looks like its stem rot but its farther up the plant then usually seen , i have one right now with stem rot cause of the rain we got 2 weeks ago was crazy such heavy down pours , but i use rose dust its a fungicide and insecticide , kills slugs , leaf cutters and many other MJ draining, stressing bugs and i will give the base of the plant a lil puff of the rose dust so if the weather changes to wet and humid i dont get stem rot 99.9 pecent of the time , but theres alot of things to use for your plant like a fungicide to stop the rotting try your local walmart or garden store