Overview;
Onion downy mildew is seen as a fuzzy fungal growth on the leaves of the onions.
This can be followed by total collapse, i.e. in drier conditions the leaves die back progressively without visible fungal growth.
The disease is transmitted by wind-blown fungus spores from host plants that were infected the previous autumn, these then infect the onions.
Spores can remain in the soil for five years or more.
The disease is most common during warm, humid summers.
Control;
Early planting can greatly reduce risk of fungal disease.
No fungicides are available to the amateur gardener to cure / eliminate this disease!
To control the problem, remove infected plants when the disease is seen, and do not grow onions for five years on land where the disease has occurred.
Clear away all debris at the end of the season that otherwise might act as a ‘host’ for the spores during the winter months.
Planting onion sets in pots / trays in autumn and growing on under glass over winter can help to reduce the risk of infection.
Plant these out in spring for a crop that matures before any infection might strike,