United Kingdom 
Why are my husband's potatoes always powdery when cooked. Do we eat them too soon?
21 Jul, 2010
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It is due to the variety that he grows, Elsie. These potatoes are descibed as being 'flourey' and cook up as you describe. At the other end of the fimrness scale are potatoes that are 'waxy' and these remain firm.
http://www.jbaseedpotatoes.co.uk/home.html
The link is to a seed potatoe seller and describes the texture of all the varieties that he sells.
21 Jul, 2010