Stemphylium blight: A leaf miner scar
Pathogen: Stemphylium vesicarium
Symptoms
Since this fungus is usually found co-infecting with Alternaria porri, symptoms are identical or at least very similar to purple blotch. However, Stemphylium leaf blight lesions appear to be a darker, more olive-brown to black color than do purple blotch lesions. Lesions are often more numerous on the sides of onion leaves facing the prevailing wind.
Disease development
Extended periods of leaf wetness from dew formation, rainfall or overhead irrigation during bulb formation and development can result in severe leaf blighting. Bulb size can be greatly reduced due to loss of foliage. Infection is usually limited to leaves and does not extend down to the scales of the bulb.
Disease management
Chemical control with fungicides is effective in reducing disease development. Long term rotation with unrelated crops may reduce losses. Also, good field drainage and reduced plant density may lessen disease severity.
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