Powdery mildew

Figure 1: Symptoms on leaf showing leaf curl and white fungal growth

Figure 2: Fruit with powdery mildew on the left compared with a healthy fruit on the right

Cause

The fungus Podosphaera aphanis.

Symptoms

The fungus can attack petioles, leaves, flower, calyxes, fruit stalks and fruit.

Leaves:

• White patches of fungal growth develop on the lower surface of the leaf. Under favourable conditions, the patches enlarge and merge to cover the entire lower surface.
• Leaf edges curl upwards, exposing the white, powdery fungal growth (Figure 1).
• Purple to reddish blotches may also develop on leaves.
• Tiny, round, black fungal structures (cleistothecia) may also be present on the underside of the leaves.

Fruit:

• Pathogen colonises the fruit and produces white mycelial growth on the seeds giving it a powdery appearance (Figure 2).
• Surfaces of the fruit harden and may crack.

How it spreads

The white mildew contains large numbers of spores, which may be carried long distances by the wind. Dry weather, high-humidity conditions, and temperatures between 15^oC and 27^oC favour disease development.

The disease may also be introduced through:

• alternative hosts growing close to strawberry cultivated fields
• symptomless runners.

The fungus survives on diseased ratoon crops via overwintering fruiting bodies (cleistothecia).

Crops affected

There are several genera of fungi that causes powdery mildew diseases

• Erysiphe
• Leveillula
• Oidium
• Podosphaera
• Sphaerotheca
• Uncinula.

These affect several horticultural crops, trees and woody ornamentals.

Control

• Plant certified runners.
• Monitor powdery mildew incidence regularly.
• Remove infected fruit to minimise spread of spores.
• Use a recommended fungicide program.

Chemical registrations and permits

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