Use a suitable insecticide or cover young plants with horticultural fleece as soon as they are planted out or emerge above the soil. Try the resistant variety 'Dynamite'. Their feeding activity causes loss of vigour, wilting and general stunting. The aphids are pale yellow but are often well hidden beneath a secretion of white, waxy fibres. Affected plants should be watered with a suitable insecticide, or choose a variety showing some resistance, such as 'Lakeland', 'Avoncrisp' or 'Avondefiance'.
Crop rotation is essential.
Lettuce downy mildew Yellow, angular patches appear on the outer leaves and then turn brown. On the lower leaf surface, corresponding with each area of discolouration, an off-white, slightly fluffy fungal growth may be visible. The affected leaves die off, and if seedlings are attacked they may be killed. Damp, warm conditions encourage this disease, so try to improve air circulation and avoid wetting the foliage. Control is difficult, as the disease may not be noticed until it's well established, but a fungicide containing mancozeb can be used.