I have grown potatoes in large pots. One of my pots is looking very poorly. There are black spots on the leaves, hardly any flowers are forming and small growths (some up to the size of a walnut) are growing on the stems. The growths are the same colour as the stems and have leaves growing out of the top of them. The other normal leaves on the plant are not curling up. Is this blight or some other disease? The other potatoes in my garden are fine, although I am keeping this pot well away from them. Could this be a problem with the seed potatoes?