1
JANUARY/FEBRUARY 2003
Encouraging
beneficial
AM fungi
in vineyard soil
WHY AND HOW
A
BY Kendra Baumgartner,
USDA Research Plant Pathologist
Davis, CA
rbuscular mycorrhizal (AM)
fungi are beneficial organisms
that have a close association
with the roots of a variety of
plants, ranging from corn to redwoods.
They have been shown to increase crop
yield and nutrient uptake, enhance
resistance to plant pathogens, and to
stabilize soil structure.
AM fungi live in symbiosis with
plants. This means they live in close
association with each other, enjoying a
mutually beneficial relationship. AM
fungi live partially within the roots of
their plant hosts. The plant provides
the fungus with carbon (a product of
photosynthesis), while the fungus
provides the plant with phosphorus
and other nutrients absorbed from the
soil.
Other direct benefits provided by
mycorrhizal fungi to some plant hosts
include improved drought tolerance 2
and resistance to specific pests and
diseases.3 AM fungi are biotrophs;
they cannot live in the absence of a
plant host except as dormant spores.
Arbuscular mycorrhizal fungi are
so named because they form a fungal
structure known as an arbuscule.
Arbuscules are tree-shaped structures
that the fungus forms inside plant
cells. These structures are responsible
for nutrient exchange between the
plant and the fungus.
Some AM fungi also produce a
structure known as a vesicle, which is
thought to have a function in nutrient
storage. Vesicle production was once
believed to be common among all AM
fungi, which were then called vesiculararbuscular mycorrhizal (VAM) fungi.
Later research identified a family of
supposedly VAM fungi that didn’t produce vesicles. Since all types produce
arbuscules, the name was amended to
become AM fungi.
Arbuscules are connected to
strands of AM fungal tissue that
extend into the soil (called extraradical hyphae). These hyphae absorb
water and nutrients for the fungus
and the plant. They extend the root
system and, in effect, increase the
plant’s root surface area.
WINEGROWING
AM fungal hyphae are finer than
plant root hairs, which allows them to
penetrate smaller soil pores. This
allows a plant with AM fungi to absorb
water and nutrients from areas of the
soil profile that roots can’t reach, providing plants with a higher tolerance
for drought and increased uptake of
certain nutrients, such as phosphorus.
Extraradical hyphae can colonize
multiple roots of either the same plant
or different plants, forming a mycorrhizal network. This network can
mediate the transfer of water and/or
nutrients between different plant
species. In one study, soybean plants
received nutrients via the AM fungal
network from adjacent weeds following an herbicide application.6
AM fungi produce thick-walled
spores on extraradical hyphae. The
spores are released into the soil surrounding roots colonized by AM fungi
and are spread by anything that moves
soil, including wind, water, and
machinery. Spores germinate near
roots in response to root exudates,
including various phenolic compounds (flavonoids in particular).
After germination, the AM fungus colonizes fine roots (germinating spores
do not colonize woody roots).
Soil fumigants kill AM fungal
spores. Deep, uniform soil fumigation
may result in slow recolonization
(greater than two years after fumigation) of grapevines by indigenous AM
fungal species and can lead to stunting
in some nutrient-poor soils.5
Research on AM fungi and grapevines in both the greenhouse 1,4,7,8 and
the field 5 has shown that mycorrhizal
vines with certain AM fungal species
have greater shoot and root growth
when compared to non-mycorrhizal
vines. Rootstocks and ungrafted
cultivars kept free of AM fungi were
approximately two to three times
smaller than mycorrhizal vines.4
Mycorrhizal vines had denser, more
compact root systems, presumably due
to the presence of numerous extraradical hyphae that branched out in the
soil.7
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JANUARY/FEBRUARY 2003
WINEGROWING
In greenhouse experiments in which
various rootstocks were colonized by
different AM fungal species, some
were found to have higher concentrations of phosphorus and zinc, but these
benefits were dependent on the AM
fungal species and the soil in which the
rootstock was planted.1
These studies, along with other
research conducted on many other crops,
demonstrate that the specific combination of plant species, fungal species,
and soil has a significant effect on how
a plant benefits from AM fungi. Some
combinations of plant, fungus, and soil
may increase zinc uptake, while others
may improve drought tolerance.
In some combinations, the plant
may not benefit at all. If the AM fungus
provides the plant with nothing, the
fungus starts to act like a parasite.
Plants do have some control over this
situation, although the mechanism of
this control is not known. For example,
under high rates of phosphorus fertilization, plants are found to have fewer
AM fungal structures in their roots.
With adequate levels of phosphorus,
the cost of “feeding” AM fungi (providing them with carbon) outweighs
the benefit (more phosphorus).
Since the combination of plant, fungus, and soil affects how a plant benefits from being an AM fungal host, it
would seem important to find the combination that works best for specific
agricultural crops. This is, however,
easier said than done. Finding the ideal
AM fungal species for grapevines
would require identification of AM
fungal species from healthy vines in
the field, propagation of inoculum, and
subsequent inoculation of vines grown
in the greenhouse to determine which
species are the most beneficial.
Identification of AM fungi is very
difficult. Since they cannot be cultured
on artificial nutrient agar in the lab,
they have to be grown on roots of living plants. AM fungal taxonomy is
based on microscopic, morphological
characteristics of the spores, and since
many species are undescribed, keys are
useful only for identifying some of the
species.
Microscopic spores of arbuscular mycorrhizal fungi are produced on the roots of mycorrhizal
plants and are spread to new roots with the help of wind, equipment, rodents, and anything
else that moves soil. Three arbuscular mycorrhiza fungal spores are connected to each other
by vegetative fungal tissue.
Propagation of inoculum and subsequent evaluation of the effects of specific AM fungal species on grapevine
growth in the greenhouse takes
approximately one year to accomplish.
Further greenhouse testing would be
required for evaluations in different
soil types. Finding the right combination of AM fungal species for grapevines may not be necessary, since AM
fungi occur naturally in vineyards
worldwide.
When land is cleared for vineyard
establishment, AM fungal spores and
roots from mycorrhizal hosts that previously inhabited the site serve as
inoculum for grapevines. Most AM
fungal species are thought to have
broad host ranges. Nearby grapevines
can serve as a source of AM fungal
spores. AM fungi also occur on the
roots of field-propagated nursery
stock. Therefore, even if soil is fumigated before planting, AM fungi will
eventually colonize young grapevines.
Vineyards become colonized by AM
fungi naturally. There are things you
can do to encourage AM fungal populations if you are concerned about a
lack of them in your vineyard.
• Plant a cover crop. Most vineyard
cover crops are hosts of AM fungi (with
the notable exception of mustard).
Maintenance of cover crops during the
winter will likely result in a higher concentration of AM fungal spores in vineyard soil than will be found in vineyards
where no cover crop is grown. Cover crop
mixes are likely to encourage a greater
diversity of AM fungal species, which
may increase the chances of encouraging
those that are best for vines in your soil.
• Mow the cover crop in spring instead
of incorporating it into the soil (if your
cover crop is not too competitive). Soil
disruption breaks up mycorrhizal networks, which are important for providing mycorrhizal plants with increased
nutrient and/or water uptake.
• Plant with field-grown nursery
stock. Vines raised in the field have
AM fungi in their roots.
Commercial inocula of AM fungi
are available. These products are more
3
JANUARY/FEBRUARY 2003
WINEGROWING
practical for use in newly planted
rather than established grapevines.
Adding inoculum may be more
important when planting into fumigated soils. Be sure to obtain the
freshest inoculum possible (within
days or weeks of it coming off the
production line) and apply it immediately. The longer the spores stay in
the container, the lower their chances
of germinating once they are placed
on plant roots.
AM fungi likely have an unappreciated role in helping grapevines tolerate
drought induced by deficit irrigation
and in the maintenance of sufficient
root growth and nutrient uptake in
marginal vineyard soils.
Adopting practices that encourage
AM fungal populations is easy, since
these beneficial fungi naturally colonize grapevine roots. However, these
practices should not be substituted for
nutrient amendment, irrigation, or pest
management, especially when deficiencies or diseases/pests are already widen
spread in the vineyard.
Reprinted from:
References
1. Biricolti, S., Ferrini, F., Rinaldelli, E.,
Tamantini, I., Vignozzi, N. 1997. “VAM
fungi and soil lime content influence rootstock growth and nutrient content.” Amer.
Journal of Enology & Viticulture 48: 93–99.
2. Fitter, A. H. 1988. “Water relations
of red clover, Trifolium pratense L., as
affected by VA mycorrhizal infection and
phosphorus supply before and during
drought.” Journal of Experimental Botany
39: 595–604.
3. Hooker, J. E., Jaizme-Vega, M., Atkinson, D. 1994. “Biocontrol of plant pathogens
using arbuscular mycorrhizal fungi.” In:
Gianinazzi, S., Schüepp, H., eds. Impact of
Arbuscular Mycorrhizas on Sustainable Agriculture and Natural Ecosystems. Basel,
Switzerland: Birkhäuser Verlag. p 191–200.
4. Linderman, R. G., Davis, A. A. 2001.
“Comparative response of selected grapevine rootstocks and cultivars to inoculation
with different mycorrhizal fungi.” Amer.
Journal of Enology & Viticulture 52: 8–11.
5. Menge, J. A., Raski, D. J., Lider, L. A.,
Johnson, E. L.V., Jones, N. O., Kissler, J. J.,
Hemstreet, C. L. 1983. “Interactions be-
tween mycorrhizal fungi, soil fumigation,
and growth of grapes in California.” Amer.
Journal of Enology & Viticulture 34:117–121.
6. Mujica, M., Fracchia, S., Ocampo, J.,
Godeas, A. 1999. “Influence of the herbicides chlorsulfuron and glyphosate on mycorrhizal soybean intercropped with the
weeds Brassica campestris or Sorghum
halepensis.” Symbiosis 27: 73–81.
7. Schellenbaum, L., Berta, G., Ravolanirina, F., Tisserant, B., Gianinazzi, S.,
Fitter, A. H. 1991. “Influence of endomycorrhizal infection on root morphology in
a micropropagated woody plant species
(Vitis vinifera L.).” Annals of Botany
68:135–141.
8. Schubert, A., Cammarata, S., Eynard,
I. 1988. “Growth and root colonization of
grapevines inoculated with different
mycorrhizal endophytes.” HortScience 23:
302–303.
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JANUARY/FEBRUARY 2006: Which mealybug is it and why should you care?
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MAY/JUNE 2006: Balanced soils play vital role in high-quality wines
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