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Symptom

Brown or black spots

Spots with clear edges usually mean a pathogen. Size, colour, rings and halos narrow it down, and so does the plant: tomatoes, potatoes and brassicas each have their usual suspects.

Use a hand lens: rings, halos, black dots in the centre or powdery pustules each point to a different disease. Note the weather too; most spots follow wet spells.

Likely causes

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

Recorded on Broccoli

  1. Beet leaves with round, red-rimmed leaf spots Fungal disease Common

    Fungal leaf spots

    Round spots, often with a red or dark rim, on beet, chard and brassicas.
  2. Basil leaves yellowing and blotched with downy mildew Fungal disease Less likely

    Downy mildew

    Angular yellow-brown patches, fuzz underneath.
  3. Blackened, frost-damaged shoot tips on a young plant Light & temperature Less likely

    Cold and frost damage

    Dark patches after frost or chilling.

Less likely on Broccoli

  1. A tomato leaf with brown target-like early blight spots Fungal disease Common

    Early blight

    Brown target-ringed spots on older tomato and potato leaves.
  2. A tomato leaf with many small grey-centred Septoria spots Fungal disease Common

    Septoria leaf spot

    Many small grey-centred spots with black dots, low on tomatoes.
  3. Potato leaves with dark water-soaked late blight patches Fungal disease Possible

    Late blight

    Large, water-soaked brown patches spreading fast.
  4. A pepper leaf with small, dark bacterial spots Bacterial disease Possible

    Bacterial leaf spot

    Tiny, dark, angular spots, sometimes with halos.
  5. Allium leaves streaked with orange rust Fungal disease Possible

    Rust

    Raised orange or brown powdery pustules.
  6. A fruit with a sunken, dark anthracnose lesion Fungal disease Less likely

    Anthracnose

    Sunken dark spots, pink spores in wet weather.
  7. Squash bugs and bronze eggs under a pumpkin leaf Pest Less likely

    Squash bugs

    Yellow specks turning brown on squash leaves.