Late Blight: how to recognise and treat it

Disease

Symptoms

Pale green to brown spots with water-soaked edges on the leaves that spread fast; leaves collapse, shrivel and turn brown, and stem lesions are brown to almost black.
In damp weather a fine white downy growth appears around the edges of the spots on the leaf undersides.
Greenish-brown, greasy-looking patches on green or ripe tomatoes that stay firm at first; potato tubers have a reddish-brown rot under the skin that later turns soft.

Cut off and bag affected leaves daily in dry weather; remove badly infected plants whole. Bury them deeply or put them in council green waste or the rubbish — do not compost.
Where copper fungicides are approved for home use, spray them on healthy plants before blight arrives and repeat in wet weather; they cannot cure existing spots.
On potatoes, cut off the foliage (haulm) at ground level once about a quarter of the leaves are affected or the stems show lesions, then lift the tubers about two weeks later.

Where approved for home gardens (e.g. in the US), a protectant fungicide with chlorothalonil applied before symptoms appear, every 5–7 days in cool wet weather; it does not cure lesions. In the UK no blight fungicides are available to gardeners. Follow the label dose and pre-harvest interval.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Grow cultivars with good blight resistance (e.g. Sarpo potatoes; tomatoes such as 'Mountain Magic' or 'Defiant'); none is fully immune.
Keep the leaves dry — the pathogen needs several hours of leaf wetness to infect; earth up potatoes well to shield the tubers.
Rotate for at least four years (no tomatoes in that soil either); destroy volunteer potatoes, leftover tubers and cull piles before spring.

Figures checked against these sources.