The Asian citrus canker in the Indian Ocean

Fruits early loss, alteration of their external quality and defoliation are symptoms caused by Asian citrus canker. Caused by bacteria of the genus Xanthomonas^[1], the disease can cause up to 50% yield loss on the most sensitive species in the tropics. Due to the severity of the symptoms it induces, it is classed as a quarantine pest in several countries including the USA, Australia & EU with a ban on exports of fresh fruits for producing countries without an effective eradication strategy.

While it does not cause the death of plants it infects, citrus canker can cause major damage. It affects all aerial parts of susceptible hosts in particular the growing organs: leaves, branches and fruits. Lesions appear on fruits and leaves as spots which then turn into small slightly raised pustules. The disease is characterized by the presence of a yellow halo surrounding the lesions. On fruit, the lesions do not penetrate the epidermis more than 1 to 3mm, and internal quality is not affected. A visual diagnosis is not enough because of possible confusion of symptoms with other diseases of citrus in the region (scab, sigatoka, black spot).

Research shows that within pathogenic strains, the range of infected citrus species and varieties varies as well as the severity of the disease. Further characterization of the bacteria can help in choosing citrus varieties. In order to better characterize these bacteria, the scientific community has agreed to set an infra specific subdivision called the pathotype. This term refers to strains within the Citrus genus that each have a range of different natural hosts (these strains include some related genera in the Rutaceae family). Four genetic lineages of citrus canker have been identified worldwide. Among them, a new group of strains has recently been identified in several countries of the Indian peninsula, but from an academic perspective and in terms of agricultural impact it is still largely unknown. It is not yet known if it is present in the southwestern region of the Indian Ocean (SWIO), which trades with the Indian subcontinent.

In the SWIO islands, Asian citrus canker was introduced at different times. Its presence was noted in the 1940s in Mauritius and Rodrigues, in the 1960s and Moheli in the 1970s in Reunion. In Seychelles the disease is present, but had not been the subject of a thorough characterization. It was introduced into Mayotte less than ten years ago. Epidemiological measures were launched under the ePRPV project (Enlargement and sustainability of the Plant Protection Network) to characterize the strains present in the SWIO islands to determine if certain inter-island trade could be dangerous. It provides a modern analytical approach to data acquired from 1980-1990.

This study, conducted by bacteriologist Olivier Pruvost and his team, has confirmed the presence of the disease in all territories of the region including some islands where the status of the disease was unknown (Anjouan and Grande Comore). In the region, only Madagascar has not yet officially observed the disease. Genotyping analysis, currently in the final stages of implementation, enabled strains from different countries in the region to be compared genetically (Reunion, Seychelles, Comoros, Mauritius, Maldives). They have demonstrated a considerable genetic homogeneity and a very predominant presence of a single line of the bacteria on all islands. Unfortunately, this group has the highest potential for invasion. The strains found in Mayotte exhibit a very high genetic similarity with certain Anjouan strains, suggesting an epidemiological relationship between the two. Another group, pathogenic to acid citrus fruits specifically (key lime, Tahiti lime and kaffir lime) has been sporadically identified in a bacterial collection originally from Mauritius. A collection in Mauritius in 2015 is being analyzed to see if this group of strains is also present there. This project also gave rise to training through the research of two students, one Comorian and one from Reunion Island (Master BEST, University of Reunion Island) and a Mauritian engineer from FAREI.

To limit the economic impact caused by this disease, control methods mainly based on prophylaxis have been implemented. They consist in providing farmers with healthy less sensitives plant varieties when establishing orchards. Lemon, grapefruit, mandarin, tangor and certain orange varieties are not very much susceptible to the disease. The satsuma mandarin and kumquat are partially resistant. Moreover, as it is an airborne disease, its spread depends on human movement, orchard and nursery maintenance and rain combined with wind. One way to limit its spread is to use a localized irrigation and establish windbreaks in plots. A preventive application of copper salts may also be used. However, strains present in Reunion are beginning to show resistances to copper. A new thesis financed by ANSES and CIRAD will start at the end of 2015 to better understand what determines resistance to copper.

^[1]  Xanthomonas citri pv. citri

En savoir plus : 

• Olivier Pruvost publications : http://publications.cirad.fr
• Description of Asian citrus canker:  : www.bio-agri.org
• Control methods for Asian citrus canker:  : www.bio-agri.org
• Research carried out by the PVBMT mixed research unit :  http://umr-pvbmt.cirad.fr
• Publication on the genetic lineages of the bacteria causing Asian citrus canker :  http://journals.plos.org