Is it like a little cauliflower curd, stuck to the stem, or does it look like a bunch of little fingers sticking down? The first might be bacterial disease called crown gall, the second might just be adventitious roots developing.

Umm. I suppose it could look a little bit like a cauliflower curd, tho it's fairly smooth & looks more like an arthritic knuckle really. I don't think it is extra root development - it seems a little higher up the stem for that. If it is crown gall, do I need to do anything, or is the condition harmless? Many thanks for your help, by the way.

Hard to tell at this point. Here's hoping not, but it might be the early stages of crown gall. If so, there is no cure, and the only way to rid the soil of the bacterium is either solarization--which requires 6-8 weeks of fairly consistent sunshine--or chemical fumigation. Maintaining a diverse population of soil organisms helps the plants to resist infection, but isn't a certain means of prevention.
On the other hand, it may just be a wart--plants get them, too--or some hopefully curable disease that doesnt happen in my area. Does anyone know if club root fungus could appear above ground?
You might want to re-post the question with pictures, if you can, so the other cabbage growers could weigh in on this.

Sorry for the delay in thanking (computer problems). Afraid adding photo is beyond my technological capabilities as yet. I sincerely hope it's not the dreaded club root, so am keeping my fingers crossed that it's something innocuous such as a common or garden (pun intended) wart.

Club root is at least preventable, since it dislikes neutral to slightly alkaline soils.