Do not plant beets, chard, spinach or amaranth in ground that held them within 3 years.
PromisingHow it works: Cercospora beticola, the leaf spot of beet, chard and spinach, survives on infested residue and in soil for up to three years and carries over on lambsquarters and pigweed; spinach downy mildew (Peronospora effusa) leaves oospores in the soil for a few years and rides on seed. A three-year break outlasts both.
Evidence: checked against 7 sources in the research-based literature.
The 7 sources
Rotate with non-host crops for at least 3 years
Beet growers wishing to prevent Cercospora leaf spot on their farm should implement a 3-year crop rotation featuring non-host crops
Rotate to non-host crops (outside of the Chenopodium family) for 2-3 years
Don't plant susceptible crops in the same spot more than once every three years.
Rotate out of land where spinach was grown for at least 2-3 years
We summarise this source in our notes rather than quote a line from it.
Remedy
Plant them in different ground, or wait until 3 years have passed since this ground last held them. Bury or remove the old leaves, keep lambsquarters and pigweed out of the bed in between, and buy clean seed.