My cactus started to turn white around 3 month ago and since then it has been spreading.
I can see white crystals on some of the thorns, it is not fuzzy but a solid and crystal-like.
It is an indoor cactus and it is placed in the window with direct sun.
It just grew a new leaf, so it is not dead yet.
Well, since my name was mentioned above, I thought, that I need to let you know, that I too cannot tell for sure what caused all these light grey splotches. There are a lot of possibilities, ranging from leaf miner insects feeding right underneath the cladode epidermis to algal spots and even perhaps a few diseases. Leaf miners, which are either the larvae of small flies or tiny moths, do cause similar lesions. There are even some leaf miners, which do feed in a circular pattern. If you take a close look at one of these insect caused lesions, you will see dark lines underneath the whitish papery epidermis. That is frass, or also known as insect excrement. The larvae of either leaf mining flies or moths are do either have a yellowish color (flies) or a pinkish one (moths).
Algal spots are of uniform color, in some cases they can be surprisingly large, i.e. if the alga has formed a symbiotic relationship with a fungus, which is then called a lichen. Lichens and algae, plus most fungi do best in humid environments. So they are not often encountered with cacti.
There are some viruses, which also produce a light-colored circular lesion, which may have the appearance of a target. Fungi also may grow this way. I am not sure, however, if there are bacteria which produce lesions like the ones present on the images.
The problem is, that it is very difficult and at times impossible to diagnose something, especially if the image does not show enough detail.
In my line of work I do use both dissecting microscopes and compound microscopes, in order to determine what fungus is causing a lesion. Sometimes even that tool does not solve the problem, because a fungus may be immature or has already shed its spores.
With the lesions shown on your Opuntia sp. plant, I would consider a possible insect infestation first and foremost. Then go with the other things mentioned in the order as covered above.
Using a hand lens or good magnifying glass, it should be possible to remove a thin slice of an affected cladode area and with a needle or probe to carefully remove the loose epidermis. You should be able to see the frass the insect larva produced. The insect may be inside the discolored area. It is very small and soft-bodied. If it already left, there will be a broken edge of the blister it caused.
Thanks so much for the detailed answer Harald!
I'll definitely take a look at a possible insect infestation and see what I find.
And then move to the other things afterwards.
If I end up unable to save it, then atleast I learned a lot for next time!
