Cercospora is a common leaf spot of Brassicas and other plant families.
The organism Cercospora is a fungal disease that causes defoliation of many plant species. Symptoms are seen on the leaves and are often gray in color, and may have concentric rings and a chlorotic halo. The affected centers of lesions dry up and often drop out as they age. When numerous spots occur on the foliage, the leaves turn yellow and may drop or wilt. Defoliation can result in reduced yields. Spores can survive in plant residues in the soil. Moist conditions or even morning dew can be enough moisture for infection. Temperatures of low 70s to mid 80s are most favorable.
Selecting disease-resistant varieties can help prevent the spread. Also, provide adequate spacing between plants for good air flow. According to the 2019-2020 Vegetable Production Handbook, there are numerous fungicides labeled for this disease. Consult your local agricultural or commercial horticultural agent for assistance with diagnosing this disease. It can easily look like many others in the field at first glance.