Susceptibility of native French elm clones to Ophiostoma novo-ulmi

More than 200 clones from native European elm species (U. minor, U. glabra and their hybrids and U. laevis) were inoculated with Ophiostoma novo-ulmi in comparison with some cultivars on two sites under six inoculation series. Crown wilt and defoliation percentage was assessed in the inoculation year, and subsequent recovery and mortality were also recorded. Multiple comparisons indicated that the results were reproducible. Species effect was generally detected, U. glabra being more susceptible than the other native species. Clone effect was always very significant with a wide symptom amplitude between clones, particularly among U. minor. In the case of U. laevis, some clones suffered high mortality rates, whereas other showed a strong recovery ability. Clones with a good behaviour could be identified among each species. Some of them showed complete recovery the year following inoculation. No effect of taxa and geographic origin was detected in the frequency of superior clones. Because no native elm clone reached the level of resistance of the best cultivars ('Sapporo Autumn Gold' and 'LUTECE Nanguen'), their cultivation in pure stand is not recommended. However, the best clones can be introduced in multi-species plantations, especially for the reconstruction of countryside hedges; they may also contribute to a breeding programme.