It looks as though your beets have a fungal disease called Verticillium wilt.  Verticillium wilt attack the roots of the beets and causes the leaves to turn brown and wilt.  You will need to treat the entire plant with a fungicide that can be bought at a local garden center or nursery.  Other remedies includes:  insuring good air circulation around and between the beets; watering around the base of the plant instead of the leaves; remove fallen leaves of infected plants;  inspect plants regularly.

Thank you

Is this fungus airborne or in the soil. If in the soil should I put straw down when my beet germinate? Would that help prevent the fungal attack?

The fungus is not airborne.  Thoroughly clean the area before planting.