I always thought it was something to do with to much nitrogen
fretilizer, but can find no referance for it.
Did find this however, doesn't answer the question, but is
interesting.
http://www.potato.org.uk/department/...ml?view_img=11
David Hill


Looking through the site that David linked to I tripped over this....
http://www.potato.org.uk/department/...f.html?item=32
Is that what you're describing?