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Onion downy mildew is a disease of onions and related crops that damages foliage and bulbs, resulting in loss of yield or even a complete failure of worthwhile bulbs to form.
Onion downy mildew
Onion downy mildew is caused by the fungus-like (Oomycete) organism Peronospora destructor, which infects first the leaves and later bulbs of onions and shallots in mild, humid weather in late spring and summer. Some perennial onions such as chives may also be infected.
It is worse in cool wet seasons and in wet areas. Closely spaced crops such as salad or spring onions are especially vulnerable.
Be alert for the following symptoms:
Resistance: Cultivars showing some resistance to onion downy mildew are now offered by seed companies supplying home gardeners, including the F1 hybrids ‘Hylander’ and ‘Santero’. These appear to offer good protection.
There are no fungicides available to gardeners for the control of onion downy mildew.
The downy mildews are a large group of plant diseases caused by microscopic, fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.
The disease is spread throughout the summer by spores produced on the infected leaves. These spores are splashed by rain, and are also carried for long distances on the wind. Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are only likely to occur during wet summers.
The airborne spores remain viable for just a short time, but the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for a long time.
The fungus can remain dormant within affected bulbs, producing spores in spring that spread the disease to newly-sown plants.
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