I planted a black elder about 4/5 weeks ago. It is dying from the bottom up [leaves turning paler color and falling off]. What could be the cause? Other shrubs planted at same time in same area are all ok. I would buy another but if I don't know what I did wrong, I could do it again.
There may be air pockets in the soil, which are preventing the roots from taking up water. This can be alleviated by flooding the area with water and tamping down the soil around the plant to collapse any air pockets.
If this does not seem to be the issue, check the drainage to ensure that your soil is not staying too wet for the plant, which can lead to root rot - often seen with yellowing, wilting and dropping of leaves.
Finally, it could be a fungus of some sort affecting your tree. Treat with a general fungicide like Neem oil.