New roots end turnning black and dry

[Orchidcork]

I have this problem in 2 cattleyas plants, the roots growing and suddenly have turnning in black in the same time in both of them.
I have other catts, what still growing good

[Blueszz]

Could it be that the top of the medium has mineral build up, that causes the root tips to burn?

[Orchidcork]

thanks for your answer, I used Leka, as in all others but only get in two of my Catts, i was thinking probally more sensible to the fetilizers?

[Orchidcork]

I check today my montsera, and have the same problem in the aerial roots, so i guess finally i did something wrong with the fertilizers and complements