Q.Ponytail Palm
I sprayed the leaves of my Ponytail Palm with Fly Spray, as there was a large black spider in there! Unfortunately, this has done some nasty damage. Most of the leaves have fallen off and what is left looks sickly. The tree is very tall 6ft or more and had just grown the first flower stalk. It’s about 10yrs old. Please, can you give me some advice to save my beautiful tree?

The chemicals in the fly spray may have been toxic to plants.
You can try washing the leaves and plant with clean water. This should remove any remaining residue of the spray and allow the plants leaves to breath.
You will need to wait and see if the plant can recover.
Here is a link to refresh you on the care requirements.
https://www.gardeningknowhow.com/houseplants/ponytail-palm/ponytail-palm-care.htm