I dismantled the apricot tree and dug it out. The ground was fertile and the root system seemed to be well splayed out with some deeper penetration around 40cm. I did spy a potential culprit. there were two crawling burrowing caterpillar like bugs nestled among the root system. There size was small compared to the tree but could they have poisoned it some how? I found the following entry
Japanese Beetle
Adult is a metallic-green and brown beetle, which skeletonizes leaves. Larvae are cream-colored grubs that feed on turf roots prior to maturity. Turf pest-control may help reduce grub populations; check turf product labels for timing and control of grubs. Traps are an option for luring adult beetles.
This description seems to fit.