16+ Beet Leaves Turning Brown. The spots are caused by beet leaf miners and about all you can do about. The beet plant growth often slows and the leaves become brittle or thick.
(too much moisture will cause the leaves to brown and turn slimy.
I suspect that your plant aborted this bloom due to the lack of sufficient sunlight. Often brown leaves are caused by environmental factors, which can be remedied by paying close attention to watering, drainage and shelter. My tap water is heavily fluorinated too. Don't use regular soil, it doesn't breathe as much and may lead to root rot (and, in turn, more brown leaves).