Source: The Sun [abridged, edited]
The annual purple blotch fungal disease affecting onions in the country has created a demand deficit of 1.1 million metric tonnes, Permanent Secretary Ernest Umakhihe stated at a workshop on management of Onion Purple Blotch. [He] explained that Nigeria contributes little to the export market largely due to [inferior] produce quality resulting from poor management of pests and diseases, and use of pesticides causing illnesses.
"The national demand gap [is] to be bridged by importation of onion and other products twice the country's annual production; exerting enormous pressure on foreign exchange earnings. This workshop is aimed at imparting the required knowledge and develop skills of our technical officers on various strategies and management practices for the control of onion purple blotch and other diseases with emphasis on the use of environmental friendly technology," he said.
[Byline: Olanrewaju Lawal]
[Purple blotch of onions (_Allium cepa_; some varieties known as shallots) and related crops, such as leek (_A. porrum_) and garlic (_A. sativum_), is caused by the fungus _Alternaria porri_. Crop losses depend on environmental conditions and may range from 25 to 90 percent. The damage may continue to develop post-harvest. Symptoms on onion include small, water-soaked lesions with purple centres on leaves, leaf bases and flower stalks, which expand and result in necrotic patches. Symptoms may be confused with stemphylium blight of onion (_Stemphylium vesicarium_; see previous ProMED-mail posts in the archives). On leek, _A. porri_ leads to white or purple spots appearing on leaves; affected leaves crack and eventually die.
The disease is favoured by high humidity and often occurs as a secondary infection on plants already affected by other pathogens or pests (for example, onion thrips). The pathogen is mostly spread with infected plant material and by mechanical means (including insects and human activities). Disease management may include fungicides, clean planting material, phytosanitation to minimise inoculum and crop rotation.
Purple blotch symptoms on onions:
Purple blotch photo gallery:
_A. porri_ asexual spores, microscopy:
Information on purple blotch of onion:
http://www.pestnet.org/fact_sheets/onion_purple_blotch_151.htm (with pictures),
https://www.greenlife.co.ke/purple-blotch-of-onion/ and via
_A. porri_ taxonomy and synonyms: