Potatoes in bags - White powder on the leaves

[fruitgum15]

I am a very new gardener and decided to grow potatoes in bags last year due to
only having a back yard and no vegetable plot. I grew carlingfords very
successfully and decided to do the same this year aswell as grow wilja, gemson
and charlotte alongside them (I haven't mixed the potatoes in the sacks as I
want to see which variety does best). My carlingfords have been in for just
over a fortnight now and are beginning to poke shoots and leaves through the
surface. However I have noticed a white powder covering the tips of the leaves
and in some cases spreading to cover most of the leaf. It only appears to
affect the carlingfords and not the other varieties. Other than this the shoots
and leaves appear strong and healthy. Any advice on how to deal with this would
be most welcome. Could it spread to the other varieties which are really
storming ahead with growth and have been well earthed up? I am so disappointed
after last years successes. The carlingfords were bought as seed potatoes and
are not last years stock. All potatoes were well chitted

[Two_Sheds]

White powder normally indicates powdery mildew. I would've said it's too early in the year, but my hardy geraniums have it already