Sounds to me that you are doing an awful lot of spraying. Are you seeing insect infestations that warrant so many sprays? None of what you are doing will help with nematodes; you need a soil chemical. I use DuPont Vydate applied pre-plant to the soil for my commercial trees; it works for me; but I'm trying to be careful what I recommend to garden growers. Ask your local nurseryman what to use for nematodes and other soil-borne insects.

I did not know that , I had a nematode problem. they are endemic in my area. My friends told me there would be a problem. I could dig them out and replant. Is Vydate restricted, requires a license? I could probably retake the state test at the supplier. My pesticide suppliers say NO treatment is available for nemetodes. the spay I use states: use at first sign of insects. That is every week. As I see white fly.

You said: "I could dig them out and replant."

If the soil where the tree is now planted has damaging nematodes, you would move those bad guys to the new site. That's a very bad thing.