My Night Blooming Cereus‘s leaves get dark spots and then dry up and die.
It’s hard to see the spots but if you look hard you can see them. Then what happens next is the tip of the leaf dries out and then expands down the leaf until the whole leaf dies. It seems to be spreading throughout my whole plant.
Certified GKH Gardening Expert
Uh-oh... This is a fungal infection that will lead to the quick death of the plant if not treated.
The first thing that you can try is to mix 1/4 peroxide with 3/4 water the next time that you water. This should only happen once a week for 2 or 3 weeks. This will help to kill of the infection. The infected material will not go away, but the spread will stop.
Make sure that this plant has PLENTY of time to dry out between watering, or is in the proper soil mix that will allow it to dry out in the correct timeframe. This problem is usually caused by overwatering.
These should become completely dry throughout the entire container before watering again. I recommend a succulent or cactus soil blend to help with this. A container that let's out all extra water is helpful as well.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm