I have two New England asters. This is their 2nd year. This year they began turning brown at the bottom of their stems early in the summer. I have read about aster fusarium wilt. I don't know if this is it, because I saw/see no yellowing. Just browning. I don't know if it is simply not enough water. Last year, however, there was a drought and they did not turn brown. Thanks for any help in this issue.

Sorry, I don't know what your problem is. The following web site may help.
https://apps.extension.umn.edu/garden/diagnose/plant/annualperennial/aster/leavesspots.html

Or you can submit a sample to the Plant Disease Clinic for them to culture your palnt material and make a diagnosis. There is a fee for this service. 

https://pdc.umn.edu/