Japanese spirea

I'm a first-time gardener, so apologies for my ignorance

I was given a perfectly healthy Japanese spirea 2 months later, it's now in this state! I thought it was infected with fungus ( burnt like leaves) so treated it with a fungicide, I also noticed "whiteflies" hence also used an aphid spray. I don't think I've overfed or overwatered it.

Help & Thanks in advance !!!!

 

At first sight I'd guess it's done for.
But if that plant was mine i would cut off all the dead growth, and leave some of the shorter shoots with good leaves.
Then i would find a semi shaded spot in a garden border to plant it in.
When removing from it's pot I'd check that nothing is attacking the roots.
If ok, then plant it in the ground with some added compost and firm gently to avoid air gaps. Water well and then leave undisturbed. It probably won't come to much this year, but if you leave it till the spring it might just surprise you.. If it comes to life I'd then top dress it with a blood, fish, and bone fertiliser or such like.
If not then at least you tried.
I've got two spireas, a 'bridal wreath' and a 'gold flame' and both have been very sturdy for many years.

 

Thank you Jymi, I don't have a garden to plant into the ground is there another option? Repot in a bigger pot.

 

Ok, in that case you could try a larger pot, but a lot will now depend on the condition of the roots.
If you take it out from it's pot carefully and all looks well, then you could put it in a larger pot with some john innes no3 compost (or mix whatever compost you've got 50/50 with some good top soil)
If the rootball is loose and wretched then you could try untangling it a bit first. (Your existing pot looks large so it might go back in it with improved compost if the roots untangle) You can only do your best and fingers crossed!