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Post Harvest Diseases:: Vegetables:: Onion

Purple blotch/scald disease: Alternaria porri
Symptom:
  • Leaves - whitish minute dots on leaves with irregular chlorotic areas on tip portion
  • Circular to oblong concentric black velvety rings appear in chlorotic area
  • Lesions develop towards the base of the leaves
  • Leaves die from the tip downwards, break at the point of infection and hang down
  • Bulbs – semi watery rot, shrinkage of the fleshy bulb scales and desiccation followed by these scales becoming dry and papery
 
  Whitish small dots Circular concentric black ring Purple blotch

Identification of pathogen:

  • Mycelium - branched, coloured and septate
  • Conidiophores - singly or in groups, septate, pale to mid brown
  • Conidia - solitary, straight or curved

Mode of spread and survival:

  • Carried through seed bulbs collected from infected field
  • Spreads mainly through air borne spores
  • Pathogen enters through stomata or wounds
  • Survives in plant debris for 8 months

Epidemiology:

  • Temp - 21 to 30oc
  • RH - 90 %
Management:
  • Seed treatment - thiram @ 2.5g/kg of seed
  • Three sprays  of Copper oxychloride - 0.25%, Chlorothalanil - 0.2% and Zineb - 0.2%
  • Mancozeb - 0.2%
  • Varieties - New Selection, Red Creole - resistant
Source of Images:
http://www.apsnet.org/publications/imageresources/Pages/fi00190.aspx
http://www.ipm.ucdavis.edu/PMG/A/D-OG-APOR-FO.001.html


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