Are you sure it's not leaf miners, which are common on spinach. Does it seem to be a "blistering" with the area being hollowed out?

Ron
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Certainly sounds like leaf miners to me. Quite common. I've never heard that swiss chard has no known diseases or pests. Leaf miners everntually get mine every year and I've also had mildew. They are one of my more troublesome veggies.

I grow lots of Swiss chard - grows year round here. Leaf miners are a problem, but not a huge one. Just remove the affected leafs. My biggest chard issue is aphids on the yellow chard - and ONLY on the yellow chard! Go figure! But I have so much that neither the miners or the aphids are too big of a problem. Fortunately for me, chard is one of the easiest things to grow. Different zones I guess! :-)

Kathy

I harvested about two gallons of seed off the swiss chard plants I let go to seed. I rototilled part of them into a strip of soil about two feet wide and 50 feet long. I now have several hundred swiss chard plants. The bugs can't eat them fast enough to keep me from getting my fill. They self seed like mad, they are now like a weed.

Mine gets that blight pretty often. I'm not sure what the cause is, some seasons are worse than others. It seems to be less present in dry weather, and chard getting too much shade from other taller plants get it much more.