How to recognise it
A white, grey or faintly purple mould-like growth is found, often first seen on the underside of leaves, sometimes accompanied by a yellowing of the upper leaf surface. When magnified they appear as lots of little fungal ‘heads’ which may be hard to distinguish from the similar powdery mildews, where the heads are more densely packed.
If you wipe downy mildew off a plant the tissue underneath is often yellowing.
Why it’s a problem
Most downy mildews are relatively harmless, although they can detract from the appearance of a plant and may eventually cause wilting of the affected areas. However, some specific types, such as lettuce downy mildew, are a more serious problem and affected plants can become stunted and deformed as a result of infection.
Where you are likely to find it
How to prevent it
Reduce any plant overcrowding to ensure that there is good airflow between plants both indoors and out, and that any indoor environments (including cold frames and cloches) are well ventilated. Don’t allow water to drip onto or rest on the leaves of susceptible plants and take care not to overwater seedlings.
Maintain good hygiene around vulnerable plants, or areas where there has been an outbreak, particularly by removing and destroying any plant debris at the end of the season to remove any overwintering fungal spores.
Ensure you thoroughly clean any tools you have been using around plants which may have downy mildew, including your hands/gloves if you have been handling the plant.
How to get rid of it
Remove any affected plant parts and destroy (not on your garden compost heap). If you have a bad infection you can use fungicides which contain mancozeb (which is the most effective against downy mildews), or copper based fungicides, on the remaining plant parts and neighbouring plants. Pay particular attention to spraying the leaf undersides.
Badly affected plants should be destroyed as they are likely to be weakened beyond the point of recovery and will only spread the disease.
Is it good for anything?!
Other useful information
There are many different specific downy mildew diseases. We have further details about: