Neem isn't going to do anything for stem/crown rot. You can try to cut above the rot & restrike, but you would have to cut well above the problem. h2o2 @ .5% to quick kill via spray. 5ml/gal of 35% h2o2 if it is in the medium as a drench. Likely the underlying problem is too much soil/medium moisture... less likely very high RH to attack upper stems. Best luck!