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Onion Downy Mildew


Onion Downy Mildew

Usually appear in

Spring to Summer

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Caused By:-     The Fungus Peronospora destructor


Onion Downy Mildew affects the onion-family of plants, including garlic and ornamental species. It is caused by the fungus Peronospora destructor that overwinters on infected plants debris and can remain in the soil for at least five years. It is often confused with powdery mildews, but is in fact unrelated.
It thrives in mild, humid conditions and needs wet weather to moisten foliage for 2-4 hours and a temperatures of 10- 12°C in order for the spores to germinate and infect plants. Overwintered plants are most at risk from early attacks. It affects the leaves and can get inside the bulbs. At first, pale spots develop on the older leaves first, later these take on a light brown or purplish shade when the weather is humid enough. When this happens it is producing spores that will spread to other plants. All the foliage is attacked and eventually destroyed. When conditions are dry the spots don’t develop into the purple stage.
Bulbs from infected crops will develop problems if they are stored. Watch out for soft, shrivelled ones and ones that sprout early. It seems to be especially common when there are mild winters and where many overwintered onion crops are grown.
Plants may not die, but may be dwarfed, distorted, pale green and bulb quality is poor and often spongy. If it does flower the seeds may not form properly and shrivel.
Downy Mildew is a biotroph which means it is a fungus that penetrates into host plant tissues without killing them, while it extracts nutrients from the living host cells. During this period it releases airborne spores from the fungus-like growth on the leaves, which disperses and infects other plants. These spores germinate in water on the leaf surface and penetrate directly into the tissues. Infection by downy mildew are both favoured by relatively cool temperatures and humid or moist environmental conditions. This disease is therefore worse under wet conditions. When the leaves finally die, the fungus produces resting spores. These can remain in the soil and may infect the roots of the succeeding crop. However, most infection occurs from wind-blown spores.
Infected tissues are often colonised by grey mould like Botrytis cinerea and this can lead to further rotting.
Picture by Howard F. Schwartz, Colorado State University,
See also Downy Mildews Brassica, Downy, Hebe, Lettuce, Pansy, Pea.
See also other Mildews American Gooseberry, European Gooseberry, Gooseberry.
See also these other Powdery Mildews Apple, Begonia, Hydrangea, Oak, Pansy, Pea, Phlox, Powdery, Quince, Rose.


Good hygiene is the first line of defence. Always remove all plant debris and allow enough spacing for good ventilation. Burn or destroy infected leaves preferably before the spores are produced. Copper fungicides may give some protection. Spraying with a good fungicide will control this disease, but thorough coverage of foliage is important in the control of downy mildew.
Weekly preventative sprays of baking soda( 1 tablespoon of baking soda a few drops of washing up liquid to 1 Gallon of water) makes an inexpensive control for powdery mildew on plants. Once the infection has taken hold baking soda offers only minimal benefits.
Some plants can be burnt by the baking soda therefore test before use on specific plants.
Spraying with a mixture of baking soda(sodium bicarbonate) or potassium bicarbonate combined with a lightweight horticultural oil is said to cure and prevent the problem.
High nitrogen fertiliser produces lush soft growth which is easier for the fungus to infect.
Crop rotation should also be practiced.
Some varieties of onion sets show some resistance to this disease.

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