brown pits on cattleya?? mites?

[honolulukid]

Dear all;

found these spots on my cattleya in january, which have been slowly spreading on one but not the other. They had been growing roots vigorously in january but now the root growth has stopped and many of the bulbs have shrivelled. On one of the cattleyas, the brown spots have grown to cover a new shoot (see pic). I peeled back the black sheath and it was clean underneath, but now the black pits are growing into the newly revealed plant tissue near the base. Only the top side of the leaves have been affected, as well as the pseudobulbs, but not the undersides. The pits looks like mite damage but i've had these plants in sphag-n-bag conditions to try to encourage root growth and i haven't seen any insects, plus there's no silvery webbing or anything.

Tried a swab with alcohol and cinnamon in case it was a fungus, with no success. I spot-treated with soap just now, we'll see how that goes. Help?! Any ideas??