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Alternaria leaf spot and head rot of Brassica crops

Quick facts

  • Alternaria is one of the most common diseases of broccoli, kale, and other Brassica crops.
  • It is sometimes referred to as black spot.
  • It affects leaves, and often results in head rot. It can be severely yield limiting. 
  • While this disease is not new, it has become worse in recent years, particularly on broccoli, and to some extent, cauliflower.

How to tell Alternaria from other brassica problems

Closeup photo of cabbage leaves. There are round brown spots with concentric rings, surrounded by yellow halos.
Characteristic Alternaria leaf lesions
  • Leaf symptoms include round, brown spots with concentric rings.
  • Spots often have a yellow halo, and can crack through the middle.
  • Spots often occur first on older leaves.
  • As the disease spreads, leaves can develop enough spots that they begin to meld together to create large necrotic areas on leaves.
  • Head rot symptoms can first appear as small brown spots on an otherwise healthy head.
  • Heads with rot symptoms deteriorate quickly.
A brown lesion with concentric rings inside. There is a crack through the center of the lesion, and it is bordered by a yellow halo.
Alternaria lesion
A broccoli head with multiple brown spots on the head, all bordered by yellow beads.
Broccoli with Alternaria

Biology

A leaf with hundreds of Alternaria lesions. In some places there are so many that they blend together and the leaf is brown and necrotic.
Severe Alternaria leaf symptoms
  • Alternaria is potentially caused by three pathogens: Alternaria brassicicola, A. brassicae, and A. raphani. Based on initial screenings, the primary species causing disease in Minnesota broccoli is Alternaria brassicicola.
  • Alternaria is a fungus. It produces spores, which can be spread via wind, water and equipment.
  • This fungus can infect any plant in the Brassica family, but tends to be most damaging in broccoli and cauliflower.
  • Alternaria can enter farms through infected seeds.
  • Once present, Alternaria can persist in residues. In some cases, Alternaria fungi will develop resting spores that allow them to survive in the soil.
  • This disease spreads in warm temperatures (60-78 degrees F), and during periods of high humidity (90% RH).

Managing Alternaria in the home garden

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Managing Alternaria on farms

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Author: Natalie Hoidal, Extension educator, local foods and vegetable crops

Reviewed by Marissa Schuh, horticulture IPM Extension educator

Reviewed in 2022

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