Cercospora Leaf Spot is caused by the fungus Cercospora beticola. It survives on infected beet debris and related host plants and spreads by spores carried by wind, rain splash and irrigation. Warm, humid conditions and prolonged leaf wetness favour infection. Spots usually appear first on older leaves as small grey or tan circles surrounded by a reddish-purple border. As the disease progresses, spots can merge and large areas of foliage may die. The plant then uses energy producing replacement leaves instead of developing the root, resulting in smaller beets.

How to Fix It
Remove infected leaves: Pick off badly spotted leaves early to reduce the amount of infected tissue producing spores.
Crop rotation: Avoid growing beets or Swiss chard in the same location for at least three years.
Clean up crop debris: Remove infected beet foliage after harvest rather than leaving it in the garden.
Keep leaves dry: Water at soil level and avoid unnecessary overhead watering.
Improve airflow: Give plants enough space for foliage to dry quickly after rain or watering.
Control related weeds: Remove susceptible weeds around the garden that can harbour the pathogen.
Choose resistant varieties: Where Cercospora is a recurring problem, look for beet varieties with resistance or tolerance.