Advice
RHS Help & Advice
Onion downy mildew
Common name: Onion downy mildew
Latin name: Peronospora destructor
Plants affected:
Allium spp, especially A. cepa (onions, shallots), A. schoenoprasum (chives) and A. fistulosum (Japanese bunching onions), but also other cultivated and wild species
What is it
Onion downy mildew is a fungal disease.
Symptoms
Leaves affected turn yellow and die off from the tip downwards. In moist conditions, a white mould develops on dead parts. This is, in turn, commonly followed by darker mould growth of other leaf-infecting fungi. Bulbs can also be infected and often shrivel in store.
Cause/biology
The fungus remains dormant in the bulbs, producing spores in the spring which spread the disease to newly sown plants. Persistent spores can sometimes survive in the soil for at least five years.
Control
It is recommended to remove and dispose of affected plants and not replanting with alliums for at least five years. Avoid damp conditions and sheltered sites and maintain good weed control to ensure airflow through the crop. Do not compost infected material.
There are no chemicals available to the amateur gardeners for the control of this disease.