[BobandJack]

One or two of my spuds have distinct yellow patches on the leaves.  Any ideas what this is?

[Goosegirl]

If it's in between the veins, it may be a nutrient deficiency or too much lime - if so, add some pelleted manure and for the latter some Epson salts (magnesium sulphate). If it is in a sort of pattern or blotches, maybe it could be some sort of virus infection? Others more in the know will, hopefully, add to this or correct me if I am wrong. Hope it's just the former!

[azubah]

A picture would help.

[BobandJack]

Well yes I guess a photo would help but I didn't have a camera with me at the time & I can't exactly nip to and from my plot as it's 18 miles away   
I don't think it's too much lime as I haven't added any & the plot hadn't been cultivated for a while prior to me using it.  I added horse manure when I was digging it over so again, it shouldn't be nutrient deficicent.  I'm wondering if it's a virus, bright yellow with quite large blotches on the outside of the leaves but only on one or two plants not all of them & I think only on one variety - Kestrel I think.  It looks like paint splashes if anything but the plant itself looks healthy enough apart from that.

[Salmo]

If the yellow is between the veins on the leaves sort of like fingers it could be magnesium defficiency. If it is on the edges with some blackening itcould be potash defficiency. If the leaves are sort of pale and blotchy it may be manganese defficiency probably induced by too much lime. Or it could just be shortage of water and the roots cannot take up enought nutrients.

Virus infection is unlikely if you used good seed.

[sunshineband]

A variegated potato? 

[BobandJack]

Mocking other people's potatoe worries isn't big & it isn't clever  Only kidding.  I found a photo of the exact thing on one of the BBC's websites but it was a pest/disease finder tool & I couldn't fathom how to get the info up, it just kept skipping to the next image, so I'm still no wiser.  Might have to look in a book!

[sunshineband]

Is this the site you used? If so, there is an interactive version and also a non interactive version which might help.

http://www.bbc.co.uk/gardening/advice/pests_and_diseases/