I received this Dypsis lutescens (palm) plant as a gift last spring, it had only two leaves and it was potted in nursery soil with sand. Recently I have noticed that the outer leaf started to form a black ring at its insertion on the stem. After new leaves have emerged I removed the outer leaf, but I have noticed that the next one had a small black dot at the insertion point, too.
At the end of the summer the plant became rootbound, so I moved it to a larger pot using general soil from the store to which I added vermiculite. The black dot became a black ring. I have removed this leaf while its lower part remained attached to the stem, becoming the brown-black ringed stump that you see in the pictures.
Now I notice that one of the leaves has a long black area on its stem (as seen on the red rectangle in the pictures). It is clear that my plant has an infectious disease, but I don't know which one.
My questions are:
What is the name of the disease?
Should I apply a fungicide like Topsin or Ridomil just in case I don't find out what disease is causing this?