GrowingHQ
Leaves

Lower leaves turning yellow

The oldest leaves — those nearest the base — losing their green while the growing tip stays healthy. Extremely common on tomatoes and peppers, and one of the least specific signs in growing: at least four unrelated things produce it, and one of them is the plant behaving perfectly normally.

Described view

Two tomato leaves photographed flat side by side against a neutral ground: one showing magnesium deficiency, yellow between the veins with the vein tissue itself staying distinctly green in a herringbone pattern, and one showing nitrogen deficiency, faded uniformly pale from the whole leaf including the veins

What causes it, and how to tell them apart

Work down the list making the observation under each heading. GrowingHQ will not guess which one you have — the observation is the answer.

  1. Magnesium deficiencycommon
    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Magnesium deficiency

    Checked 2026-08-13

    How to tell

    The yellowing is BETWEEN the veins, and the veins themselves stay distinctly green, giving a marbled look. Hold the leaf up to the light: a clear green vein network on a yellow background is magnesium, an evenly yellow leaf is not.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Diagnosing nutrient deficiencies

    Checked 2026-08-13

    If it is this, do first

    Check the feeding regime before treating. Most cases in containers follow heavy high-potassium feeding or exhausted compost, and correcting that matters more than any spray.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Feeding tomatoes

    Checked 2026-08-13

  2. Aphidsoccasional
    source

    Horticultural consensusmoderate confidence

    • UC Statewide Integrated Pest Management Program — University of California Agriculture and Natural Resources: Aphids

    Checked 2026-08-13

    How to tell

    Turn the leaf over. Aphid damage comes with the insects themselves, sticky honeydew, and often distorted young growth at the tip — yellowing without any of those is not aphids.

    source

    Sourced guidancehigh confidence

    • UC Statewide Integrated Pest Management Program — University of California Agriculture and Natural Resources: Aphids — identification

    Checked 2026-08-13

    If it is this, do first

    Wash the colonies off with a jet of water and check again in a week.

    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Aphids — control

    Checked 2026-08-13

  3. Late blightuncommon
    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Tomato blight

    Checked 2026-08-13

    How to tell

    Blight does not begin as clean yellowing. Look for dark brown-black patches with a paler margin, often starting at a leaf edge, spreading within days, and appearing on stems as well. Yellowing that stays yellow for a fortnight is not blight.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato and tomato blight — symptoms

    Checked 2026-08-13

    If it is this, do first

    If dark patches are spreading, act the same day: on potatoes cut off the top growth, and keep any nearby tomatoes dry and under cover.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato blight — managing an outbreak

    Checked 2026-08-13

Where GrowingHQ holds more for one plant

This page lists the causes across every plant. The plants below are the ones where something about that plant changes the answer — different candidates, or a different observation that separates them.

Vegetables

Fruit

Herbs

Houseplants

Ornamentals

Commonly reported on