Mycorrhizae Overview
by J.C. Meneley, Ph.D.
AgBio Inc.
Mycorrhizal fungi are specialized microbes that form a
close association with most plants that grow on land.
Mycorrhizal fungi are the most researched group of
beneficial fungi in science with over 15,000 publications
on the subject. They naturally colonize roots and
dramatically increase the root absorptive area resulting in
an impressive increase in uptake of water and major and
minor nutrients. Once plants become mycorrhizal, there is
no need to reinoculate since the fungus becomes a permanent
part of the root system.
Plants with mycorrhizal roots use fertilizer more
efficiently and are more tolerant to drought, high salts,
soil compaction, heavy metals, and organic toxins. In
addition, plants are more resistant to nematodes and
diseases. There have even been reports that demonstrated
resistance to insects.
There are 2 main groups of these important beneficial
fungi. Endomycorrhizal fungi are microscopic and grow
inside roots. They send out their own fungal roots and
search the soil for nutrients, returning those to the
plant. The plant, in turn, provides a home for the
mycorrhizae, forming a symbiotic association. Endos form
reproductive propagules inside roots and spores outside in
the soil.
Ectomycorrhizal fungi live on the root surface. They
sometimes appear as a whitish growth. They form spores in
underground structures (e.g. truffles) or above the ground.
Certain forest mushrooms are mycorrhizal fungi. Both groups
function the same way.
Endos are the most widespread, forming a root association
with over 80% of the green plants on earth. Ectos grow
primarily on conifer, oak, birch, beech, pecan, eucalyptus,
and willow roots. A few plants are not endo or ecto
mycorrhizal such as orchids, cabbages, beets, and mustards.
An excellent university site for more information is found
at http://mycorrhiza.ag.utk.edu/
Suppression of Activity
Mycorrhizae function best when inorganic soluble phosphate
fertilizer is used at moderate to low rates. In nature,
mycorrhizae search the soil for nutrients and convert rock
and organic phosphates to soluble forms and return to the
plant roots. High rates of soluble phosphate fertilizer
suppress the mycorrhizae. Reviewed here. However,
mycorrhizae perform very well when used with organic
fertilizers, rock phosphate, or low to moderate rates of
soluble inorganic phosphate fertilizer.
Likewise, certain chemical fungicides have been shown to
suppress or kill mycorrhiza. Chemicals such as benomyl,
iprodione, and PCNB should not be used. In contrast,
Pythium fungicides such as metalaxyl actually promote
mycorrhizae. Insecticides and herbicides have no or little
effect on mycorrhizae.
Biocontrol organisms like the Streptomyces in Mycostop
Biofungicide are encouraged by mycorrhizae. There are many
scientific publications demonstrating that higher
populations of beneficial organisms inhabit the root zone
when mycorrhizae are present.