Thank you for the responses.
I'm in Southern NM - high desert. I've lived here for a few years, but this is my first year attempting a vegetable and flower garden.
I have a drip irrigation system that waters morning and evening (to try and keep the water levels more even). It's been crazy hot here (100+ degrees for a couple of weeks) with no rain. The nursery worker suggested that the evening irrigation was creating humidity, as the water 'steams up' from the heat of the day. She suggested irrigating morning and possibly afternoon, as long as the leaves don't get wet, to avoid moisture at night.
The lowest leaves are dead, and the next 3 tiers of leaves are affected. I think I'll remove the most damaged, and try the fungicide solution on the rest (after testing).
Thanks for help!