GrowingHQ
Leaves

Spreading brown or black patches on leaves and stems

Dark, water-soaked-looking lesions that enlarge over days, often with a pale halo, sometimes with a white fuzz on the leaf underside in damp weather. On potatoes and tomatoes in a wet summer this is the sign that matters most.

When this is urgent

The patches are spreading noticeably day to day in warm humid weather on potatoes or outdoor tomatoes. Late blight can destroy a crop in under a fortnight, and the window in which cutting the potato haulm still saves the tubers is measured in days.

Described view

Strawberry foliage and fruit in one frame: a berry and adjoining leaf tissue carrying the dense grey-brown fur of botrytis with the tissue beneath collapsed, and separately a leaf with dry, sharply-edged brown scorch along the margin and between the veins with no fungal growth on it at all

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. Late blightvery common
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato and tomato blight

    Checked 2026-08-13

    How to tell

    Speed and weather. Blight lesions expand within days during warm humid spells, spread from plant to plant across a row, and affect stems as well as leaves. A spot that has looked the same for two weeks is something else.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato blight — symptoms

    Checked 2026-08-13

    If it is this, do first

    On potatoes, cut and remove all top growth immediately and leave the tubers in the ground for about two weeks before lifting.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato blight — managing an outbreak

    Checked 2026-08-13

  2. Magnesium deficiencyoccasional
    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Magnesium deficiency

    Checked 2026-08-13

    How to tell

    Severe magnesium shortage can brown the tissue between the veins after prolonged yellowing, but it works from the oldest leaves upward, does not spread between plants, and never affects stems.

    source

    Sourced guidancehigh confidence

    • University of Minnesota Extension — University of Minnesota: Nutrient deficiencies in vegetable crops

    Checked 2026-08-13

    If it is this, do first

    Look at the leaf immediately above the affected one. A gradient from bad at the bottom to healthy at the top points to nutrition, not infection.

    source

    Horticultural consensusmoderate confidence

    • Royal Horticultural Society: Diagnosing nutrient deficiencies

    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.

Fruit

Houseplants

Commonly reported on