The roots have been severely damaged by inconsistent watering. It is hard to under water, but easy to overwater this plant, especially when it is potted in garden soil in a large pot, as Sally wrote. A single episode of heavy watering will not cause root rot. Keeping it constantly damp will.
Root rot is not the same as having a fungus problem. There is no reason to treat yours with neem oil because the roots are not diseased. Pot them separately or together in a small pot in a very porous Cactus type potting mix. Keep the mix just slightly damp to promote new root growth. Good sunlight and warm temps will also help.
