Downy mildew (latin name – Peronospora destructor) develops on leek (Allium porrum) when humidity is high and night temperatures are low. These conditions are usually seen in the autumn.
Downy mildew is characterized by pale–green, yellowish to brownish areas of irregular size and shape (oval to cylindrical) on infected leek leaves. Masses of spores are produced on the surface of the leaves, which turn from transparent to greyish, and then rapidly become a violet colour. Leaves become girdled in the region where mildew develops and the leaves collapse, resulting in dead leaf tips. The dead leaf tissue is often colonized by purple blotch, which is dark in color and obscures the downy mildew.
Downy mildew seldom kills leeks, but the leek growth may be reduced.
The relatively cool, moist weather that the UK has been experiencing favours the development of downy mildew in leeks.
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