Night blooming Cereus has parts of leaf that are dark brown & dry on one leaf at the top end of the plant. What could cause this?
This is an old plant. Thinking it might need new soil.
Certified GKH Gardening Expert
This appears to be a fungal infection. It can happen from the soil remaining wet for too long, without a dryout between.
This can be treated with a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you with the care of the plant: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm