All of a sudden I found this problem on some brandywine tomato seedlings - only on the brandywines.
No marks on stems, no concentric rings (early blight), no visible problems at the edges (drought stress), no tan center (septoria), no fuzz (late blight). Any ideas?
name that affliction
How many leaves are affected, for I see just the one?
If there are many you might consider what's called leaf edema which is due to too much moisture, but if this plant was treated the same as all of your other tomato plants in exactly the same way, then I don't know what it could be, other than a Brandywine that isn't a Brandywine and what you're seeing is the result of a cross, and or rugose foliage.
If that's the only leaf infected, or if very few leaves are showing these symptoms, I suggest you very carefully cut and slip them into plastic bags, seal the bags and throw them in the trash. Then wash your hands before handling other plants.
I don't know what you have, but just incase it's a fungus, I would nip it in the bud.
When watering tomatoes, try not to get the leaves wet.
I didn't look at it under an enlarged photo but did just now and I can faintly see some lesions on the leaf surface but impossible to tell what they are at this point, either fungal or bacterial or spray residue, etc.
But again, with only one leaf showing it I don't know really, what to say since it it's a foliage disease those are translitted via wind and rain, so lots of leaves should be infected.
Another possibility is if you bought the Brandywine plant, since it's the only one to have this problem, and it came already infected or sprayed.
I know you said no Early Blight, no foliage diseases, but I do see something there in the early stagees.
So was the Brandywine plant different from all your other plants in some way, either by being purchased, or???
If you look really closely to the full photo, the other two, smaller leaves to the left of the large leaf also have the same lesions. The healthy leaves around appear to be from other plants--they are different types of leaf. I would segregate that plant from the others, if it isn't too late now that it is a week after the original posting.
i believe i have found the underlying problem: phototoxic compost. meaning, i used [my] compost that was not mature enough and it was stealing nutrients from the tomatoes. once the tomatoes were switched to another medium, they recovered quite nicely.
a good lesson in patience. i will be setting some compost aside to age for a year for seedlings for next spring.
Good to hear they are coming around. So the Nitrogen is still being tied up in the under composted compost. I've had that happen in my garden, but didn't realize what it was until it was too late. It doesn't surprise my either that the PLs were the ones that showed symptoms first.