I grew this opuntia cactus from a seed of a cactus pear which I ate. It is about 5 years old. Growing indoors in a condominium apartment facing east. It has never been taken outside. Recently it has started having these white discolorations on its pads. I showed the picture to Master Gardeners at Canada Blooms on Friday, but they could not help so asked me to send the picture in. I might add, that I tried to scratch some of the damaged texture off but no powder came out. Unfortunately, the plant looks ugly and I am sad to toss it in the compost. Is there any way to save it?
Thank you for contacting the Toronto Master Gardeners. We are impressed that you grew your Opuntia or Prickly Pear cactus from seed! Judging from the photo, there are several possible causes behind the white discolouration: physical damage, sunburn, a fungal or bacterial infection, or an insect infestation.
You said, when you scratched the “damaged texture” of the cactus pads that “no powder came out”. From the photo, it looks like the cactus is scabbing over some previous abrasion to its surface. Perhaps the plant was knocked over and suffered some injury to its surface? Or did it get accidentally splashed with some hot liquid during cooking, or chemical agent during household cleaning? Maybe it got roughened up a bit during repotting? Has the plant been moved closer to a heating source such as a hot air register, as this may cause one side of the plant’s outer surface to dry out? These would be examples of the physical damage category.
The other possibility is sunburn. A houseplant is more tender and sensitive to sunlight than its outdoor counterparts. When it is subjected to a sudden change in light level, its surface tissues can get a sunburn. We have had a couple of hot sunny days. Is your cactus located close to a window, and is the white discolouration limited to the side of the plant facing the window?
Alternatively, the white discolouration can be caused by a fungal or bacterial infection. Is the site where your cactus located humid? Often times a healthy plant can contract an infection through the wound(s) of a physical injury. In any event, monitor your cactus to see if the discolouration spreads. If it doesn’t, then you have nothing to worry about. If it does, then your plant has an infection. Unfortunately fungal or bacterial infections are difficult to eradicate. Your best bet is to find an uninfected stem or pad and repropagate from it, and throw the rest of the plant away. For the propagation, use only new potting medium and pot(s), and thoroughly disinfect your tools before and after use.
Optunia species, Prickly Pear, are susceptible to Optunia Bugs, Chelinidea vittiger, so take a very close look at your plant with a magnifying glass, to determine if you have an infestation. If you do have these little pests, your best course of action is to take your vacuum cleaner and suck as many of them off as possible. Following this, a good spraying with an insecticidal soap should kill off any immature adults. There can be as many as 5 generations per year so it is important to take both of the above actions every couple of weeks.