If I had 800 plants I would mail or drive a sample to a university sponsored Plant Pathology lab.

Here is one in Auburn--click on the plant form to see what they need to know and how to collect the sample. (cost $20)

http://www.aces.edu/dept/plantdiagnosticlab/

The link below is for one in Birmingham--looks like is costs $10 or so.

Here are some pictures to look at:
http://aggie-horticulture.tamu.edu/tomatoproblemsolver/leaves/index.html

Here is a link that might be useful: Plant Diagnostic Lab

By the way, forgot to say that it does look similar to alternaria stem canker but I would never guess with that many plants.

I had less that 100 plants and paid $20 to have a diagnosis of Tomato Spotted Wilt Virus confirmed.