Disease of the Month - Alternaria Leaf Spot of Brassicas

  Hort Snacks - May 2017
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 A.K.A. Black Spot

Causal Organism(s): Alternaria brassicae or Alternaria brassicicola

Crops Affected: Cruciferous vegetables, including cabbage, broccoli, cauliflower, Brussels sprouts, Chinese cabbage, kale, kohlrabi, rutabaga, turnip and cruciferous weeds

Disease Cycle:

  • Fungal pathogen, which can be seed, soil or wind-borne or transferred mechanically
    • Contaminated seeds (with surface spores or internally infected) may produce infected seedlings, but not all of the time
      • Seed surface-borne spores may be viable for up to a couple years, with internal infections remaining viable for over a decade
        • This is the most likely way that new infestations are introduced
    • The pathogen may also survive on infected crop debris or on cruciferous weed hosts
      • This is likely the primary method of survival from year to year
    • Spores are produced on infected tissues and may travel via the wind for long distances (over a mile)
    • Spores may also be spread mechanically by equipment, people, animals or some insects (e.g. early flea beetle feeding)
  • Spots may appear first on older leaves, and will then spread to marketable parts of the plant
  • Spores are black and sooty within the spots
  • If conditions are favorable, the disease will develop where a spore lands, whether on leaf or head tissue
  • Yellow, dark brown to black circular leaf spots which exhibit concentric rings (light and dark rings; target-like pattern)
    • Lesions start off yellowish, turning tan coloured prior to sporulation
    • The centres of lesions may drop out
    • Spots may coalesce into larger necrotic areas
    • Infected leaves will often drop off if infections reach 50 percent of the leaf surface infected
  • Spotting of cauliflower curds is very obvious (also observed on broccoli florets)
    • Black, sunken spots develop on the florets, spreading to encompass large parts of the head
    • Spots may be tiny to several centimetres across
  • Fallen leaves (and active lesions) are covered with dark brown/black sooty spores
Alternaria leaf spot lesions on broccoli leaf
Black, sunken Alternaria lesions on cabbage
Black spot (Alternaria) on cauliflower head

Conditions Favoring Disease Development:
  • Wind, water splash (rain or irrigation) or equipment can move the spores within the growing season
  • Prolonged periods of leaf wetness and moderate to warm temperatures (15-25C; 59-77F) favor development
  • High humidity and temperatures over 14C will lead to spore development
  • Check new and old leaves and growth throughout the field, starting at emergence, assessing the percentage of leaves that are infected and the number of lesions per leaf
  • Note any black spotting on cauliflower or broccoli florets on mature plants
  • Some varieties may be more resistant to the pathogen and may be a good choice if this disease is an issue
  • Start with disease-free seed
    • Seed treatments (hot water and fungicide) can reduce internal and surface-borne spores on seeds, with fungicide treatments controlling surface-borne infections
  • Maintain a good rotation to non-cruciferous crops
    • Control cruciferous weeds to minimize introduction to fields and spread within the fields
  • Ensure good air circulation within the plant canopy with appropriate plant spacing
  • Avoid overhead irrigation if the disease has been identified within the field
  • Incorporate crop residues to encourage decomposition and breakdown of tissues
  • Straw mulch may be useful in reducing introduction of the pathogen from soil-borne infection
  • Ensure that product going into storage (e.g. cabbage) is free from disease
    • Store cabbage below 4C (39C) to prevent new infections
    • Ensure that storages are properly cleaned and disinfected prior to storage
  • Protective applications of fungicides may be used prior to disease detection and development
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on April 27, 2017.