I have in another thread explained how calcium is crucial to the immune system for plants when it comes to fending off fungus and mold.
If you have root damage or a medium with an incorrect pH it may be hard to correct the issue, via the roots alone.
What you can do is make a foliar tea in the following way. Hardboil a few eggs in their shells, you can take out the eggs to eat, but keep the water.
use between. Top up the water with tap water until you have 1 liter of water to this add between 100 and 200ml of milk. It can be any dairy milk, fresh or expired.
The milk is a further source of calcium and is also antifungal.
Put it in a spray bottle, and spray down your plants.
This puts the antifungal milk on the outside of the plants, and increases the calcium inside the skin tissues, to bind with pectin and form a layer that the fungus can not penetrate when it sends out its runner for food and water.