Downy mildew of Lettuce

Bremia lactucae

Lettuce downy mildew is a disease caused by the fungus-like micro-organism Bremia lactucae. This is a common disease on both greenhouse and outdoor lettuces. Although indoor lettuce may be attacked at any season, outdoor lettuces are affected from early summer until mid-autumn.
Bremia lactucae has many strains and affects many other plants including: Centaurea (cornflower), Cineraria,Gaillardia and globe artichokes. But only those from lettuce and very close relatives can infect lettuce.

You may see the following symptoms: 
On leaves: Yellow patches on upper leaf surfaces, often angular because they are limited by the veins. A fuzzy growth of whitish mould on the corresponding underside. Later, the infected tissues turn brown and die

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