It looks like ants have taken up residence. You will probably lose part of our plant as they hollow out the stem. I've had this happen in tomatoes and removed the stem they occupied. After removing that stem, use a bait to control ants. You may wish to coat the base of clean stems with tanglefoot or petroleum jelly to deter further visits while you wait for the bait to knock back the colony. Note that petroleum jelly will wear off over time.