HI, I’m in Toronto, the chard is growing in a raised garden bed. I’ve built up the soil using compost, sheep manure and black soil. There is a garden wall to the East that puts the bed in shadow until afternoon. Then the bed has full light for the afternoon. I regularly water the bed. Every year when I plant either chard or spinach, the leaves develop a dried out patch, that then spreads, and ultimately the leaf withers. I’ve attached a photo to show the damage on the chard growing right now. You may notice, I’ve put a cut-out milk carton into the earth around the chard, in the hopes that would stop any slugs or cutworms. However, the problem persists. Could it be insects? Any light you can shed on this problem, and epsecially any potential cure, would be appreciated, thanks
From your photo it appears that your Swiss Chard is being attacked by leaf miner. Leaf miner flies will lay their eggs on the underside of the leaf and when the larvae hatch they will begin to tunnel in between the leaf surfaces and feed on it’s tissue. Damage appears as winding trails in the leaf tissue. As the mines enlarge, they may merge and form large light-coloured blotched areas. Feeding lasts 1-3 weeks. The larvae may pupate in the leaf or in the soil and the ¼’ long gray flies emerge in 2-4 weeks.
Remove and destroy all leaves that have brown patches. Do not compost but place in yard waste bags to be picked up by the city. If you see clusters of white elongated eggs, you can brush them off. Once you have picked off all the leaves cover the crop with a floating row cover. In the future covering the crop with a floating row cover after germination will prevent damage.
It i possible from the picture that there may also be some slugs involved, but it is hard to say from the photo. It sounds like you are doing your best to catch those critters.
Good luck with the rest of the season.