Peony leaves have wilted. What to do?

Hello, last autumn I bought a collection of 8 peonies from J.Parkers. I planted them in pots and all successfully started growing. Unfortunately several days ago one peony's leaves have wilted and I do not know why and what to do. Have a look at the pictures.
Could anybody help with advice please?
Thanks in advance.

 

Thank you for the prompt reply and advice. I will move the peony to a lightly shaded area and will let you know if it survives.

 

Maybe Google Peony Wilt which unfortunately is what your plants look to have. I'd move it away as far as possible for the others (in a pot still? so not too difficult to achieve). I have many different varieties of Peony and occasionally the odd 1 gets an attack and usually recovers altho some sections will often die. I know it's slightly "shutting the stable door after the horse has bolted" but Peonies are 1 of a certain number of plants which you are better to buy from a specialist nursery and regrettably pay a bit more for. In the past I have bought relatively cheap plants with very seldom good results. The good news is that if you get healthy plants you wiil have them for just about ever. I have Peonies which originally my father bought and they are now going on for 60 years old and still growing beautifully with little ....ok no real assistance from me.
Personally I have found Peoines do much better planted in the soil rather than in pots. I had some Tree Peonies growing in huge 1/2 Whiskey barrels and after a few years they were not putting on much growth so I took my courage in my hands and moved them (Peonies aren't thrilled about being moved so if needed do so with care). After being moved into a flowerbed they were decidedly happier and put on huge amounts of growth to my delight.

 

Thank you for the advice. It's my first experience with peonies and I felt completely lost when one started having wilted leaves.

 

Hello again,
If it is a sort of a fungus could water spray with soda help?

 

Thank you very much.