Improving identification of a fungal pathogen of citrus to prevent it from arriving in Europe
The Mycology Unit of ANSES's Plant Health Laboratory has developed a PCR test to distinguish the fungus responsible for citrus black spot from that of a recently discovered related species. This test will improve the ability to detect this pathogenic fungus, thus preventing it from being introduced into Europe.
Citrus black spot disease affects most plants in the Citrus genus, especially lemons and oranges. It is caused by the parasitic fungus Phyllosticta citricarpa. While the presence of the fungus does not affect the taste of infected fruit, the black spots that appear on them make them unfit for marketing. In addition, the disease reduces the productivity of fruit trees by also affecting the leaves and by causing premature fruit drop. This disease therefore has a major economic impact.
Today, the fungus is present on every citrus-growing continent except Europe. To prevent its introduction, the European Union has classified it as a quarantine pest: imported citrus fruit must be free of the fungus. The risk of introduction is heightened by the fact that contaminated lemons were discovered in Tunisia in 2019. "This was the first time that the fungus had been detected in orchards in North Africa, showing that it can adapt to the Mediterranean climate," explains Renaud Ioos, head of the Mycology Unit at ANSES's Plant Health Laboratory. "It could therefore spread among citrus crops in southern Europe if it were introduced."
A new species of pathogenic fungus discovered
In 2017, a species related to Phyllosticta citricarpa was discovered by an international scientific team. It has been named Phyllosticta paracitricarpa. "These two species are very closely related and none of the available diagnostic tests were able to distinguish between them," explains the scientist. "Only Phyllosticta citricarpa is currently considered to be a quarantine pest. Work is now under way to determine whether Phyllosticta paracitricarpa can also cause citrus black spot. However, as long as it has not been officially declared a quarantine pest, it is not legally possible to prohibit plants carrying this fungus from being brought into the country. It is therefore important to be able to differentiate between the two species."
The Mycology Unit, which is the national and European reference laboratory for plant pathogenic fungi, has therefore developed a new test capable of distinguishing between the two species. To do this, it joined forces with a team from Zhejiang University in China, which possessed strains of Phyllosticta paracitricarpa. "Finding parts of the genome specific to just one of the two species was complicated and time-consuming," continues the unit head. "We had to sequence the entire genomes to compare them." The scientists succeeded in finding genetic differences, confirming that these were two distinct species and enabling the scientists to develop a PCR test to distinguish between them. This is in the process of becoming an official detection method in France, and its use will be encouraged in other European countries.