Black stuff invading my orchid

The "black stuff" is probably due to a bacterial infection. Such infections often occur due to bacterial infection that is secondary to mechanical damage on the leaf or stem. Dendrobium hybrids that are derived from the phalaenanthe and sphatulata section of Dendrobium will get bacterial infections called black rot if water remains standing on areas that are damaged and temperatures are low for them , that is under 60F, for an extended period of time. Under natural conditions these plants are subjected to wet/dry cycles where the leaves can be wet for a considerable time, however this happens at the warm temperatures that these plants are adapted to. In their natural habitats, winds and the swaying of the stems means that most standing water either drains away or dries off. My advice would be to avoid standing water on the growing tips or in the bases of leaves.

 

Once black rot starts it can be devastating. Only cutting every piece that is black and a generous margin around it can stop it. However, sometimes it can be self limiting. There is no saving that growing point. It is already gone. However the base of the cane appears to be in good shape. If the rot has not penetrated the vessels of the cane all the way to the base, the cane will eventually produce a new growth from the dormant buds that lie just over the part of the cane that produces the roots. With these dendrobiums, the best you can do is not water them when temperatures are cold, and under no circumstance allow water to pool around new growths when temperatures are very cold. You can also tip the plant on its side so that water drains naturally from the leaf axils. I have seen this dendrobium growing happily near the sea shore on palm trees that have the consistency of concrete and with all their roots exposed. These plants loathe media that remains soping wet for a long time.