Hello,

Blossom end rot in tomatoes is caused by calcium insufficiency.  The transport of calcium through a tomato plant is affected by inconsistent water uptake as well as calcium availability in the soil.

Lets look at watering first.  Did you supplement rainwater?  Cass county was very dry in May, wet in June, dry in July and wet in August.  Field grown tomatoes can exhibit blossom end rot with drastically varying water uptake.

Root damage can also affect water intake and therefore calcium transport.

Cool spring weather can affect calcium levels in plants, because at cooler temps, the calcium is bound more tightly to the soil and cannot be taken up by the plant.

Finally, if high nitrogen fertilizer is applied and the plant grows too quickly, calcium transport can be erratic.

Here is an MSU Article that address blossom end rot:

https://www.canr.msu.edu/resources/blossom_end_rot_tip_sheet

Hope this helps.