I have encountered similar thing happening on some of my recently started carambolas. Lost about 20 seedlings, the seeds were planted in two small pots and upon germination started to have this browning right at the soil line . I watered and sprayed them with fungicide and this stopped.
In the past when I have germinated carambolas I have had the same thing happen, the plants that had the browning but survived ,later were thinner at the brown place ,compared to the rest of the trunk. They did fine after that...
Its some kind of damping off fungus I think, and having seeds germinating at high density like I did, surely does not help ....
