Something affecting new growth of these Mammillarias

[ralphrmartin]

I've had this problem too. Mainly affects Mammillarias, but I have also seen it on Notocacti, and even the odd Rebutia.

[AllanA]

I have had this problem in the past and I was told it was caused by mites, however, not red spider mite.

[David48]

Thank you. If it is another mite, I presume spraying with a miticide will deal with it just the same.
Terry S., I like your hypothesis, a genetic switch off, what is striking is that the plants became like "inermis or nuda" versions at least temporarily. Maybe the mechanism was something interfered with the gene(s) dealing with the creation of spines.

[Alexander]

If it is another mite, I presume spraying with a miticide will deal with it just the same.

Not necessarily every miticide. I had some tiny mites on a mesemb and tried Kanemite (Acequinocyl), which is great against spider mites, but it didn't work. I had to use fenpyroximate to get rid of them.

[David48]

About three weeks later spines have started to grow on the previously bare tubercles of two of the plants. Maybe light intensity was a factor.