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Symptom

Mottled or mosaic leaves

A mosaic of light and dark patches, rather than distinct spots, usually means a virus. Nutrient shortages and weedkiller can look similar, but make a more regular pattern.

Look for regularity. Viruses make irregular mosaics on scattered plants; nutrient shortages make repeating patterns on the same leaves of every plant.

Likely causes

Ordered from the usual answer to the less likely one. Each links to the full problem.

  1. A courgette leaf with yellow-green mosaic mottling Viral disease Common

    Cucumber mosaic virus

    Mosaic and puckering on cucurbits, peppers, spinach; aphids around.
  2. A tomato leaf with light and dark green mosaic mottling Viral disease Common

    Tomato mosaic virus

    Mottled, sometimes fern-like leaves on tomatoes and peppers.
  3. A bean leaf with mosaic mottling and puckering Viral disease Common

    Bean common mosaic virus

    Mosaic, blistered, cupped bean leaves.
  4. An older tomato leaf yellow between green veins Soil & nutrients Possible

    Magnesium deficiency

    Regular yellowing between veins on old leaves.
  5. Tomato leaves twisted and cupped from weedkiller Damage Less likely

    Weedkiller damage

    Twisted new growth after spraying nearby.