Hello, and welcome to the Forum.
In only two months or so, it's either too much water or not enough water*. Tell us about the plant's irrigation, initial planting and rootball, and soil conditions... but the true answer may lie beneath the surface, literally.
* Root rot is a special case of overwatering leading to the same end as underwatering. Major trunk or rootcrown damage at the time of planting is terminal underwatering regardless of how much you water.