the leaf, greyish-purple or dirty white; on the upper side yellow, angular spots bounded by the veins
Pathogen	Ascomycete fungi of the order Erysiphales — ectoparasites growing on the surface of the tissue	Fungus-like organisms, oomycetes of the order Peronosporales — parasites penetrating deep into the tissue
Favourable conditions	Warmth and high air humidity, but with a DRY leaf surface — wetting is not required	Cool and damp; infection REQUIRES water or dew persisting on the leaves
Nature of the coating	Superficial, dry, powdery — can be rubbed off with a finger	Fluffy, felt-like, more strongly bound to the tissue, arranged in patches between the veins
Effectiveness of sulphur	Sulphur and bicarbonate work effectively	Sulphur ineffective; specific fungicides against oomycetes are needed (e.g. copper-based)

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Sanitation and sanitary pruning	Ongoing removal and destruction of infected leaves and shoots, together with autumn clearing of debris, limits the source of spores and overwintering fruiting bodies. A supporting method that will not replace spraying under heavy pressure.	Throughout the season, thoroughly in autumn
Cultural practice and resistant cultivars	An airy, sunny position, correct spacing, thinning and moderate nitrogen limit the development of the disease; choosing resistant cultivars is the cheapest lasting protection.	At the planning and planting stage
Potassium bicarbonate and oil-plant preparations	By contact they change the conditions on the leaf surface (pH, an oil film) and inhibit the growth of the mycelium. Effective at low and moderate infection, they require regular repetition.	From the first symptoms, every 7–10 days

Chemical methods

Method	How it works	When to use
Sulphur (contact fungicide)	A mineral contact and preventive product, also permitted in organic growing; it inhibits spore germination. Do not apply in hot weather or on sulphur-sensitive species.	Preventively and at the first symptoms
Systemic fungicide (triazoles / strobilurins)	Penetrating, systemic action: DMI block sterol biosynthesis, QoI the fungus's respiration. For heavy infection, and essentially with rotation of FRAC groups so as not to select for resistance.	Under high disease pressure

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.