What is downy mildew?

Downy mildew is a disease of the foliage, caused by a fungus-like (Oomycete) organism. It is spread from plant to plant by airborne spores. It is a disease of wet weather as infection is favoured by prolonged leaf wetness.

A range of common edible and ornamental plants can be affected, including brassicas , carrots, columbine (Aquilegia) , foxgloves, geum, grapevines , Veronica (Hebe) , busy Lizzie (Impatiens) , lettuces , onions , pansies , parsnips, peas, poppies, rhubarb, roses, spinach and tobacco plants (Nicotiana).

Click on the links above to go to profiles of the disease on some of the common host plants.

Downy mildews usually have narrow host ranges comprising of just a few related plants. For example, the downy mildew affecting brassicas is a different species from the one attacking pansies.

Take a look at the photo gallery below for pictures of symptoms on different host plants.

Unlike p owdery mildews , plants affected by downy mildew are not so easy to recognise. Look out for the following:

Discoloured blotches on the upper leaf surface. These may be pale green, yellow, purple or brown, depending on the plant affected. The blotches sometimes have straight edges (e.g. on lettuce) if they are bordered by the leaf veins

A mould-like growth on the underside of the leaf, corresponding to the blotch on the upper surface. This growth may be white, grey or purple, depending on the species of downy mildew

On some plants (e.g. pea, pansy) the growth is easy to see with the naked eye. On others (e.g. foxglove, rose) it can be difficult to see, even with a hand lens

Severely affected leaves may shrivel and turn brown (e.g. tobacco plant), or turn yellow and fall prematurely (e.g. Impatiens, rose)

Occasionally, other plant parts can be affected, e.g. cauliflower curds, Impatiens flower buds, pea pods

Severely affected plants are often stunted and lack vigour. In some cases (e.g. columbine,Impatiens, tobacco plant) the plant may die