Xanthomonas axonopodis pv. allii is a new bacterium on Allium crops: addition to the EPPO Alert List
In the last 20 years, severe outbreaks of bacterial blight have been observed on Allium crops in several parts of the world. The disease was first found in Barbados in 1971, and then in 1975 on several islands of the Hawaiian archipelago. In the 1980s it was found in Brazil, Cuba and Mauritius. From 1990 to 2000, it reached USA, Venezuela, South Africa and Japan. Recent work done in France and Florida (US) showed that, in these different parts of the world, the disease was caused by a new bacterium for which the name Xanthomonas axonopodis pv. allii has been proposed. Studies on its host range showed that, so far, only Allium crops were susceptible. It was also demonstrated that the bacterium was transmitted by Allium seeds, and that the use of contaminated seeds (even with low infection level) could lead to field outbreaks.
Xanthomonas axonopodis pv. allii (an emerging disease of onion and garlic crops)
Why: A new bacterium, Xanthomonas axonopodis pv. allii, causing damage to Allium crops has been reported from several parts of the world as an emerging disease.
Where: The disease was first observed in Barbados in 1971, and then spread to other continents (America, Africa and Asia).
EPPO region: absent.
Africa: Mauritius, Réunion, South Africa.
North America: USA (California, Colorado, Georgia, Hawaii, Texas)
Central America and Caribbean: Barbados, Cuba.
South America: Brazil, Venezuela.
On which plants: Allium species (A. cepa (onion), A. sativum (garlic), A. porrum (leek), A. schoenoprasum (chives), A. ascallonicum (shallot), A. fistulosum (Welsh onion)). The disease tends to be more severe on onions.
Damage: 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 high temperatures (higher than 27 °C) and severe outbreaks usually occur shortly (7 to 10 days) after a period of humid, rainy weather. In USA, yield reductions of 20 % or greater are commonly observed in affected fields.
Dissemination: X. axonopodis pv. allii is seed-transmitted. Within crops, wind and irrigation (in particular overhead irrigation) can ensure further spread of the disease. Dissemination is favoured by storms with hail. The bacterium can also survive on crop debris and volunteer Allium plants. It may be also disseminated by infected debris adhering to workers and equipment.
Pathway: Allium seeds, bulbs for planting from countries where X. axonopodis pv. allii occurs.
Possible risks: Allium crops are widely grown in the EPPO region. Severe outbreaks have been reported with crop losses from countries where the bacterium occurs. Control measures are available (use of healthy seeds and bulbs, destruction of volunteer onions, destruction of plant debris, rotations, chemical control) but should be applied in combination. For the moment, no routine diagnostic test is available for testing Allium seeds. Although more data is needed on the amount of imports of Allium seeds (or bulbs) from infected countries and on the potential of establishment of this disease in Europe, the introduction of infected seeds (or bulbs) presents a risk to Allium crops, particularly in the south of Europe.
EPPO RS 2005/063
Panel review date - Entry date 2005-04
Humeau L, Roumagnac P, Soustrade I, Gagnevin L, Degas J, Jeuffrault E, Pruvost O (2004) Une maladie émergente de l’oignon à la Réunion. Le dépérissement bactérien causé par Xanthomonas axonopodis pv. allii. Phytoma – La Défense des Végétaux, no. 573, 28-30.
Roumagnac P, Pruvost O, Chiroleu F, Hugues G (2004) Spatial and temporal analyses of bacterial blight of onion caused by Xanthomonas axonopodis pv. allii. Phytopathology, 94(2), 138-146.
Colorado State University. Cooperative Extension. Xanthomonas leaf blight of onion by H. Schwartz and D. H. Gent. http://www.ext.colostate.edu/pubs/garden/02951.html