Seasonal development
and life cycle—Big vein
Big vein is caused by an unidentified viruslike agent. This agent is soilborne, and is introduced into
lettuce plants by a soil fungus (Olpidium brassicae) that attaches itself to lettuce roots. Even
though the agent is soilborne and therefore always present in infested fields, disease severity may vary
greatly from season to season. Big vein is more prevalent and severe during cool weather.
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Plant
on the left is infected with big vein
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