Normally we've only seen late blight in any of our gardens. Late blight is a fungal disease that thrives in cooler weather. As the weather cools in late summer, late blight sets in from the ground up and quickly takes down the tomato plants. All the tomato plants get thrown away at the end of the season to try to control it. Flea beetles do attack the lower leaves on my tomato plants which eventually yellow, so I've always wondered if this allowed an entry point for the infection.
The greenhouse might have actually promoted the disease since it was very hot and humid in there, and the plants were definitely packed in way too tight so there was no air flow.
And here's a picture of some interesting bugs I found on the underside of a tomato leaf: