Genetic variation among clones of Larix decidua polonica for resistance toLachnellula willkommii.

Forty clones of European larch selected from four native stands in the Bliz yn region (central Poland) were tested for resistance to Lachnellulla willk ommii. In spring 1991, artificial inoculation (mycelium on sorghum straw in serted into a hole drilled in twigs) was conducted in three French sites (O rleans, Peyrat-le-Chateau and Cendrieux). Each clone represented by between 3 to 15 11-year-old ramets received between 20 to 40 inoculations. The dev elopment of larch canker symptoms (bark necrosis, pathogenic resins, fructi fication) was observed for three growing seasons. The overall infection rat e (Ti, defined as the proportion of inoculation points showing at least one of the symptoms) varied from 10 to 30 % according to the site. This confir ms the rather weak to moderate susceptibility to canker of the polonica rac e and reveals a strong site effect on the infection success and further dev elopment. A broad clonal variability is shown (worthy of mention is the abs ence of an immune clone) together with a sufficiently high level of generic control (h(bs)(2) : 0.53-0.66 for Ti) for the selection of clones for resi stance to be conducted efficiently. Nevertheless, two main factors somewhat limit the scope of this study : i) the instability of ranking of clones ov er sites and ii) a large within-clone variability. Progress in the experime ntal use of this artificial inoculation technique is expected from a better understanding of the host-pathogen interaction process. In particular are suggested : i) the definition of optimal ecological conditions for the path ogen expression (for the choice of the experimental site) and ii) a limitat ion of undesirable experimental noises - e.g. it is hypothesised that the v igour of the inoculated twig can influence the infection success, in which case the standardisation of twig size is necessary. ((C) Inra/Elsevier, Par is.).