Interest in trees, whether in our streets, parks or forests, has in­ creasedconsiderablyin thelast 20 years or so. One reason for this has been the decline and dying of forests, which caused great concern about our environment during the 1980s. Because ofthe prominenceofthis event,which is nowblamedon abiotic factors, it is all too easyto forget that the life oftrees is also affected by a multitude of biotic factors: viruses, bacteria, fungi and animals. These may have very different relationships with trees, but are usually deleterious. The fungi playa particularlyimportant part, and during the course of their evolution they have developed various abilities and strategies in order to