diseased swiss chard, beets, spinach in a community garden

(Question)

I have a plot in a downtown Toronto community garden and something is wrong with all the swiss chard, beet and spinach plants. We have no idea what it is or what to do about it. Leaves seem to turn brown or brownish. Most of us amended our soil with a mushroom compost (perhaps mixed in with horse manure) that was delivered to the garden.

(Answer)

The problem in your garden could be a fungal disease called Cercospora leaf sport. The following information is from Mother Earth News.

  • “Cercospora leaf spot is a fungal disease that causes light brown patches surrounded by purple halos to form on leaves of chard, beets, and sometimes spinach. Warm, rainy weather favors outbreaks. Keep plants properly spaced to promote good air circulation and promptly remove any affected leaves.
  • Slugs often chew holes in chard leaves and rasp grooves in the ribs, feeding at night and resting through the day in mulch. Trap them in beer-baited traps, use an iron phosphate slug control product, or try repelling them by surrounding your chard plants with crushed eggshells.
  • Viral diseases cause new growth to be small or distorted, with unusual crinkling of leaves. Plants sometimes outgrow infection. Watch affected plants for a week or two and pull out those that show no signs of improvement”

For more in depth info see https://www.motherearthnews.com/organic-gardening/growing-swiss-chard-zmaz09amzraw.aspx

And/or https://extension.umass.edu/vegetable/articles/cercospora-leaf-spot-swiss-chard-beets-and-spinach