Downy Mildew - Leeks/Onions

Pathogen

Fungus

Hosts

Onions and leeks

Symptoms

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 systemically 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.

Development

Mostly associated with onions but can also be found on leeks, garlic and chives.

Infections typically develop from late May into summer. Usually first noticed 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.

Importance

An economically important disease of onions.

Control

• Clean seed, sets and bulbs

• Rotation

• Removal of volunteers and discarded plant debris

• Use of fungicides at first sign of infection

• Decision support systems have been developed for this disease

Onion downy mildew - field symptoms

Field symptoms on onion crop

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