Certified GKH Gardening Expert
Are you seeing any other symptoms? Normally, brown stems is an indication of stem rot fungus, which is common in soils that do not drain well. I would check your drainage and ensure that the plant is not in overly wet soil. If you catch this early enough, before the stems begin to turn blackish in color, you can improve the drainage (or move to a drier location) and prune out the infected stems/leaves. You can also treat with fungicide, though there is no guarantee this will cure the problem. Once the plant is too far gone, all you can do is pull it up, dispose of it and start again.