Purple blotch: lesion on the neck region of an onion plant.
Pathogen: Alternaria porri
Symptoms
When bulb infection occurs, it is normally through the neck. If the fungus invades the bulb, lesion on the neck region can be observed.
Disease Development
The fungus over-winters as mycelium in leaf debris and cull piles. Spores are formed during humid nights and leaf wetness periods greater than 12 hours. As the morning dew dries, spores become air-borne and are disseminated to susceptible onion tissue. 1-4 days are needed for symptoms to develop after infection. Disease development is greatest during prolonged periods of leaf wetness.
Disease Management
A fungicide spray program with broad spectrum protective fungicides applied prior to infection can provide good protection. Minimizing leaf wetness by using surface rather than sprinkler irrigation, good field drainage and correct plant spacing can reduce disease development. A rotation out of Allium to unrelated crops for several years can reduce disease as well.
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