Sorry to see this.
Azaleas can take so much, then reach a critical level and die back dramatically. It seems this plant is on that flight path. Anything from hereon is emergency room work. In this case the cause seems to be, as
@Glaucus points out, likely a fungus infection caused by multiple factors.
Here’s what I would do, others may have differing opinions.
Recall as the foliage surface area decreases the need for water also decreases. Also fungus like it wet, azaleas like the media just moist. So the goal is to reduce the moisture level in the media to the bare minimum, while providing moisture on the foliage...
Cut off all affected parts to healthy wood… (clean tools afterwards.)
Mist all foliage and trunk thoroughly with a solution of 1 tsp 3% Hydrogen Peroxide in 16oz of water. Use an old toothbrush to lightly scrub the area on the trunk of concern.
Take the plant outside in a mostly shaded area, out of the rain. (If this is not possible one could try to do this inside, but the chances of survival are lower. Azaleas are outside plants. In the case of keeping it inside, get the plant out of the sun and use a small fan to circle the air around the plant. Low flow)
Water lightly with 3 tsp of 3% Hydrogen peroxide solution in 16oz of water, enough to penetrate the top layer of the soil, not enough to soak the entire root ball. This is counterintuitive as likely the root ball is wet, but the H2O2 has to be delivered to the roots to do any good at this point. Wait about an hour, then ‘chock up one side of the pot about 4 - 5 cm to lower the water table inside. Then switch the chocked up side daily.
Continue misting foliage 2 x 3x daily timed to dry by nightfall , but do not water the media until it becomes barely moist. Then use the higher concentration H2O2 solution to water the surface as needed.
I wish you good luck on this effort.
Cheers
DSD sends
