cereus tetragonus problem
I have had this plant for several years and it has always been happy and health. It about 10" in wide and a foot tall: quite a fat little plant. Recently, it got brown spots on it in a few places. The local gardening store told me to simple prune it off and the plant should be fine. I did that and waited about a month before replanting. Now all hell has broke loose, as the plant has an area on the lower portion that has turned a brown color and it's developed what looks like white fuzz but is actually hard. Anyway - I don't know what to do to help it. Any advice?
When plants are showing brown spots, it normally signals the onset of a fungal infection. Treating the plant with a fungicide (neem oil works well) should help take care of this - if it has not progressed to the point of no return. Additionally, it could be an insect pest, like scale or mealybug, affecting your plant. Again, neem oil can help. Here is more information on neem oil: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm