Elm Yellows in Europe

Abstract

Fifteen years of observation of natural Elm Yellows (EY) infections on native and introduced elm species living in the Mediterranean area have led me to conclude that: 1. EY is not a dangerous disease on native European elm populations, but there are sources of pathogen inoculum and efficient insect vectors scattered throughout Italy, which may lead to epidemic outbreaks if susceptible hosts are introduced, 2. The majority of Asian species that were introduced into Europe (in order to increase native elm resistance to Dutch elm disease) show higher susceptibility to EY; therefore, they introduce a new complication into conventional breeding work, 3. Conventional breeding work based on clone constitution can fail if selection for low susceptibility to EY is not included. Clones with intermediate EY susceptibility must be supported by adequate cultural practices in order to compensate for weakening resulting from the disease, 4. The possibility of working with native species without the need for recourse to foreign germplasm argues in favor of using genetic engineering techniques for improvement work, 5. The phytoplasmas that are the agents of EY in Europe are very similar to those found in North America. Furthermore, in Europe they are often present in association with phytoplasmas of other groups. Future work should investigate the level of virulence of these strains and identify the insect vectors, which so far remain virtually unknown.