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Downy Mildew - Leeks/Onions

Close up, close up of leaf symptoms & field symptoms on onions.




Onions and Leeks


Irregular lesions are initially found on the older leaves. The leaf area under these early infections are at first a pale green colour which later become visibly more yellow. These lesions later develop a typical downy mildew grey/purple mycelium containing fruiting bodies (sporangia) which can spread spores by rain splash or over longer distances on the wind. In severe attacks whole leaves can be lost. Infections can further spread systematically to the bulbs of onions and shallots which become soft and shrivelled adversely affecting bulb quality and size. Affected areas of plant tissue can be further colonised by other bacterial or fungal pathogens.


Mostly associated with onions but can also be found on leeks, garlic and chives. Infections typically develop from late May into summer. Usually first notices in foci which if untreated spread out to affect wider areas of the crop. Can survive to infect subsequent spring crops via infected plants or bulbs or as resting spores in the soil which can survive for several years.

Favourable Factors

Cool wet seasons especially in areas of high rainfall.


An economically important disease of onions.


Identification and Management of onion and leek diseases

Find out more information on the key disease threats to your onion and leek crop. For each disease you will find out the importance of the disease in terms of potential yield penalty, how to identify the disease in its early stages and our advice on the best control strategies.

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