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Onion downy mildew is caused by the fungus-like (Oomycete) organism Peronospora destructor, which infects first the leaves and later bulbs

These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes. of onions and shallots in mild, humid weather in late spring and summer. Some perennial

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs. onions such as chives may also be infected.

It is worse in cool wet seasons and in wet areas. Closely spaced crops such as salad or spring onions are especially vulnerable.

Be alert for the following symptoms:

• The affected leaves turn yellow and die off from the tip downwards
• In moist conditions a white, and later purplish mould develops on affected parts of the leaf
• This, in turn, is commonly followed by darker mould growth of other leaf-colonising fungi
• Bulbs
  These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes. can also be infected and often sprout prematurely or shrivel in store

Non-chemical control

• Avoid overcrowded crops, damp conditions and sheltered sites, and maintain good weed control to ensure airflow through the crop
• Do not compost
  Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own. infected material
• It is recommended to remove and dispose of affected plants and avoid replanting with onions or shallots for five years
• It is important not to allow any bulbs
  These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes. to remain in the soil from year to year. The fungus remains dormant
  Dormancy refers to a period of inactivity in plants, during which they slow down or stop their growth, conserving energy and resources. This is a natural process that helps them survive harsh conditions, such as winter or drought. Dormancy in seeds is a mechanism that prevents them germinating until environmental conditions are favourable for growth. in infected bulbs, producing spores in the spring which spread the disease to newly sown plants
• Any soft, mouldy or otherwise suspect onion sets should not be planted
• Onions sown in late summer for over-wintering can carry the disease from year to year
• Onion sets for over-wintering, planted in late September, should be less vulnerable to infection and acting as a source of the disease for crops sown or planted the following year
• However, if possible separate over-wintered crops as much as possible from spring ones
• Similarly, perennial
  Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs. onions should be separated from other onion crops in the vegetable garden

Resistance: Cultivars showing some resistance to onion downy mildew are now offered by seed companies supplying home gardeners, including the F1 hybrids ‘Hylander’ and ‘Santero’. These appear to offer good protection.

Chemical control

There are no fungicides available to gardeners for the control of onion downy mildew.

The downy mildews are a large group of plant diseases caused by microscopic, fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews .

The disease is spread throughout the summer by spores produced on the infected leaves. These spores are splashed by rain, and are also carried for long distances on the wind. Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are only likely to occur during wet summers.

The airborne spores remain viable for just a short time, but the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for a long time.

The fungus can remain dormant

Dormancy refers to a period of inactivity in plants, during which they slow down or stop their growth, conserving energy and resources. This is a natural process that helps them survive harsh conditions, such as winter or drought. Dormancy in seeds is a mechanism that prevents them germinating until environmental conditions are favourable for growth. within affected bulbs

These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes. , producing spores in spring that spread the disease to newly-sown plants.