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Causal Agent: 

Fungus (Alternaria brassicicola)

Characteristic Symptoms:
 

All aboveground parts of the plants are affected.
   
Lesions on leaves start as small, dark spots that gradually enlarge and turn brown to gray with concentric zones resembling that of a target.  Lesions may have purplish or blackish border, chlorotic halo is evident if surrounding area of the leaf is still green. 
   
Older plants are more susceptible that the younger ones.
   
Conditions for Disease Development
 
Wet weather conditions, relative humidity above 95% and temperature range of 28-31°C are necessary for spore germination and infection.  
   
The pathogen can survive on infected plant debris and on the soil and are spread by wind, rain splashes and runoff water.
   
It can survive in the soil.
   
The bacteria can spread by insects and by cultural practices such as irrigation and farm equipments/machinery.
   

Infection is through damaged areas, often caused by fertilizer burn or hail injury in the field, but can be associated with the damage during harvesting.
   
Management and Control:
 
Maintain good field sanitation, remove infected plants/plant parts and dispose properly.
   
Crop rotation with non-host crop for at least 3 years to reduce the amount of primary inoculum. 
   
Remove volunteer and weedy crucifers that may serve as alternate hosts for the pathogen.
   
Spray with copper-based fungicides (e.g. Cupravit®, Vitigran Blue®, Nordox 50®), Mancozeb (Dithane M-45®) or Chlorothalonil (Daconil 2787®) at the initial onset of the disease.
   

References

Compendium of Brassica Diseases by APS (2007)

To view other diseases,  click here.

Need more help?  Ask the Doctor.