What is wrong with my potatoes

could be blight, check the blight site

Have a look at this

No 161/2 magnesium deficiency looks like yours, what do you think 

What a very usefull site. 

Yes it does look like it. Is there a remidy?

Just a thought but could they just be ready for harvesting? They are Yukon Gold which is a second early variety.

Mumofstig, that was a fascinating link. Thanks for posting that.

Just a thought but could they just be ready for harvesting? They are Yukon Gold which is a second early variety.

In my (limited) experience, that is normal, provided it happens round about the time you would expect to start lifting them. That's what happened to my spuds last year and this. For example, all of my King Edwards have started to go like that from the bottom up over the last two weeks and yet the adjacent Picasso are still fully green. So, to my mind, the KEs are ready to start lifting and using Epsom salts would be a waste of time and money. On the other hand, if that had happened much earlier, then I guess some intervention would have been useful.

I have a pear tree that looks just like the pic could it be suffering the same

The same web site also has details of problems with other vegatables/fruit.  For pears see:-

http://www.hbci.com/~wenonah/min-def/pear.htm