Possibly sick Mammillaria & Peruvianus Monstrosus

[RomanZ]

Hello all,
Have these two Cacti which are possibly sick.

1. The Mammillaria (quite large one) - has something on its top-middle part which slowly spread . I also added picture of it when it was fully OK.
2. Peruvianus Monstrosus (hope i identified it correctly) - has some strange powder-like attachment which can be removed with brush but also seems like some kind of sickness.

Both placed in sunny places (sun for few hours a day).

Thanks in advance for your help.

[Brontosaurus]

The Peruvians looks like scale to me. I would scrub all the white flecks off and then spray rubbing alcohol all over it.

[hegar]

I agree with Brontosaurus on the cause of the problem your Peruvianus cactus is experiencing. Those off-white specks seem to be armored scale insects. You can try mechanical removal and rubbing alcohol, but you may find out, that this is a tedious treatment, which may not lead to the elimination of the scales. It also needs to be repeated, because you may miss the tiny eggs these insects produce. Scale insect immatures are called crawlers. Those will possibly spread to your other plants. So it might be a good idea to place this heavily infested cactus away from the other cacti, basically a quarantine condition.
Because I know how tough it is to eradicate scale and mealybugs, I prefer to use a systemic insecticide. There are a number of those available to the home gardener. This kind of insecticide can be applied to the top parts of the plant, but more importantly it can be added and applied with the watering can. The plant picks up the chemical with its roots and translocates it throughout the whole plant. The problem with this kind of treatment is, that bees and other beneficial insects might be affected, if they do use pollen for example as a food source. Also, there are probably some insects, that have developed resistance to the pesticide.

The Mammillaria sp. shown does have a damaged crown. That is the part that is most delicate. I do not see, however, an insect problem there. It could be an environmentally caused thing. For example, if the plant was indoors and was then moved outside into the bright light, the sun could have damaged the new growth.

Harald

[RomanZ]

Thanks guys for your detailed responses. Much appreciated.
Regarding the Peruvianus Cacti, I will clean it "mechanically" with alcohol and brush us recommended and with the chemical insecticide introduced in the water to prevent it from re-appear again.
The Mammillaria case is more problematic.. not sure how to prevent it from spreading further. I will try to place it darker place with less sun.

[ElieEstephane]

Regarding the Peruvianus Cacti, I will clean it "mechanically" with alcohol and brush .

Not too rough though. You don't want new wounds for diseases to enter.
As for the mammillaria, what i would personally do if it's spreading it to "spoon" the diseased areas and the apex. This way it will produce many more offsets and the mother plant wouldn't grow further to show the damage. That's just me though!
Best of luck!