I'm afraid I disagree with pretty much everyone. I would say its fungal (sclerotinia maybe), caused by too much moisture and not enough ventilation (and of course spores). The thinner diameter is indicative of the cell walls breaking. Is it harder there than in the green areas? Woody, I mean? As the plant grows the stem will get thicker. It looks to me that the plant persevered over the sclero rot. It's unbelievably common in greenhouses and it's commonly overcome. No chemical treatment will stop an established infestation as far as I know. Rockwool is good for sclero and pythium. It maintains a constant humidity around the base. Using perlite and coco seems to work better for me. Sclerotinia can cause the stem to become woody and thus reduce water flow. This can reduce yields or even lead to further complications down the road. Obviously reduced water intake is going to cause problems. I had a few plants that suffered this when young, but they pulled out of it. But, they produced less and smaller fruit. Still enough for us, so it wasn't a big deal. The stems stayed thinner at the base and I'm guessing the bottle neck is what caused the reduced yield. Those plants were in rockwool. In fact, I have a bunch of rockwool starter cubes here if anyone lives near New Whiteland, IN. I won't use them because of this problem and the higher rate of damping off. There are cheaper, superior alternatives. If I were you, I would keep these plants going, but consider an alternative method later to see the difference. If it weren't for the thin section, I would then have also said it was salt buildup. I get that a lot, but not with the thinning, scabbing issue.

Need a picture, please.

Finally getting back to this! I have better pictures, including one of the shrub as a whole. Here goes: The shrub (you can sort of see the white bit on the bottom right of the plant): Close-up of one of the affected stems (there are two in total, close to each other): Thanks again for any assistance. I'm not sure whether to cut the stems to the ground (the upper growth appears as healthy as the rest of the plant, FWIW), or wipe off the white stuff, or leave it alone...?

Possibly aphids of some description. You can just run the whole thing under the tap and carefully slosh the leaves and stems in water.