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Fungal leaf spots

Many fungi cause leaf spots: Cercospora on beet and chard, Alternaria and ring spot on brassicas, and others on strawberries, hydrangeas and sunflowers. They are rarely fatal but spread in wet weather and weaken plants.

Fungal diseases
Beet leaves with round, red-rimmed leaf spots
Beet leaves with round, red-rimmed leaf spots

What it looks like

Spots with defined edges.

  • Brown, grey or tan spots with distinct borders.
    Round spots Brown, grey or tan spots with distinct borders.
  • Cercospora on beet has pale centres and red rims.
    Red margins Cercospora on beet has pale centres and red rims.
  • Spot centres can dry and fall out.
    Holes Spot centres can dry and fall out.
  • In bad years spots join and leaves die.
    Merging spots In bad years spots join and leaves die.

How to confirm it

Check pattern and weather.

  1. Distinct spots

    Clearly bounded, not diffuse yellowing.

  2. Wet weather

    Worse after rain and overhead watering.

  3. Older leaves

    Usually older leaves first.

What to do

Tidy and dry.

  1. Remove spotted leaves

    Pick off and bin affected leaves.

  2. Water at the base

    Keep leaves dry.

  3. Space plants

    Allow air to move.

  4. Clear debris

    Remove old leaves after harvest.

Prevent it next time

  • Rotate crops for 2 to 3 years.
  • Grow resistant varieties.
  • Avoid overhead watering in the evening.

What not to do

Do not leave infected debris over winter.

In the growing guides

Where the guides mention it, with plant-specific advice.

Sources

  1. Cercospora leaf spot of Swiss chard, beets and spinach, UMass Amherst Extension