Brassavola seedling leaves got blackish lines along midrib

[Roberta]

I'll jump in with my ... Scale definitely can develop a shell (not baby scale)... underneath would be eggs that can develop into more scale. A fungicide is of no use whatsoever against scale, which is an insect. For a first pass, clean the leaves with soapy water (a few drops of dish soap in a sprayer of water works) Does that remove it? then spray the leaves in all the crevices, leave them drippy wet. If insects, the soapy water clogs their pores and drowns them. The treatment, however, only work on the adults, likely not larvae and certainly not eggs. So the treatment needs to be repeated over multiple weeks. Scale, however, is very stubborn - it that's really what it is, time for an insecticide, preferably systemic like Bayer's. And THAT also needs to be repeated. If it is actually a pest, it is NOT one and done.