Loyal readers will no doubt remember the great Beetroot Mystery of 2006. Every year since we’ve observed this pattern of leaf damage on spinach, chard, and beetroots. None of the remedies we tried had any effect. Finally the mystery has been solved and the culprits identified: leaf miners. I’m surprised we hadn’t heard of these ubiquitous pests before.
Unfortunately there’s no remedy beyond removing affected leaves in an attempt to limit the population. On the bright side, leaf miners usually don’t kill the plant; though they can spoil much of your spinach and chard harvest if left unchecked.