I was looking at your very helpful blog and was wondering if you had any insight to the below. My cactus recently had a bit of scale and once I removed it with a tooth brush it began to discolor with brown/black spots. I’m not sure if this is caused by the scale or if it is rotting and what my next steps should be. I bought some organic neem oil and treated it on Saturday evening but wanted to see with you if you think this is the right approach or what you would recommend. (I have attached a photograph for your reference) do you think there is any possibility this cactus could live?
Additionally I have another cactus potted in the same pot which appears to be healthy but I wanted to see if you think it is ok to leave it or if I should repot the ‘sick’ one.
I look forward to hearing from you and thank you in advance for your help!
I can’t tell what is going on from the photo. That wouldn’t have been caused by the scale. Generally we don’t recommend using a toothbrush since the bristles can be too firm – a soft paintbrush dipped in alcohol is sufficient to remove scale. It is possible that the skin of the Cereus was damaged and now has a fungus or other rot-related issues, but I can’t be sure. Neem wouldn’t have caused it unless you sprayed in direct sun, but it would help with any fungal issues. Or it can also be something entirely unrelated to the scale removal.
I would definitely separate the two plants, clear off all the soil from the clean one’s roots, and plant in a new pot with fresh cactus soil. If you live near Berkeley you could bring it in and we can help you with that.