Peronospora destructor

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Peronospora destructor - symptoms on onion (click on image to enlarge it)
Author(s): Gerald Holmes, Valent USA Corporation
Source: IPM Images

Peronospora destructor Casp. 1860

This oomycete is widely distributed in temperate regions and causes downy mildew on garlic, onions and other Allium species. The disease is economically important and outbreaks frequently develop under cool and humid conditions. Symptoms develop first on the older leaves as greenish-yellow to brown areas. Under wet conditions, infected leaves form a gray-violet mycelium and spores on the leaves.

The spores (conidia) disperse through the air and infect other plants, causing new disease cycles. Under moist conditions, one cycle is typically completed within 2 week. In autumn, durable oospores are formed which overwinter in the soil. Viable mycelium might also persist in plant debris or in onion bulbs left in the field.

Vernacular names
• Deutsch: Falscher Mehltau der Zwiebel
• English: downy mildew of onion
• Español: mildiú de las cebollas
• Français: mildiou de l'oignon