Monitoring of the incidence of Dutch Elm Disease and mortality in experimental plantations of French Ulmus minor clones

The results of 16 experimental plantations of Ulmus minor clones of the French national collection are presented as a complement to a previous work (Collin et al. 2020) on the outcome of the French Programme for the Conservation of Native Elm Genetic Resources. A total of 710 elms from 38 clones were planted in three different regions of France using two types of experimental designs and exposed to natural infection by Dutch Elm Disease (DED). DED infection and subsequent mortality were monitored in rectangular monospecific plantations (“plot-tests”), comprising at least 100 elms (10 clones). Linear plantations of 15 to 36 elms intermixed with other trees and shrubs (“hedge-tests”) investigated the feasibility of using native field elm clones in hedge reconstruction projects. After at least 9 (up to 17) years of experimentation, overall DED infection frequency was 33%, with scores above 63% in the oldest plantation and in two fast-growing tests. The overall mortality in the 232 diseased trees was 21%, reaching 29% in the oldest plantation and 64% in a hedge-test on high quality soil, suggesting a possible effect of the very fast growth of the trees. A few clones showed an interesting lower infection frequency or some ability to recover, whereas some others were found quickly infected in several tests and could serve in future experiments on clone attractiveness for DED vectors. Practical conclusions for genetic resources conservation consist of recommendations for the establishment of conservation plantations using regional clones.