Avocado

Letitia starts with ...
Hi,
I have some young avocado trees & I have been loosing them. They start getting a yellowing of the new growth branches & then go a blackish colour. When you cut the branch it is similar to what die back in roses looks like. I have been watering every couple days with these heat waves as they are only newly planted (within the last 2 months). They were all looking good but now they all (3 of them) are looking like this & 1 is an advanced tree still in a pot. Is it over watering as someone suggested or is there something else that could be going on? How do I fix it as I really don't want to loose them. They are all different varieties too.
Thank you so much,
Letitia Schwarzrock