I’ve had this cactus sitting in direct sunlight by a large west facing window for 2 years. I water about once per month. At the beginning it was entirely green but slowly it has accumulated this orange/white powdery looking substance which now covers most of the plant. What could this be?
This is a challenging case! I discussed the cactus’ symptoms with other Toronto Master Gardeners. It looks like you have more than one cactus in the pot – up to 5 or perhaps a couple of plants and a few cactus offshoots or “pups”. They appear to be Mammillaria elongata (ladyfinger cactus). Here are some ideas and suggestions:
Powdery mildew? From your description (it’s hard to see from the photo), your cactus may have powdery mildew, which is a fungus. It looks like the flesh of each cactus is creamy in colour, with the spines being orange. If it is powdery mildew, I’m not sure why the powdery substance is orange-y in colour, it is usually whitish.
If it is powdery mildew, you can spray the plant with insecticidal soap or with a copper fungicide, although given the spiny nature of the cactus, it may be hard to treat the entire plant without missing spots. Spray the plant the best you can, and follow product directions. You may need to reapply the product (again, according to directions).
Crowding, competition for nutrients? The cacti are quite crowded in the current container. Repot the plants using a fresh cactus potting mix, which should offer better drainage than regular potting soil. You can get cactus mix from a nursery or big box store. Also, to ensure good air circulation, if possible, give each plant more space in the pot, or plant them in more than one pot. I’m not sure from the photo if you can separate the cacti, or if they are all offshoots of one plant – if the latter, then they would need to remain in a single pot, just slightly larger. Also, I’d suggest switching from the green plastic to a non-glazed terra cotta (clay) pot, to ensure moisture is wicked away from the plant roots.
When repotting, check the roots – are they in good shape? If some are rotting, then the plants could be lacking nutrients. Cut off dead/mushy roots. The cacti could have outgrown their pot – check if the plants are rootbound, they would be densely packed and sometimes circling the container. If so, they may not be absorbing nutrients as well as they should, which could contribute to the plant flesh looking paler. Untangle the roots as best you can without breaking them. If rootbound the plants could also be competing with each other for nutrients from the soil.
Give the newly transplanted cacti at least a week to acclimate to their new conditions before resuming watering.
Air circulation for good plant health: Regardless of whether this is a fungal infection or something else, it is important to maintain good air circulation around the plant. For example, use a small fan if you have one. On a warm day, opening a nearby window will help, although the plant should not be in the direct path of a draft. Isolate the cacti from other plants to give it breathing room and (if the problem is powdery mildew) to avoid spread of the fungus.
Watering: You mention that you water the plants about one a month, a better approach may be to water only when the soil is dry. Allow the soil to get dry between waterings, then water thoroughly so that excess water escapes through the pot’s drainage holes. Now that our days are longer (it’s early April), the plants will likely need a bit more water.
Some references indicate that in cases where a cactus is watered via spraying/misting, there could be a (powdery) mineral buildup from the water. If you do spray or mist the plants, consider switching to distilled water or rainwater, and do not use a spray.
Other causes? Cacti can be susceptible to sunburn, but we would expect to see spots, not powdery material all over the plant. And it’s been in the same position for 2 years and hasn’t been burned. This also does not sound like an infestation of mealy bugs or spider mites (which would look like tiny cotton balls or white, stringy material).
To properly treat your cactus, you should be certain of what is ailing it. Although powdery mildew is a possibility, this does not seem to commonly afflict cacti. For this reason, I suggest that you contact the Toronto Cactus and Succulent Club. Members might have some helpful ideas for you.
For general information on cactus care, see Toronto Master Gardeners. Growing Cacti and Other Succulents: A Toronto Master Gardeners Guide.
All the best in keeping your cactus healthy!