[Lesleyk]

I have large yellow marks on my potato leaves (not all of them, just a few of the first ear lies) which Hessayan suggests could be mosaic virus, or a magnesium deficiency.  From the pics it looks more like the m virus.  Is there any treatment for this?  Will it affect the crop?  I have, as a precaution, given the plants a feed with a tonic, but not sure if this will do more harm than good if magnesium deficiency is not the culprit.  Could do with some advice from someone who knows what they are talking about!

[mumofstig]

could we have some pics please, stand more chance of getting an answer 

[Salmo]

It is not unusual to see yellow marks on the first leaves as they emerge. Slight frosting can also give yellowing. I very much doubt if it either mosaic virus or magnesium deficiency at this stage.

Mosaic virus either comes in infected seed or is spread by aphids later in the season. The symptoms are typically yellow mottling all over the plants and the plants soon fall behind non-infected plants although they do not die and will still produce potatoes. Google potato mosaic virus on Google images.

Magnesium deficiency occurs when the plant is growing rapidly and the roots cannot keep up. The older leaves are affected first with typical yellow stripes between the leaf veins (google also). Sometimes there are also small brown spots which are sometimes mistaken for blight.

[ptarmigan]

I was going to say leave them to it for a bit, or give a bit of a liquid feed (comfrey or seaweed or something similar).  Without a picture it's hard to say but it's probably just the cold and slightly inhospitable weather at the moment.

My potato leaves are looking a little yellow at the moment too.

[Goosegirl]

Deficiencies tend to show themselves as a regular pattern on the leaves either in the veins or between them, whereas viruses show a mottled pattern like someone has splashed something on them.