Stems rotting, losing whole plants please help!

Hey guys, really could do with some help got a problem ive not had to deal with before.

Medium Coco plus
feed A+B canna
6x600w hps setup
15 litre pots hand fed

So i noticed a couple arms were dying on a plant but the rest of the plant looks lovely. A few days have passed and another plant has started dying but again the rest of it looks lovely.

I went to move the branch that was wilted and it just snapped clean off the plant it was almost bone dry. Where it connects to the stem was rotten (see pictures)

I dont know if its a humidity issue or some sort of fungus but if anyone has seen it before and knows best how to combat it id love some help.

also all the stems seem red/purple but they have been that colour since they were babies. the rooms never really been below 15-17c

i have removed the two plants effected and the rest of them seem ok ive checked the stems and seem normal but im really worried

I don't know how old the plants are but this looks like a fungal infection to me. My initial thought is that the pathogen has entered the plant and your plant is probably in some trouble. Looks as if the infection has been going along for awhile. Check the canopy areas.See if they show similar signs.

The treatment phase could be difficult. If it is botrytis you may be screwed as treatment probably will not eradicate it from the plant. If you are close to harvest you might need to cut the plant as this could lead to things like bud rot which is nasty shit for your yield. Got any more pictures??
Good luck