Abstract
Key message
Appropriate silvicultural practices combined with the use of resistant Central European provenances can reduce the prevalence of larch canker caused by
Lachnellula willkommii
(R. ...Hartig) Dennis, a major disease affecting larch plantations in France. However, cascading translocations have resulted in frequent admixture in European larch (
Larix decidua
Mill.) and subsequent certification errors regarding the origin of plant material. Our study highlights the urgent need to improve the certification process of seed orchards.
Context
The recent history of European larch has been marked by translocations of plant stock within and beyond its native range. In order to increase stand resistance to larch canker disease, it is strongly recommended by French authorities to plant Central European provenances. However, a recent disease outbreak questioned the resistance of Central European provenances and the origin of the forest material used in these plantations.
Aims
Our goal was to assess the effects of cascading translocations and mixing between larch gene pools on genetic composition of plantations and on their vulnerability to the disease.
Methods
In the outbreak area, we checked the genetic origin of the trees and we estimated the percentage of Central European trees, disease prevalence and severity.
Results
Intra-site genetic diversity was high. Genetic composition did not match with geographic origin certificates. A high proportion of trees could not be assigned to either the Alpine or Central European gene pools. These admixed trees were as resistant as Central European trees. Geographic origin turned out to be one of the main drivers of canker prevalence, along with abiotic factors.
Conclusion
We need a precise knowledge of the origin of material used in seed orchards to mitigate canker disease and adapt forests through assisted migration.
Historical results from provenance trials tended to position Sudetan larch (Larix decidua Mill. sudetica) populations among the least susceptible to larch canker (Lachnellula willkommi (R.Hartig) ...Dennis), in sharp contrast to Alpine populations. However, a recent large-scale outbreak of larch canker in artificial French stands planted with Sudetan larch brings into question this conclusion but also, as a consequence, the value of Sudetan larch seed orchards to mass-propagate risk-limited forest reproductive material. Clonal material from the French breeding population and seed orchards was investigated for its susceptibility to larch canker. As a first step, a genetic structure analysis of this population (220 clones) was conducted: it revealed its genetic heterogeneity, with 53.1% of the clones classified as "pure" Sudetan, 3.1% as Alpine; 1.5% as Central European and 42.3% as introgressed. Following artificial inoculation, "pure" Sudetan clones appeared generally less susceptible to canker than Alpine clones; admixed clones behaved in a similar way to the pure Sudetan material. However, the broad clonal variability observed within each population prevents the sole use of genetic origin of clones to rogue the most susceptible ones within orchards. Artificial inoculation is, so far, the only reliable way to properly assess clonal behaviour towards canker and thus support genetic roguing of orchards.
