Disease threats to onions originate in seedbeds, and the main culprit is damping off. This disease is caused by a number of different fungi and fungus-like organisms, especially the various races of Pythium, as well as Fusarium and Rhizoctonia.
Although damping-off can also be a problem in directly seeded onions, it’s more likely to occur in seedbeds due to the microclimate created by the higher plant population and seedbed covers.
Damping-off favors damp, semianaerobic conditions. Seedbeds should therefore be raised, and the soils should have a healthy organic content and good structure.
Be careful not to over-irrigate. If you do, and plants start falling over from damping off, drench them with a fungicide containing copper and keep the beds relatively dry.
DOWNY MILDEW
Another destructive foliar disease is downy mildew. It is caused by the aptly named fungus Peronospora destructor and can destroy a crop within a short time under favourable conditions.
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