Xanthomonas leaf blight of onion

Xanthomonas leaf blight of onion (Xanthomonas axonopodis pv. allii) is a bacterium causing damage to Allium crops has been reported from several parts of the world as an emerging disease. It was first observed in Barbados in 1971, and then spread to other continents (America, Africa and Asia).

Symptoms
Leaf lesions initially appear as white flecks, pale spots, or lenticular lesions with water-soaked margins. Lesions quickly enlarged, becoming brownish, with extensive water-soaking. As the disease progresses, lesions coalesce causing tip dieback and extensive blighting of older leaves. Reduction of foliage leads to stunting of the plants and smaller bulbs. In case of severe outbreaks, premature plant death is observed. The disease is favoured by temperatures higher than 27°C (80°F) and severe outbreaks usually occur 7-10 days after a period of humid, rainy weather. In USA, yield reductions of 20 % or greater are commonly observed in affected fields.

Treatment
Control in the US is most often partially achieved through a massive use of copper bactericides sometimes amended with ethylenebisdithiocarbamate fungicides, but this strategy is economically and environmentally not satisfactory.

Prevention
Crop rotation should be practiced, however, the longevity of the pathogen’s survival in the field in association with crop debris or in the absence of Allium spp. and weeds is not precisely known. Overhead irrigation should be avoided and furrow or drip irrigation should be considered as alternative irrigation modes. No Allium cultivar has been consistently characterized as resistant to ''X. axonopodis pv. allii'' and no source of resistance has been identified to date.