A Night-blooming cereus leaf got partially damaged including the center stem- that section has developed some
brown bumps on it- I've now cut it off. However the uneven-shaped bumps have gone further down to the undamaged part of the leaf-- and I just found 2 very tiny black insects on it. I cleaned entire plant with alcohol and q-tip swab. What further to do?
Certified GKH Gardening Expert
That is a fungal issue, not likely related to the insects, but not impossible either.
Usually this is caused by overwatering, or leaving the soil wet for too long. It appears from the photo, that the soil is a little heavier than these plants like, and fully saturated. Two things that will not fare well for this plant.
Although these are not true cacti, and they do like moist soils, the soil must be light with organic amendments such as compost. This thicker soil will not allow the roots to breathe properly. I would try a cactus soil amended with compost.
Once you repot, you can begin treatment. This particular issue is a little more aggressive, so if the first treatment does not work, you will have to turn to copper fungicides.
After repotting, make a solution of 1/4 peroxide, and 3/4 water. Water this in, and do this two more times each after the soil dries out almost completely down to 3 or 4 inches or slightly more. So, roughly, once per week for 3 weeks after repotting.
This article will give you more information on caring for these plants: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm