Can genetic resources save the common plane tree?

The plane tree (1), Platanus × acerifolia, is a hybrid species belonging to the Platenaceae family, originating from a cross between the American plane P. occidendalis and the oriental plane P. orientalis (from Asia and Europe). It is widely used in Europe as an ornamental tree along roads and waterways – contributing to the beauty and spirit of villages, cities and landscapes in South of France; this is particularly well illustrated with the Canal du Midi (opened in 1681; classified world heritage of the humanity of the UNESCO since 1996). With more than 42 000 plane trees just along the Canal du Midi, the plane tree is of particular historical and cultural importance in South of France. However, this species is currently threatened by a fungus, as all the treatments tested against this pathogen were revealed unsuccessful.

Les platanes abattus le long du Canal du Midi — Figure 1 : Many plane trees have already been cut down along the Canal du Midi (Midi Libre)

Progression du chancre coloré du platane dans le Sud de la France — Figure 2 : Extent of the canker strain disease in South of France, since it was introduced in France in 1945, with dates of the first recorded cases of plane trees infected by *Ceratocystis platani* (d’après Fredonra.com)

The canker strain is an incurable disease, specific of the plane tree, which has already caused the death of 50 000 plane trees in France, during the past 50 years. Ceratocystis platani is the Ascomycete responsible for this disease. The only way to limit the spread of the disease is to cut down any contaminated tree (Figure 1). The fungus can enter trees through the slightest wound in the air system and/or the roots (mainly because of root anastomose: physical and functional merger of roots with the nearby plane trees). The disease originated from North America, and was introduced in France at the end of the World War II during the American landing in Marseille, with ammunition boxes made with contaminated wood. The epidemic was reported in 1945, and identified 25 years later only. It is currently diagnosed in the South of France (to the North of Lyon – Figure 2), in Corsica, Switzerland, Italy, and also in Greece (2) and Armenia. The choice to plant clone varieties only has widely favored the epidemic, limiting the genetic diversity within the population, which would have favored the emergence of resistant genotypes.

La disparition du feuillage, l’écorce grise et craquelée, ou les taches violettes le long du tronc sont des indicateurs d’une attaque possible par le champignon — Figure 3 :No leaves, grey and crackled bark, or purple trails are good indicators of any putative attack by *Ceratocystis platani*, but only necrosis represent good symptoms of infection which can be seen only after the tree being cut down (photos FREDON Rhône-Alpes)

The maintenance of plane trees was relatively simple in the past; nowadays, in order to limit the epidemic, road agents have to disinfect each cutting instrument before and after each pruning, and then treat all carved branches with antifungal solutions. In addition, continuous monitoring of the trees must be provided to locate any new sources of infection, in order to react quickly and destroy them as soon as possible. But unfortunately, it seems that this devastating epidemic would not be stopped: the historic landscapes of Southern France are then condemned to major changes, comparable to the epidemic that ravaged elm trees (Ulmus) throughout the Northern Hemisphere since 1925.

This is why the French National Institute for Agricultural Research (INRA) has conducted a research programme in order to exploit genetic resources useful to introduce natural genetic resistances present in the American plane tree P. occidentalis (in collaboration with a tree breeder). In 2003, they made crosses between resistant P. occidentalis genotypes and P. orientalis which is particularly well adapted to the French climate (3). They obtained a resistant hybrid genotype which can be now used as a new variety to be planted in South of France: Platanor® Vallis Clausa (co-obtaining INRA / Nurseries ROUY Imbert). This new common plane tree should replace all the plane trees which have been cut down. Interestingly, the hybrids were demonstrated as resistant to other major diseases (anthracnose, powdery mildew) and insects (like the tiger of the plane tree from the Tingidae family). However, the growth of these newly planted trees will take time, and the Meridional landscape, that we are so proud of and that pleases so much our English neighbours (Peter Mayle, A Year in Provence) will be disrupted for several decades, until the hybrids look like their predecessors.

The plane tree facing the canker stain is a very good example to illustrate how it can be useful to exploit natural genetic resources. This also reminds us the necessity to preserve and conserve genetic diversity (and biodiversity in general), which represents a fantastic source of solutions to problems which are still to be guessed…

References

(1) Vigouroux A. 2007. Le Platane – Portrait, botanique, maladies. Editions Edisud (ISBN 978-2-7449-0704-3).127 p.
(2) Ocasio-Morales R.G., Tsopelas P., Harrington T.C. 2007. Origin of Ceratocystis platani on Native Platanus orientalis in Greece and Its Impact on Natural Forests. Plant Disease 91: 901-904. DOI: 10.1094/PDIS-91-7-0901
(3) Vigouroux A. & Olivier R. 2004. First hybrid plane trees to show resistance against canker stain (Ceratocystis fimbriata f. sp. platani). Forest Pathology 34: 307-319. DOI: 10.1111/j.1439-0329.2004.00372.x