Cercospora leaf spot is a common disease on Long Island. It has been described as the most destructive foliar disease of beet.
Disease development is favored by rain because pathogen spores are moved by wind and rain, which also provides leaf wetness for infection. In addition to frequent rain, other favorable conditions include relative humidity of at least 90% and temperature above 75 F. It is especially destructive in Swiss chard and beets grown for greens as affected leaves are unmarketable. The fungal pathogen also can infect spinach.
Symptoms are small, light tan to brown, round to sometimes angular spots whose border is often purple to red. The center of spots observed on Long Island were lighter in color on the beets than the Swiss chard, and only the beets had the distinctive border. Spots were so numerous on some young Swiss chard leaves that they had coalesced and caused distortion.
Sources of the pathogen include contaminated seed, infested debris from previous crops and the related Chenopodium weeds that are also susceptible. The pathogen produces specialized survival structures (sclerotia) in infected leaves that enable it to survive in soil up to 2 years. Thus it is important to know where Cercospora leaf spot occurs to plan rotations.
Note that a different pathogen causes Cercospora leaf spot in carrot, parsley, celeriac, celery, dill and fennel.
Cercospora leaf spot of beet
Cercospora leaf spot of Swiss chard