Clueless student seeks solution to dying mint! virus?

Hi everyone, great forums! :)

I just wanted to ask about my mint plant (mentha arvensis? it has been a while since my botany course!)—this is the second one I have bought, which at first, has grown voraciously. However after a few months it has developed some mottling on its leaves. Now my new one started the same, and the leaves have started dying from the top down. I keep it between two window panes and well watered. I thought at first it may have received too much sunlight/heat from the galsshouse effect of the window but now I think otherwise.

I've pruned the dead parts off as there are still new shoots growing but they are also showing signs of the disease (?). I bought it from a supermarket - would it be more prudent to go to a garden centre to avoid this contamination(?)?

Many thanks in advance for your suggestions, please find pictures of affected plant below :)

 

It's an outdoor plant - I'd agree with your first suspicions that it is due to the heat and dry air between those window panes.

 

I would agree, but keep it confined when you plant it or it will spread where you don't want it and it is very hard to kill it all.

 

Remove the two window panes. Its being roasted by the "greenhouse effect".