Jane, I don't know the cause. Some say fungal infection and yet some say lack of vitamin B. I took B complex, did not work, took biotin, did not work either. It did improve on the TeaTree + Jojoba oil. It used to be just on my right thumb, but now it looks my hands' fingers and toenails are having vertical lines as well. Could it possibly be dehydratin as well, i don't know. I have to get back on using tea tree and jojoba again. Melly
