How can I tell if I have a fungus or water problem?

[Jlryan]

Help!
I have over 70 tomato plants (never planted on this land before). All of them have yellowing, browning branches at the bottom that eventually just dry out and break off. I'm unsure whether this could be a fungus or if it's due to periods of heavy rain and high heat. They also don't seem to have grown for the last couple weeks. I have several green tomatoes although even some (not all) of the blossoms that were once white seem to have dried up.
Thank you in advance!