If exotic parasites such as Dutch elm disease or chestnut bark disease are imported, the consequences can be much more serious.
Trees are in permanent contact with microorganisms, which have very different functions.
Some of them are beneficial, such as mycorrhizal fungi or antagonists of the pathogenic soil flora, nitrogen-fixing bacteria or microorganisms assisting mycorrhization. Others eliminate the vegetational remnants and recycle their elements. Finally, some are tree parasites. Among the latter the most threatening are the fungi and some bacteria and viruses. The development of populations of parasites depends almost always on the climate.