Onion - Downy Mildew


Downy mildew is seen as shades of green, yellow, and or brownish areas of irregular size, and shape, on infected leaves.

Fruiting bodies and spores called sporangia (fuzzy fungal growths) develop on the surface of the leaves.

These spores are carried by the wind from host plants that were infected the previous autumn.

Downy Mildew Examples

The spores appear at first as transparent to greyish blotches, that quickly change to a violet colour

Leaves become ringed with mildew, and then collapse.

In drier conditions the leaves die back progressively without visible fungal growth.

The disease rarely kills the plant, but bulb growth may be substantially reduced.

The neck of the affected bulbs may become soft and affect the bulbs keeping quality.

The disease has also been known to infect chives, garlic, leeks and shallots.

Depending upon the weather temperature the condition generally appears at any time between May and July.

The disease is most common during warm, humid summers.

Early planting can greatly reduce risk of fungal disease.

Spores can remain in the soil for five years or more.


No fungicides are available to the amateur gardener to cure 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.

The disease seems to occur more frequently on seed sown varieties as opposed to setts.

This is perhaps due to the setts being heat treated prior to purchase.

If true, then buying heat treated setts could be an alternative method of control.

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