🍂 Alternaria Leaf Spot

Alternaria spp.
Dark spots with rings — but if it is tomato or potato, you are probably looking at its famous cousin.

Signs and symptoms

Dark brown to black leaf spots, often with concentric rings or yellow halos, that expand and merge until leaves yellow and drop. In severe cases spots reach stems and pods. Wet foliage and plants weakened by rain and moisture stress are favoured. By host: A. brassicicola and A. brassicae on brassicas, A. cucumerina on cucurbits, and A. alternata on many hosts.

Alternaria leaf spot on Chinese cabbage — small dark brown spots.
Alternaria leaf spot on Chinese cabbage — small dark brown spots. Photo: Jochen Kreiselmaier, Pflanzenschutzberater am Dienstleistungszentrum Ländlicher Raum, Neustadt/Wstr., CC BY-SA 4.0.
Alternaria leaf spot on broccoli — yellowing with ring-shaped brown spots.
Alternaria leaf spot on broccoli — yellowing with ring-shaped brown spots. Photo: Schlaghecken Josef, CC BY-SA 4.0.
Alternaria leaf spot on sweet potato leaf.
Alternaria leaf spot on sweet potato leaf. Photo: Schlaghecken Josef, CC BY-SA 4.0.

Not a duplicate of early blight: early blight is the tomato and potato disease caused by A. solani, with target-ring spots and fruit lesions. This guide covers the brassica, cucurbit, and bean Alternaria leaf spots — see early blight for the tomato and potato version.

Plants it attacks

Cabbage, broccoli, cauliflower, cucumber, squash, melon, beans, and — as secondary spots, cross-referenced rather than duplicated here — tomato.

What to do

  • Remove affected leaves promptly; do not compost diseased material.
  • Improve air flow and spacing, and mulch to reduce soil splash.
  • Avoid overhead watering and keep foliage as dry as practical.
  • Rotate to a non-host crop.
  • A labelled fungicide is only a later option, used per label.

Prevention

Rotate crops, space for air flow, water at the base, use clean seed and transplants, clean up debris at season’s end, and avoid wounding leaves, which opens the door to infection.

Nerdy fact: Alternaria is a master of the encore: the same genus that spots your cabbage also includes the tomato early blight fungus and a species that turns up on a huge range of hosts. The spot pattern looks familiar because the fungi are related — but the host, and therefore the advice, is what tells them apart.

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