Answer: From your description it is difficult to make a definite diagnosis of the problem. For instance, it might be damage from an insect such as leaf miner (affects spinach, beets and chard) or a fungus. However, most foliar problems can be reduced by the same steps: rotating crops, cleaning up and destroying any infected plant trash as it occurs (and all plants at the end of the season), avoiding overhead watering especially at night, and maximizing both sun and air circulation. In addition, if it is cercospora you should look for resistant beet varieties. Here is a photo of leaf problems on beets: http://www.ext.nodak.edu/extpubs/plantsci/rowcrops/pp764w.ht... for comparison.
If it happens again this summer, you might take a sample to your County Extension (652-3370) for a definite identification of the problem and suggested controls, if any.
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