Most root rot organisms favour low oxygen environments.
You can ward them off by growing in high air content mixes.
You can measure this as air filled porosity.
A gritty mix should in theory have more air space, but fine particles can also fill the pore space.
If you dump a kilo of sugar into a box of cornflakes, the sugar could fill the spaces between the cornflakes ( pore space ) and not overflow the box, so same volume less air.
Gritty components of pot mix, could also end up dense in the final mix.
https://www.sungro.com/air-porosity-and-water-holding-ability-of-media-components/I have not found Fungicides reliable to cure plants, more so it seemed to finish them off.
I have got plants back by repotting into taller pots ( more drainage and air space ). Often it seems better to elevate without repotting the root ball.
Increasing biological activity with organic fertilisers also seems to help recovery.