Not sure about root rot, don't think I've ever had it. My garden is heavily mulched with straw, and I have to fight early blight so much that I cut down the plant at soil level without removing the roots, since I avoid soil being around the plants as much as possible to avoid the blight. I didn't want to dig around and expose the several remaining plants to soil.
We have had a lot of rain off and on, the soil is quite moist (or wet). Clay type soil. My PH tested good this year, I did feed several times so maybe nitrogen was a factor? Hadn't fed in weeks tho. I've only watered twice this year (soaker hose covered with weed fabric) as the soil stayed moist to wet due to rain. Our temps have generally been quite hot (90's). We did just have a week or more of cooler temps though. Same variety next to this plant is okay.
Since I had a couple of vole-y looking holes maybe the roots on the one plant got chewed on?
The outside of the stem looked normal. The same OSU link mentioned Pith Necrosis, I wondered about that. It appears randomly which fits.
http://u.osu.edu/vegetablediseasefac...osis/advanced/