If it is in soil I guess the problem is in it,something like retaining moisture or something else(i hope someone will tell the exact reason to compare because i also want to know what cause it)Some strains are doing that or similar,i don't like it,may be they are more sensitive to some ferts or may be N....I always get back to soil,roots and feeding when I think about it.....How about your other plants?Containers,soil?
I also spray them from time to time in flower but not in latest stage.....I was living on the 7th floor and in the winter when we had central heating the air was really,really dry,no chance for mold or something.Too dry.I even stretched my laundry around them.:)