I have a few small cacti that I noticed are coated with a very fine white powder that wasn't there before. It doesn't seem to be mealybugs and it's not scale, I thought it was dust at first but it's a very fine, white powder that rubs off easily. Maybe the same white powder some succulents have for sun protection?

Photos please... white powder could be glauscesent shielding, like dudleya, or could be fungi, or could be camo like on astrophytum

Here are some photos. Some of the rubbed off spots, I rubbed the powder off weeks ago and it hasn't grown back over where I rubbed it off. I didn't think more about it until lately. Powdery mildew, maybe...? But humidity isn't an issue here and I've never seen it on any of my plants before, so I don't know.

I have no idea what this is, and I grow a lot of Opuntias. It looks very much like what someone was calling corking, but I don't think it's corking.

Thats the top layer glaucous tissue, and youve damaged it. Youll get a scab on the area it used to be. That is, if thats the state you bought the plants in.

Its probably weird due to sun exposure or weather conditions. The green marks on the plants mean they have been touched recently.

I grow a lot of Opuntias as well (dozens, actually... lol) and I've never had the white powder before. The brown scar tissue was already there when I first bought them and is old damage, no corking yet. The green marks are the green cactus itself exposed after I rubbed off the powder.

I dunno. Antifungal coating? Something like copper2sulphate? Insecticide?

Can you check with the people where you bought them? I'd love to know what it really is as I've never seen anything like that on my opuntias.

They were from my cousin, he said he got them from someone in Texas. Just in case it is powdery mildew I've moved them away from my other plants, although it hasn't made the leap to any of them in all the months they have been clustered under the eaves together. Or maybe they are just producing an excess of that powdery sunblock that some produce. Guess I will clean them completely off and see if it comes back!

Can powdery mildew even grow on outdoor cacti in dry climates?

I try to never say never, but I've never seen powdery mildew on an outdoor cactus in any climate.

That makes me feel better hopefully not! I would think it would have spread like mad by now if it was.