Answer: It could be a number of things. Beets like cool weather, so excessive heat and drought could cause this. But that's probably not the problem in your climate. My guess would be that it is related to nutrient levels. How are other plants growing in your garden? If everything looks great, then the soil is probably fine, but if things are struggling, I'd have a soil test done to check nutrient levels, and, most importantly, pH.
Your beets may be the first crop to show signs of deficiency, because they are quick growers and often one of the first things to mature. Check to be sure the plant is getting adequate (but not too much) water. If you have not fertilized the plants, you might try a dilute liquid plant food.
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