Source: CIRAD

The first symptoms of bacterial blight in anthuriums - family Araceae - are oily-looking leaf spots, which give way to yellowing and necrosis. The infection can rapidly become systemic and kill the plant. The infectious disease, caused by the bacterium Xanthomonas axonopodis pv. dieffenbachiae, almost entirely wiped out anthurium production in the West Indies in the 1980s.

The bacterium was accidentally introduced into Réunion in 1997 through contaminated plants from the Netherlands. An eradication campaign was launched, with the destruction of all the plants in affected nurseries and a ban on sales. It was made compulsory to import anthuriums in the form of in vitro plantlets, with an 18-month quarantine period to acclimatize them to the conditions in Réunion.
CIRAD launched a research programme in conjunction with the Réunion Plant Protection Service and players in the anthurium supply chain. The aim was to find more effective ways of inspecting imported plants. A reliable molecular tool is now available to detect the bacterium.

The tool is both specific and sensitive

The tool was developed in two stages. Firstly, the aim was to build a collection that was representative of the global genetic and pathogenic diversity of the bacterium. To this end, researchers collected bacterial cultures from all the zones affected by the disease. The results showed that the bacteria that affect Araceae make up a genetically heterogeneous group, not all of which affect anthuriums. The diversity was characterized using two techniques: AFLP (Amplified Fragment Length Polymorphism), which serves to compare individuals two by two for a large number of characters in the genome, and tests measuring pathogenicity on various plants from the family Araceae.

Researchers subsequently worked to develop a reliable, universal detection tool capable of detecting all the bacterial strains that may cause the disease, regardless of their geographical origin. The results were conclusive: in addition to the initial conditions, the tool proved to be specific - it does not detect non-pathogenic strains - and sensitive - it detects strains even if the plants are only slightly infected, with no visible symptoms.

Improved checks on imported plants and a 50% reduction in the quarantine period

Detection is based on a gene amplification technique (PCR): it is one of the genes of the bacterium that is detected. To this end, it was thus first necessary to identify a large number of potential target genes with the genome of the bacterium by determining which were present in the bacterium in question but not in others. Lastly, the research meant using the DNA sequence to check, a posteriori, that the target gene was indeed unique and corresponded to one of the bacterium's vital functions. This guaranteed that it would be found in almost every strain of the bacterium.

The tool has a wide range of applications. It is now possible to diagnose infection quickly. The tool can also be used on a larger scale, to monitor nurseries and check imported plants as they enter the country. Moreover, the quarantine period imposed on importers has been halved. The tool can also be applied under certification schemes aimed at producing disease-free plant material. Patents have been taken out in France and the Netherlands. Lastly, there are plans to apply to have the tool approved by the European Plant Protection Organization (EPPO) as an official diagnostic method.