Are you sure your drip system is working? They tend to clog and could be the reason for the oleander and mesquite decline.

Your neighbor would have had to spray on or near your plants to affect them unless they used some sort of soil sterilant like Triox. These products persist in the soil preventing anything from growing and can also move in soils with water. If it seeped into the root zone of your plants it could kill them.

Here is a link to a listing of labratories that conduct all sorts of tests. Look for one that lists soil or chemical.

Good luck.

Here is a link that might be useful: Labs for soil, water, plant or water testing.

thank you for the information. yes the drip system is working well all our other trees and plants are doing well except this group. thanks again, jan

Those are both pretty hard to kill, I doubt if weed killer would do it. Also they are both pretty drought tolerant and after 3 years should have deep roots to sustain themselves. We lost several grown trees in our yard that suddenly died and when we dug them out we found 2 large grub looking things in the root system. We were told these were from the palo verde beetle and that is what killed them. You may want to check with a tree nursery and see what they think.

Pam

thanks. what do the palo verde beetle and grub look like?
thanks again, jan

Here's a link to photos of Palo Verde beetle larvae and adult. They are especially fond of the Mexican Palo Verde tree but can affect other ornamentals as well.

Here is a link that might be useful: Palo Verde Beetle

thanks again I recall seeing that grub in my plants a few years back.so far I haven`t seen any sign of it but will keep watch.do birds eat them? I remember a bird went after it and it was gone.