A staggering 80 to 95 percent of all terrestrial plants form symbiotic relationships with mycorrhizal fungi. In these relationships, or mycorrhizae (mycorrhiza, singular), the host plants supply the mycorrhizal fungi carbon, and in return, the fungi help roots obtain and absorb water and nutrients that the plants require. These relationships are vital to the health of almost all plants that grow on Earth. Each group of mycorrhizal fungi interacts and colonizes its plant host in a different way, in a process so complicated that it took scientists a long time to catch on to its importance.
If you are reading this book, you are probably familiar with the soil food web, the incredibly diverse community of organisms that inhabit the soil. Most of you understand the importance of symbiotic (mutually beneficial) relationships among plant roots and a multitude of soil organisms. You are aware of the relationships among bacteria, rhizobia, and legume plant roots that result in nitrogen fixation, and you understand that bacteria can form a symbiotic relationship with plant roots. Soil-borne mycorrhizal fungi, the subject of this book, interact with plant roots in a similar way.
Mycorrhizae have been known since 1885, when German scientist Albert Bernhard Frank compared pine trees grown in sterilized soil to those grown in soil inoculated with forest fungi. The seedlings in the inoculated soil grew faster and much larger than those in the sterilized soil. Nevertheless, not so long ago (in the 1990s), the importance of mycorrhizal fungi was unknown to many farmers and gardeners--and most garden writers. We feared and loathed all fungi, the stuff of mildews and wilts. Fungi were usually considered downright evil, and most of us took a one-size-fits-all fungicidal approach in our gardens. I had been writing a weekly garden column for some 25 years when I first heard the words mycorrhizal and mycorrhizae in 1995. I was embarrassed by my lack of knowledge of these important organisms, but when I asked my peers if they had ever heard of mycorrhizal fungi, they had no idea what I was talking about. (When I first started writing about mycorrhizae, not only did my word processor program's spell checker reject the word, but my editor did as well.)