Powdery mildew

General information

Powdery mildew is probably the most common of all plant diseases. The characteristic white, powdery growth occurs on plants as diverse as cereals, trees, turf grass, woody ornamentals and most vegetable and fruit crops.

Cause

Although the symptoms of powdery mildew are similar on many hosts, several fungal species cause the disease. Many are host specific, often infecting only a few, related species. The main genera of fungi causing powdery mildew diseases include Erysiphe, Leveillula, Oidium, Podosphaera, Sphaerotheca and Uncinula.

Races or strains may develop within a species in response to using a host-resistance gene for management.

Symptoms

Powdery mildew appears as spots or patches of white to greyish, powdery growth (mycelium) on the surface of leaves and other plant parts. The mycelium is most visible on the upper leaf surface, often covering it completely as the disease progresses.

Damage from powdery mildew may take some time to develop. Efficiency is reduced in affected leaves and fruit can be scarred and damaged, causing produce to be downgraded. Severe outbreaks can cause defoliation, exposing fruit to sunburn and predisposing them to secondary rots.

How does in spread?

Fungi causing powdery mildew grow largely on the surface of plants. They are obligate parasites and obtain nutrients by sending feeding organs (haustoria) into the epidermal cells of plants.

The superficial fungal mycelium produces chains of spores (conidia) that are widely dispersed by air currents. The spores do not require free water for germination and germinate freely in relatively low humidity, including moisture from morning dews, fog or condensate. Disease development is favoured by warm, dry and especially, cloudy conditions, which limit damage to the fungus by ultraviolet radiation. Humid, wet weather slows disease progress.

Overwintering fruiting bodies, called cleistothecia, may develop late in the season, or, when conditions become unfavourable. These appear as tiny, pin-head size, yellow-gold and later brown to black bodies within the mycelium. These fruiting bodies survive in leaf litter and crevices of plants or on alternative host species until spring, when ascospores are released to begin new infections. In warmer areas, the fungi may survive on alternative hosts or as mycelium or conidia in buds and other plant parts.

Crops affected

Most horticultural crops.

Control options

Use appropriate cultural management procedures, including removal of diseased twigs and crop debris, to reduce inoculum levels.

Apply pre-infection (protectant) and post-infection (eradicant or curative) fungicides.

Plant resistant varieties when appropriate, and available.

Chemical registrations and permits
Check the Australian Pesticides nd Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this disease on the target crop in your state or location. Always read the label and observe withholding periods.