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Lettuce Downy Mildew


Lettuce Downy Mildew

Usually appear in

All Year

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Caused By:-     The Fungus Bremia lactucae


Downy mildew of lettuce is a serious disease caused by the fungus Bremia lactucae that occurs worldwide. The symptoms are yellow patches and a white mould on leaves. Under favourable conditions, a white cottony-like fungal growth that is indicative formation of fungal spores appears on the lower leaf. These patches turn brown as the leaf tissue dies. It affects seedlings and mature plants. It is a common disease on both greenhouse and outdoor lettuces. Although indoor lettuce may be attacked at any season, outdoor lettuces are affected from early summer until mid-autumn.
Downy mildews are a large group of plant diseases and despite their similar name and some similar symptoms, they are unrelated to the powdery mildews.
Bremia lactucae 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 underside of 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.
Bremia lactucae has many strains and affects many other plants including: Centaurea (cornflower), Cineraria, Gaillardia and globe artichokes. But only those from lettuce and very close relatives can infect lettuce.
Infected tissues are often colonised by grey mould like Botrytis cinerea and this can lead to further rotting.
Picture by Gerald Holmes, Valent USA Corporation,
See also Downy Mildews Brassica, Downy, Hebe, Onion, 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.


Spraying with fungicides is usually ineffective as the disease is systemic.
Remove infected leaves promptly to prevent the release of spores and contamination of the soil. As the disease often only affects outer leaves the hearts can often be harvested and the rest of the plant removed and destroyed.
Crop rotation will ensure that a build up of spores will not occur.
Avoid watering Lettuce on the leaves and grow resistant varieties of lettuce.
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.

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