ased on your description, it is possible that the brown spots on your cactus could be due to fungal or bacterial infections, sunburn, or physical damage. It is also possible that the cactus is experiencing some sort of stress or environmental change.

To help narrow down the cause of the brown spots, you could check for any signs of insects or pests on the cactus, and also inspect the soil to make sure it is not overly moist or damp. If the soil is wet, it may be a good idea to reduce watering frequency or adjust the amount of water given.

You could also try removing any damaged or infected parts of the cactus, and apply a fungicide or antibacterial solution to the affected areas.

If the problem persists or gets worse, it may be helpful to consult with a local horticulturist or cactus expert who can provide more specific advice based on the type of cactus you have and the environmental conditions in your area.

I live in the Sonoran Desert, where cacti live outdoors; I'm used to seeing the lower parts turn brown and hard and woody (lignification) as the plants mature. I guess it is part of what gives them the strength to stand upright as they gain height (I have a 20-foot-high cactus in my back garden, and there is a 30-foot-high one growing in the middle of street out front, with woodiness in the bottom few feet). If your cactus is old, it might be this (and might need repotting). But... there are other causes too. Here is a link to a website that can help you formulate your questions as you search for the cause of the browning: https://cactusenligne.ca/blogs/news/why-is-my-cactus-turning-brown

Root Holic: thank you for all the information. I don’t see any signs of pests and the soil isn’t particularity moist. I am living out in Canada so maybe it hasn’t been receiving the desired amount of sunlight these last few months ..

Marco: A 20 foot high cactus growing in your backyard sounds impressive! Thank you for the article with further information for investigation.