Note to reader: this article applies to desert cacti, not jungle cacti.
Your cactus is turning brown and you don't know what's causing it? Don't be discouraged! It’s an opportunity to explore and learn more.
For all plants, indoor or outdoor, the appearance of visible browning often indicates a disorder which can be biotic (diseases, insects) or abiotic (temperature, watering, injury, etc.). In order to properly identify the source of the problem, put yourself in detective mode, ask yourself a few questions and observe.
How often do you water? Desert cacti prefer dryness between watering and if you don't allow this break, you run the risk of root rot, which results in a brownish discoloration at the base of the stem. To avoid this problem for indoor cacti, let the soil dry at least between 4 to 5 cm before watering again. When the time comes, if you can transport your plant easily, water by capillary – that is, water from the bottom. Assuming it has holes, dip the pot in about 4 cm of water until the soil is nicely soaked. Desert cacti prefer occasional heavy watering rather than frequent light watering.
What time of year is it? Ah, this is an important question, as desert cacti require more water in the summer and very little, if any, in the winter. You have to adapt to the season, the temperature in the room, and the exposure to light. Also, be aware that a drastic increase in sun exposure can also cause browning. This should be avoided.
Which potting soil did you use? Black spots on a cactus can indicate excess moisture or poor drainage, both of which are very important for desert cacti and can be avoided by using specialized potting soils for cacti. When in doubt, with good gloves, take the plant out of its pot, observe the moisture content of the potting soil and its composition, and change the potting soil if necessary.
Has the plant been damaged by a physical injury (by a tool, by a fall, human mishandling or a prick from a nearby cactus)? In this case, it is most likely that the damage has caused browning. You can remove the affected part or allow it to scar.
Other causes can explain the appearance of brown or black spots, such as poor aeration or a fungal disease. Sometimes, it is simply the normal aging of the plant that is responsible. This is the phenomenon of lignification.
The proper care of a cactus is inherent to its genus and species. It is therefore always best to learn about the specifics of your cactus by researching its Latin name. You will find great satisfaction in deepening your knowledge and feeling that you are giving the right care to the designated plant.