Plant Diseases – White or Tan Spots on Brassica Leaves

pak-choi-white-leaf-spot

White leaf spot is a fungal disease of cruciferous vegetables (brassicas) caused by the pathogen Mycosphaerella capsellae, which is also known as Pseudocercosporella capsellae. The distribution of this disease pathogen is worldwide, it can be found in many countries with temperate climates, where brassicas are grown.

The image above shows a pak choi (Chinese cabbage) plant displaying the symptoms of white leaf spot disease.

Detailed below are the characteristics of this plant disease and instructions on how to manage it.

 

Plants Affected by White Leaf Spot Disease

The white leaf spot fungus affects crops from the Brassicaceae (brassica, crucifer, mustard, or cabbage) family.

Certain brassica varieties are more susceptible to this disease, these include Chinese cabbage (bok choi, pak choi). mustards, turnip, rutabaga and canola (oilseed rape).

Less susceptible brassica varieties include cabbage, cauliflower, broccoli, collards and kale.

Horseradish and radish are also susceptible to this disease.

The disease can overwinter in volunteer crop seedlings (self-seeded susceptible crop plants) and susceptible weedy brassicas such as wild radish (Raphanus raphanistrum), wild mustard (Sinapis arvensis), and shepherd’s purse (Capsella bursa-pastoris).

 

Symptoms

The white leaf spot fungus produces tan (light brown) to white spots, approximately 10 mm in diameter, up to 20 mm, which are first observed on older leaves, usually in autumn and early winter when conditions are wet. As the season progresses, the spots on the leaves darken to become grey or black. This disease can attack the leaves, stems and pods of susceptible plants.

 

Favourable Factors

Disease emergence is favoured by wet leaves and cool temperatures. Temperatures of 10°C – 15.5°C (50°F -60°F) with moist conditions created by rain, dew or irrigation promote disease development.

 

White Leaf Spot Disease Cycle

The white leaf spot fungus survives on plant remains and seeds of infected plants, and its spores are spread by wind and water splash.

Primary infection spread – When prolonged wet weather conditions occur over autumn or winter (due to rain, dew or irrigation), and temperatures are between 10°C – 15.5°C (50°F -60°F), the lesions on the leaves produce sexual spores which are wind-borne and can spread over long distances.

Secondary infection spread – Later in the season, water splash caused by rain or overhead irrigation spreads asexual spores which will cause further disease in the next season.

The fungus can also survive by overwintering in self-seeded crops or brassica weeds such as wild mustard, wild radish, and shepherd’s purse, which can act as a source of infection to infect newly-planted crops.

 

Management of White Leaf Spot Disease

Cultural controls are integrated pest management (IPM) practices which disrupt the environment of the disease, reducing establishment, reproduction, dispersal, and survival. These cultural practices are the most effective methods currently available for control of white leaf spot in organic farming systems.

 

Cultural controls

  • Plant hygiene – remove any diseased plants immediately to prevent disease spread and build-up in the planting location.
  • Crop rotation –  rotate out of brassica crops for a minimum of 3 years.
  • Weed control – eliminate host plants that can overwinter disease by removing volunteer crop seedlings and susceptible weeds such as wild mustards, shepherd’s purse, pepper-grasses and wild radish.
  • Incorporation of crop residues – after harvest of crops, immediately remove any plant debris or bury it into the soil to allow it time to rot.
  • Seeds – use clean high-quality seed, use hot water treatment of seeds before planting by soaking in water at temperature of 50°C (125°F) for 20 minutes.
  • Isolation– avoid planting next to areas infected in the previous season to prevent infection by wind-borne spores.
  • Watering – avoid overhead sprinkler irrigation if possible as this causes water-splash spread which makes the disease worse.
  • Minimise cross-contamination – only work in garden beds when leaves are dry to prevent spread of disease through contact with hands, tools or clothing.
  • Plant health – provide adequate nutrition to reduce crop stress.
  • Row spacing and orientation – reduce potential for disease by increasing row spacing or plant spacing within rows, and orienting rows into the predominant wind direction.

 

References

  1. Government of Western Australia, Department of Primary Industries and Regional Development, Agriculture and Food division –
    Diseases of vegetable brassicas, 1 October 2018
  2. Oregon State University, Pacific Northwest Extension Publication, PNW Plant Disease Management Handbook – Cabbage and Cauliflower (Brassica sp.)-White Leaf Spot and Gray Stem
  3. Victoria State Government, Agriculture Victoria – Asian vegetables, Note number: AG0633, Published: June 2000, Updated: March 2011
  4. Penn State University, PlantVillage project – Chinese cabbage
  5. Crop Pro, Identification and Management of Field Crop Diseases in Victoria – Canola
  6. Oregon State University – White Leaf Spot and Gray Stem in Crucifer Seed Crops in Western Oregon,2014, Cynthia Ocamb
  7. University of Arkansas, Division of Agriculture Research and Extension, Agriculture and Natural Resources – FSA7549, Diseases of Turnip and  Mustard Greens, Sherrie Smith

About Angelo (admin)

Angelo Eliades is a presenter, trainer, writer, permaculture consultant, urban permaculture pioneer and food forest specialist.
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