I have potatoes in tubs all in fresh multipurpose compost with added Osmocote, religiously watered. In the last 2 weeks some leaves have yellowed between the veins and turned brown. There is no sign of any mould. Dr Hessayon’s book (The Vegetable Expert) suggests it could be magnesium deficiency. If you think this is likely, I would be very grateful if you would recommend a product to boost Mg levels.
Osmocote should deliver enough magnesium but it may have been leached. You could try some liquid feeding in about every fourth or fifth watering, The standard feeds, such as Miracegrow, contain the full range of plant nutrients and should address deficiencies.