Dark greasy spreading patches on my potato leaves with white mould in cool damp weather

Dark greasy fast spreading patches with a white mould on the leaf underside in cool damp foggy weather are late blight, the most destructive disease of potato, and it is urgent because it can destroy a field in days. Act immediately. If no disease is present yet but the weather is favourable, apply a protectant fungicide such as mancozeb or chlorothalonil as a preventive. Once you see active spots, switch to a systemic or translaminar fungicide such as cymoxanil plus mancozeb, dimethomorph or a metalaxyl based mix, and repeat as per label since the disease moves fast. Remove and destroy badly hit plants, avoid overhead irrigation, and earth up well so spores do not wash down to the tubers. Do not delay even one or two days in blight weather. Since potatoes are stored and eaten, follow the label dose and pre-harvest interval, and follow your KVK or state advisory which often issues blight alerts.