Large black or brown spots on potato leaves can be a sign of late blight. This disease causes heavy damage to the potato crop. It is a rapidly spreading disease. Fullstop for its control, 30g. Or Sofia, 35-40 ml. or Entracal, 25-30 g. Spray it by mixing it in every 15 liters of water. After 3-4 days, 1 tablet of booster and punch, 10 gm per 15 liters of water. Mix and spray.