Indoor cacti growing white substance and becoming soft

Asked September 10, 2019, 2:14 PM EDT

I recently moved to Fort Collins from Tucson. I have been here since the beginning of July and I thought that I had successfully moved the cactus here without much difficulty. Last week I noticed a little white soft spot, but it was on the part of the cactus that didn't get as much sun, so I rotated it thinking that might be the problem. I was gone over the weekend and when I got back it had quickly spread. There is also a little bit of sap like substance I can see. The only thing I have found as an explanation is fungal or mealbug infestation, but I wasn't sure that this was the same thing. I'm afraid it is not salvageable, but any recommendations? I hope the attached picture is sufficient to see what I am talking about.
Thank you!

Larimer County Colorado

2 Responses

Hi ! In looking at the photo which you sent, while I cannot confirm from a photo, it looks to me as though your cactus might have mealy bugs, I am sorry to say. They are a common pest of indoor plants, and it is difficult to get rid of them. This would seem to go along with your observation of how quickly this infestation spread, as mealybugs can multiply extremely quickly, with the females of some species being able to lay 600 eggs, while the females of other species produce live young, like aphids do. If you touch the white substance and it is sticky, it almost for certain is mealybugs, as the sticky substance is "honeydew" or their excretion. Mold can also grow on the honeydew. It is important to isolate the infested cactus from other cacti or other plants that you have. Mealy bugs can move from one plant to another. You should consider discarding this plant if you have other cacti nearby. If you do not have other plants nearby and you wish to try to treat it, you can try an insecticide labelled for mealybugs, such as an insecticidal soap or horticultural oil. There is also a systemic houseplant insect control product that contains the neonicotinoid, Imidacloprid.l You wouldn't want to use this on a flowering plant outdoors that would be visited by pollinators, but you could consider it for an indoor cactus not being visited by bees, etc. There are some natural insect enemies that are predators of mealybugs, such as parasitic wasps, which are used in commercial greenhouses, but they are not practical for use by a homeowner.
If you wish, you could bring a sample of the cactus with the white infestation on it, into the Extension Office in Fort Collins, at 1525 Blue Spruce Avenue, for one of the master gardeners to examine it. There are master gardeners on duty in the Extension Office on Mondays, Wednesdays and Fridays from 10 am to 1 pm. If you do choose to bring in a sample, just be sure to seal it up in a plastic bag, so if there are insects on it, they can't escape. Thank you !

I had a typo in the name of the ingredient in the systemic houseplant insect control product. It is Imidacloprid. Again, though, I would not advise its use on any outdoor blooming plants that are visited by pollinators. But, on your cactus indoors, (not in a greenhouse) could be an appropriate use. Thank you.