By David Marks
Lettuce are generally very healthy plants and the main reason for failure is lack of water and not harvesting correctly. However they do occasionally suffer from pests and diseases and the main ones are described below.

Amongst the most common diseases of lettuce are mildew and moulds. Greenfly can sometimes be a problem and slugs or snails can do great damage especially to young crops. Root aphids are not visible to the naked eye and can also do great damage


This is a fungal infection which is commonly spread in the air on windy days although occasionally it can live in the soil. If left to itself the damage caused will attract moulds which further damage your lettuce crop.

Yellow spots on the upper side of the leaves are the first signs of infections. As the fungus gains a hold the undersides of the leaves will develop small white patches of mould. The damage is only to the outer leaves but if left untreated will attract various other moulds.

There are no approved chemical treatments available to the amateur gardener for downy mildew on lettuce. To Prevent the spread of the disease remove any affected leaves and burn them, only the outer leaves are affected the rest of the lettuce will be unaffected and is perfectly edible. It is best to harvest the entire plant leaving no trace of the stalk and roots in the ground.

A major contributing factor in downy mildew is planting lettuce too closely together so thin seedlings to the correct distance apart. Downy mildew thrives in damp conditions where there is no or minimal air circulation.


A variety of small flies are attracted to lettuce especially new seedling and young shoots. They eat the leaves and stems and sap the strength from the plant. The goo they excrete is sugary and attracts a variety of moulds.

The small flies are visible to the naked eye so detection is not difficult. The flies will congregate around the growing tips and young seedlings.

There are chemical sprays available to the amateur but a leafy plant such as a lettuce will have residues of the chemicals left in the leaves so this form of treatment is not recommended. Systemic insecticides are much more liable to enter the leaves rather than just remain on the surface because that is the intention behind a systemic insecticide.

The best treatment is to inspect the plants every few days and squash the flies as soon as they are noticed. Spraying gently with a jet of water will also wash off a surprising amount of the flies.