do you live in an HOA and if so are there any rules there that would help with this?
Most states you can cut off anything that hangs over into your property, but that wouldn't fix this issue.
I have never heard of tree sap being so bad that it covered things in the neighboring yard and killed other plants. Correlation is not always causation, yes the sap may be there, but do you know that is what is killing the plants?
The neighbors may not be able to afford to take them down or treat them. If they are large as it sounds, it could literally take gallons and gallons of solution and special sprayers/equipment to treat them-yearly= lots of money. They may also be against using chemicals for obvious reasons.
I don't know the laws but in my opinion which isn't worth anything
you have no more right to have something in your yard than they do. Trees drop leaves, nuts, sap, ect, it's their nature and so basically an 'act of god' so to speak. Other than trimming what is on your side, I don't see that the neighbors are responsible for anything. Leaves, sap, fruits, and bugs ect always will end up in a pool from all over the surrounding area, that is just part of caring for a pool. Just move your plants to a different area of the yard and see if they better.
