Two classes cover nearly every root partnership you will find in soil. Ectomycorrhizae wrap the outside of woody root tips. Endomycorrhizae grow inside living root cells, and that second group goes by the name of its biggest subgroup, the arbuscular mycorrhizae. The types of mycorrhizae in your garden bed or your woodlot land in one of those two camps.
I pulled the leaf litter back under the young oak at the fence line last October. The feeder roots came up pale and swollen, and each one forked again and again into short stubby tips. A soft white sheath covered every tip, and the sheath rubbed off on my thumb like damp felt. You are touching fungal tissue when that happens. Most of the fungi that build the sheath are mushroom-forming basidiomycetes. The caps that push up around that oak each fall belong to the same organisms working the root tips in June.
The ectomycorrhizae vs endomycorrhizae split comes down to where the fungus stops. An ectomycorrhizal fungus builds a mantle over the whole root tip. Then the fungus pushes threads between the outer root cells to form a Hartig net, and every strand of it stays outside the cell wall. Arbuscular fungi go one step further. These fungi grow into the cells of the root cortex and branch there into small tree-shaped structures. Those branches are short-lived, under 15 days in most cases.
More than 4,000 fungal species form ectomycorrhizae. You find them piled onto a short list of woody families: pines, oaks, birches and myrtles, plus the trees close to them. That list looks huge when you walk through a forest and small when you see it on paper. Only 3% to 4% of vascular plant families pair with ectomycorrhizal fungi at all. Walk a pine stand and almost every trunk around you runs on that small club.
The other side of the split reaches much wider. Arbuscular mycorrhizal fungi sit in the Glomeromycota, and they partner with almost 90% of plant species. Most of what you plant on purpose sits on that side of the line. The odds alone put your beans, your grass and your squash there. You will never spot these fungi the way you spot a mushroom under an oak. Arbuscular mycorrhizal fungi make microscopic spores underground and put nothing above the surface, so bare soil gives you no clue.
Both types of mycorrhizae show up in bags at the garden center, and that difference should change how you buy an inoculant. A product built on Glomeromycota spores aims at the arbuscular side, which makes it the wrong match for a pine or an oak seedling. Check the species list on the label before you pay. A good bag names the fungi in it, and the names tell you which of the two camps you are buying. If the label names no species at all, put the bag back on the shelf. Matching the type to the plant rules out the wrong bag. It says nothing about how well the product was made.
Two classes give you a fine place to start, and they hold for most of what grows in a yard. The picture gets finer up close. Penn State describes five categories rather than two. The extra three are the ericaceous, orchidaceous and ectendomycorrhizae, and each of them has a host list of its own. Blueberries, heaths and orchids sit on those odd ones, so a plain ecto or endo label tells you little about your soil.
Read the full article: Mycorrhizae: What Works and What Is Hype
