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.