Certified GKH Gardening Expert
There is many things that can cause this. The first culprit is normally fungus. You can treat fungicide for this. Pests are another common cause. A pesticide will manage this. It could also be under watering, over watering, root rot or a few other things. Can you send us the way the leaves look before they turn brow? This will help us better pinpoint what the issue is.