We Get Questions
My son is very worried about his cactus. He has had it for about two years and it started to turn black (please see attached photos) two weeks ago. We live in Wisconsin, and his cactus receives about 8 hours of sunlight a day. Any advice would greatly appreciated.
The cactus is mutant Gymnocalycium that is grafted on top of a Hylocereus stem (the green part). It is a chimera pair so that the bottom graft can feed the top bright-colorful part that lacks chlorophyll since it was likely exposed to gamma radiation to kill the chlorophyll and bring out the wild otherworldly color. The sad reality is the mutated part is generally short-lived because it has compromised immunities and can’t build all the proteins it needs.
The black infection showing in the photos is likely a fungal infection (it could also be a virus). If it is a fungus it may respond to being treated with a fungicide. We use Neem Oil, which is usually effective, while having low (to none) toxicity issues around mammals (us, kids, pets…). Neem Oil is used in toothpaste and cosmetics. You should be able to find a ready-to-use Neem Oil product at your local nursery. Follow the directions and spray it down well. Retreat after a week. Hopefully it will stop the infection, but the top graft will always be scarred. If the infection continues the top graft may fail and turn all black; if it does cut it off and treat the green base with Neem. The Hylocereus base is actually a cool jungle cactus that can be treated more like an orchid and if it starts growing new arms it can eventually bloom and even fruit, which are those cool and tasty looking “Dragon Fruit” you might have seen at the grocery store.