Hi gang,
I’ve been growing tomato seedlings directly in promix herb and vegetables for about 3 grow seasons. I like it since I don’t have to fertilize until they go outside 2 months later. The one problem I’m encountering is that a every year some plants get scarring on the stem that grows and scars the branches/underside of the lower leaves. I accidentally grew one out last year and it produced but I don’t recall if it was stunted since I have so many plants.

What do you guys think it is? Do I toss the plants? Should I change my starting mix?

I’m all but certain it’s Intumescence (edema.)
It’s a physiological disorder related to lack of uvb radiation. That is why my plants seem to recover when I plant them outside.
I hope this helps others. I had been told it was canker or early blight but my plants weren’t dying and the pictures didn’t match my plants.
https://ag.umass.edu/greenhouse-floricu ... tumescence

Looks like edema to me. I recommend putting a fan on them while inside and lower your watering

The strange thing is, I usually see the edema or intumescence on leaves before it's on stems and leaf veins. I believe it's worse on stems, because the vasculature can be damaged, permanently. But the 'scarring' effect is obviously shallow, if it is broken cells they are near the surface.
These effects can vary a lot depending on the variety. Some complex genetics controlling the reaction to irregular or excess water flows.

I have been battling with this problem some years now, when I started to use LED lights for growing seedlings indoors. Some varieties grow fine with LEDs while others get this scarring and drop lower leaves. The worst year I lost quite many plants, before I found ways to prevent it. While many other plants grow well with LEDs, I do still keep tomatoes under a fluorescent lights or have at least one fluorescent tube with the LED tubes. I did also notice that my habit of bottom watering the seedlings after transplanting is also making the problem worse. The ones suffering had really small roots and I believe that the bottom watering was causing the soil being dry on the top and soggy on the bottom, which prevented the roots from growing.

There are many studies about the lack of UV-B causing it.
https://www.sciencedirect.com/science/a ... 3817305484
https://journals.ashs.org/hortsci/view/ ... e-p712.xml

A quite new study mentions rapid changes in watering being the cause.
https://www.jstage.jst.go.jp/article/ec ... f/-char/ja

Sari

100% edema. You see how the branches also twist where they are affected. When the stem is affected it's pretty bad. Maybe bury it deep, above the affected part.