Lights and shadows of a possible strategy to cope with alien and destructive forest pathogens: the example of breeding for resistance to Dutch elm disease in Italy

Since the onset of the 20th century, two pandemics of Dutch elm disease (DED) destroyed native elms throughout Europe and North America. The disease is caused by two invasive fungi, Ophiostoma ulmi (Buisman) Nannf. and Ophiostoma novo-ulmi Brasier, which appeared one after another and were new to science. From the late 1920s, in Europe and in the US, researchers strove to find natural resistance to DED in native elms, but their efforts yielded ephemeral success. The resistant cultivars obtained in the 1930s by hybridizing European elm genotypes, were defeated by the second pandemic. The inclusion of Asian resistant species in breeding programs finally produced resistant second-generation hybrids. In Italy a program to breed resistant clones for the Mediterranean climate, was started in the mid 1970s. The successful use of Asian species in the country encouraged an in-depth assessment of their adaptability to local climates for broadening the genetic base of breeding. Selection of superior genotypes reduces genetic variation. However, when breeding is designed to obtain multiple genotypes for diverse conditions and uses, variation can be maintained. The case of elm is paradigmatic. Since elms have many uses, and an important one is as ornamentals, breeding included the selection of genotypes with fast growth, and attractive crown shape and foliage. To meet all needs and provide genetically variable cultivars to deal with climate change and new diseases, genetic resources were broadened. Native elms with good aesthetic qualities were crossed with DED-resistant and adaptable Asian genotypes. The program produced resistant clones adapted to summer drought and winter floods, yet endowed with notable ornamental features. Five of these clones were patented. A similar strategy including both the crossing of European Fraxinus species and of native with non-native resistant genotypes, may be successful against Hymenoscyphus fraxineus, the invasive agent of the European ash dieback epidemic.