Downy mildew

Downy mildew
A grape leaf showing downy mildew and powdery mildew
Scientific Classification
Kingdom:	Chromalveolata
Phylum:	Heterokontophyta
Class:	Oomycetes
Order:	Peronosporales
Family:	Peronosporaceae

Downy mildew refers to any of several types of oomycete microbes of the Peronosporaceae family that are a particular problem for growers of brassicas, grapes and vine-type vegetables.

Cucurbit downy mildew (caused by Pseudoperonospora cubensis) is specific to cucurbits (e.g. pumpkin, squash, courgette). The disease is one of the most important diseases of cucurbits worldwide.

A serious disease in cool, damp seasons.

Symptoms

The initial symptoms of downy mildew appear on leaves as light green to yellow spots, then as it develops it becomes a downy grey mould. Leaves slowly die back and shrivel from the tips. Onion bulbs infected become soft and are not suitable for storage.

Treatment

None. Should the disease be confirmed on site, immediately dispose of diseased material in sealed bags or bins and arrange for it to be destroyed.

Prevention

Good practice:

• Try to ensure any starting plant material is disease free.
• Provide good air circulation and ventilation in glasshouses.
• Try to avoid long periods of high humidity and leaf wetness.
• Avoid watering crops late in the day.
• Apply a routine preventative fungicide programme to minimise disease risk, especially during periods when environmental conditions are favourable for downy mildew infection.
• Practice good nursery hygiene by cleaning up plant debris between crops and at the end of the season and use appropriate disinfectants.

Examples

Broccoli (Peronospora parasitica)

Cucumber (Pseudoperonospora cubensis)

Grape leaf (Plasmopara viticola)

Lettuce (Bremia lactucae)

Onion (Peronospora destructor)

Parsley (Peronospora umbelliferarum)

Raspberry (Peronospora sp.)

Sage (Peronospora lamii)