This disease was observed in a home garden recently on table beet (Figure 1). I was surprised considering how dry it has been. Cercospora leaf spot also affects swiss chard. Symptoms include circular leaf spots that may have a reddish margin. The center of the lesions may start off a light brown and turn to gray after the fungus (Cercospora beticola) begins to sporulate. Under conditions conducive to disease, the lesions can coalesce and result in loss of foliage. Yield and quality of the crop can be reduced.
Figure 1. Cercospora leaf spot of beet.
Cercospora leaf spot is favored by rainy weather or overhead irrigation and temperatures from 77 to 95°F. The spores are readily dispersed in rainy, windy weather. One reference I found said that Cercccospora leaf spot can start with 90% relative humidity—that is, leaf wetness may not be necessary. That might be why this disease is present in this dry spell.
Cercospora may in survive crop residue over winter. The fungus may also be seed borne.
Resistant cultivars are available. Fall tillage and crop rotations of 2 to 3 years should help to lessen disease severity.
Several fungicides are listed in the 2020 Midwest Vegetable Production Guide for Commercial Growers including copper compounds, some of which may be allowed in organic certifications.
Synthetic fungicides include: Products with the active ingredient chlorothalonil (e.g., Bravo®, Equus®, Initiate®); Cabrio EG®; Flint®; Fontelis 1.67 SC®; Merivon®; tebuconazole products such as Monsoon® and Onset®; Quadris® and Satori®; The Midwest Vegetable Production Guide has more information. Always consult the label before every pesticide application.