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Symptom

Yellow leaves

Yellow leaves have many causes, from water and feeding to pests and disease. Where on the plant the yellowing starts, and whether the veins stay green, narrows it down quickly.

A monstera with a lower leaf turning yellow

Start with the soil: watering causes more yellow leaves than anything else. Then look at which leaves turned first, old or new, and at the pattern on the leaf.

Likely causes

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

Recorded on Rosemary

  1. A snake plant with soft, yellowing leaves collapsing in soggy compost Watering Common

    Overwatering

    Soft, yellow lower leaves in soil that stays wet.
  2. Yellowing plants collapsing in waterlogged soil Watering Possible

    Poor drainage and waterlogging

    Whole beds yellow after wet weather in heavy ground.
  3. Pale, leggy seedlings stretching towards a window Light & temperature Possible

    Too little light

    Pale, stretched growth leaning to the light.
  4. A colony of green aphids on the underside of a leaf Pest Possible

    Aphids

    Yellow, curled leaves with insects and honeydew underneath.
  5. A leaf mottled pale by spider mites, with fine webbing Pest Possible

    Spider mites

    Speckled, then yellow-bronze leaves in dry heat.
  6. A plant lifted from its pot with brown, rotting roots Fungal disease Possible

    Root rot

    Yellowing with wilting; roots brown and soft.

Less likely on Rosemary

  1. Pale yellow lower leaves on a slow-growing plant Soil & nutrients Common

    Nitrogen deficiency

    Oldest leaves pale evenly, veins and all; the plant looks hungry.
  2. An older tomato leaf yellow between green veins Soil & nutrients Possible

    Magnesium deficiency

    Yellow between green veins on the older leaves.
  3. Leaves turning yellow between green veins Soil & nutrients Possible

    Iron deficiency (lime-induced chlorosis)

    Yellow between green veins on the youngest leaves; soil often alkaline.
  4. A tomato plant yellowing and wilting on one side Fungal disease Less likely

    Fusarium wilt

    Yellowing on one side of the plant, stem stained brown inside.
  5. Basil leaves yellowing and blotched with downy mildew Fungal disease Less likely

    Downy mildew

    Angular yellow patches with grey fuzz underneath.
  6. A tomato leaf with brown target-like early blight spots Fungal disease Less likely

    Early blight

    Yellowing around brown ringed spots on tomato and potato.
  7. A soil pH test kit beside a garden bed Soil & nutrients Less likely

    Wrong soil pH

    Deficiency signs that feeding does not fix.