Onion Downy Mildew
Onion Downy Mildew
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Peronospora destructor, or Onion Downy Mildew, is a leaf blight that can affect Onion and Shallot plants and bulbs. Chives can sometimes be affected too. Unlike Powdery Mildew, Downy Mildew is not a fungus but a fungi-like organism. The disease is most common during cold, wet periods of the year, typically in early spring but infections may occur in early summer depending on location. Onion Downy Mildew thrives when plants receive poor ventilation. The main symptoms include leaf yellowing followed by death from the tip down. When conditions become wet an off-white mould begins to form on the dead parts of the leave, which ultimately turns purple. Beneath the mould will be yellowing which may go through to the top surface of the leaf.
An off-white/ purple mould appears on the tips and edges of leaves. Beneath the mould will be yellowing which may go through to the top surface of the leaf. The mould appears all around the leaves and on the dying tips. Bulbs kept in storage may shrivel, soften and some mould may appear.
Spores are spread during wet conditions.
Initial leave yellowing.
White-greyish/purple mould on tips and undersides of leaves.
Leaves curling up and falling.
Bulbs in storage shrivel, soften and can sprout early.
Spores germinate on the exterior of a wet leaf and enter through the stomata where they grow and impregnate the tissue. Once mature, they will produce a sporangiophore out of the stomata and release new spores into the air. This typically occurs during wet conditions in the summer. The disease can also spread to the bulbs where they can remain dormant through winter, infecting newly sown plants in the spring. Resting spores are slightly different to those transmitted through air currents. The resting spores are produced in plant tissues and are released into the soil when the diseased plant matter begins to decay. They're much more resistant compared with airborne spores, persisiting in soil for years after.
Good hygiene around plants,e.g. removing dead leaves from the base of the plant, burning infected material etc. Remove affected plants straight away and do not compost. Make sure soil drainage is sufficient. Increase airflow around and through the plant. This can be achieved by regular weeding, providing sufficient space between plants and choosing not to sow seed in sheltered areas. In a glasshouse, avoid prolonged leaf wetness by watering in the mornings and not watering over the foliage during Winter. Rotating crops can also reduce the risk of infection. It's recommended to allow five years before replanting onions following an outbreak of Onion Downy Mildew. Sow seed into fresh compost. Avoid planting crops in spring near autumn-sown plants. The cultivars ‘Hylander’ and ‘Santero’ show some resistance and can be purchased from retailers as seed.