Not sure what it is. I usually pinch off such stuff and put it in the burn pile, hoping that will prevent it spreading.
Garden’s alive Catalog has pictures of various diseases and advice on treatment, so when I want more info, I usually start there.
Check the underside of the leaves for aphids with a magnifying glass.
If you have other plants arround , this could be the beginning of an infestation .
Prevention is better than Control , especially if the plants are young .
Sound like you got some wilt virus. Not alot you can do as it’s in the dirt, but I’ve take peroxide and water 1/2 and 1/2 and sprayed it on the plants. Seems to slow it down.
Was watching this video last night and the host mentioned this website below for vegetable diseases and maladies.
http://vegetablemdonline.ppath.cornell.edu/factsheets/Tomato_List.htm
The leaves on one or two of our tomatoes have tiny light dots, like they were burned.
Thrips or Aphids? When you hold a leaf up to a strong light, and look at the leaf underside, do you see any damage? If so, what does it look like?