Polyphasic Characterization of Xanthomonas axonopodis pv. allii Associated with Outbreaks of Bacterial Blight on Three Allium Species in the Mascarene Archipelago

Abstract

Based on the number of new reports during the last two decades, bacterial blight of onion (Allium cepa) is considered an emerging disease. The causal agent, Xanthomonas axonopodis pv. allii, is pathogenic to several Allium species after inoculation, but outbreaks worldwide have been primarily reported on onion. We describe a unique epidemiological situation in Réunion Island, France, with concomitant outbreaks on three Allium species, onion, leek (A. porrum), and garlic (A. sativum). There was no host specialization within Allium spp. among strains associated with the three host species. Based on amplified fragment length polymorphism (AFLP) and restriction fragment length polymorphism, strains associated with these outbreaks in Réunion Island were highly related genetically to strains isolated from diseased plant samples and contaminated seed lots in the neighboring island of Mauritius, where the disease has occurred since 1984. All AFLP haplotypes were identified as X. axonopodis pv. allii based on polymerase chain reaction analysis using specific primers, biochemical tests, and/or pathogenicity tests. Two genetically related groups of strains (A and B) that can be distinguished by AFLP, differential utilization of three carbon sources, and xanthomonadin pigment production were detected initially after establishment of the pathogen. In less than 10 years after the establishment of the pathogen there was nearly an extinction of group A strains in Réunion Island, suggesting differences in fitness between strains in the two groups.

From Local Surveys to Global Surveillance: Three High-Throughput Genotyping Methods for Epidemiological Monitoring of Xanthomonas citri pv. citri Pathotypes

“…Fragments with fluorescence above a threshold set to 500 relative fluorescence units were scored. This threshold was found to be suitable for minimizing scoring discrepancies among DNA replicates in earlier studies (2,53). Only fragments detected for both DNA replicates were scored as positive in the data matrix.…”

Multiplex Nested PCR for Detection ofXanthomonas axonopodispv. allii from Onion Seeds

“…allii (41). First identified in Hawaii (4), this pathogen emerged in the 1990s and 2000s in several countries, including Brazil (25), Venezuela (55), the Caribbean Islands (29), different states of the continental United States (16,28,47,50), Japan (17), the Republic of South Africa (52), and the Mascarene Archipelago (Mauritius and Réunion Islands) (33). Consequently, X. axonopodis pv.…”

“…Yield losses ranging from 20 to 50% have been recorded under conditions conducive to efficient development of disease (28,50 (41), as well as at least two Citrus species (11). Worldwide, most outbreaks have been reported on onion, but outbreaks have also affected leek and garlic in the Mascarene Archipelago (33) and Welsh onion in Japan (17). Compared to some Xanthomonas pathovars, X. axonopodis pv.…”

“…Compared to some Xanthomonas pathovars, X. axonopodis pv. allii is phenotypically and genetically diverse (11,12,33,41). Onion seeds originating from diseased fields were identified as a possible pathway for X. axonopodis pv.…”

“…Inoculum associated with seeds contaminated at a rate of 4/10,000 has been used as the primary inoculum in field epidemiological studies (43). Recently, onion seeds were identified as the most probable pathway for the introduction of the pathogen into Réunion Island from the neighboring country Mauritius (33). Currently, the pathogen is isolated from onion seeds or from plant material using semiselective media (42,51) and a subsequent identification step consisting of pathogenicity tests and/or molecular typing techniques (12,33).…”