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  • From: Lawrence London <lfljvenaura@gmail.com>
  • To: permaculture <permaculture@lists.ibiblio.org>, marketfarming <marketfarming@lists.ibiblio.org>
  • Subject: [permaculture] FUNGI AND MYCORRHIZAE FREQUENTLY ASKED QUESTIONS: Dr. James Traquair
  • Date: Tue, 12 Feb 2013 21:22:32 -0500

FUNGI AND MYCORRHIZAE FREQUENTLY ASKED QUESTIONS: Dr. James Traquair
http://www.ibiblio.org/london/orgfarm/faqs/fungi-mycorrhizae.faq

FUNGI AND MYCORRHIZAE

FREQUENTLY ASKED QUESTIONS:

Dr. James Traquair


1. What are fungi and how do they differ from bacteria ?

2. What is the ecological role of fungi in soil and on plant surfaces
?

3. How do agricultural practices affect the fungi in soil ?

4. What are mycorrhizae and what is their role in agriculture ?

5. What is biological control in the context of plant disease
management ?

Special Topics

Key Words

Selected References

Other Related WEB Links

Special Topics:
* Microbiology of Composting
* Rhizosphere Microbiology
* Nutrient Cycling
* Non-target Effects of Pesticides
* Allelochemistry
* Crop Residue Management

Return to Top of Document


Return to FAQ Table of Contents

Return to PMRC HomePage
_________________________________________________________________


1. eukaryote, filamentous, yeast, chitinous walls, glucans, conidia,
spores, sclerotia

2. saprophyte, parasite, pathogen, biotroph, necrotroph, mildew, root
rot, damping off ,antibiosis, mutualism, symbiosis, Rhizoctonia
solani, Cylindrocarpon destructans, Pythium spp., Thielaviopsis
elegans, nutrient cycling

3. crop rotation, selection pressure, amendment, enrichment,
anaerobic, tillage, pesticide residue, drainage,host specificity,
propagule survival, dispersal

4. Glomus spp.. Gigaspora spp., vesicular-arbuscular mycorrhizae,
endomycorrhizae, ectomycorrhizae, growth promotion, mutualism,
nutrient absorption, fertility, phosphorus azygospore, chlamydospore,
disease suppression, drought tolerance,carrier, agarose beads,
pelletization, peat-based potting medium, obligate symbiont, colonized
root fragments

5. rhizosphere, mycorrhizosphere, rhizosphere competence, competitive
saprophytic ability, actinomycete, antibiotic, toxic fungal
metabolites, Sporothrix spp., Tilletiopsis spp., hyperparasites,
antagonists, competition, allelopathy, chelation


Brief Responses to FAQs:

1. The Fungi

Several diverse microorganisms are classified within the broad group
of protists called the fungi. However, they do share some typical
features that distinguish them as fungi different from other microbes.
These distinguishing features are based on the physiology and
morphology of cells. The fungal cell is distinguished from the
bacterial cell by its size (generally greater than 1-2 um in diameter)
and its eukaryotic structure similar to that of plant and animal cells
(cells having distinct membrane-bound organelles such as nuclei,
mitochondria). Bacterial cells which lack these organelles are termed
prokaryotic. Fungal cells divide by mitosis (asexual reproduction) and
by meiosis (sexual reproduction); bacterial cells divide by binary
fission.

Like animals, the fungi are heterotrophic organisms that cannot
manufacture their own food by photosynthesis as plants and algae can.
They require oxygen for growth (aerobic) and generally prefer an
acidic environment (below pH of 7) unlike the bacteria which are
anaerobic and aerobic and generally grow in basic environments ( at or
above pH of 7). Fungi utilize preformed organic material from other
organisms as sources of energy and building blocks for their cellular
sysnthesis. Soluble nutrients are absorbed from the growth substrate
following the breakdown of complex polymers by extracellular enzymes
(proteinases, cellulases, pectinases etc.) secreted by fungal cells.

The fungi have diverse morphologies especially in spore production
which is the basis for identification. But, they are commonly
recognized as the yeasts (single-celled thallus), the molds
(filamentous thallus called a mycelium consisting of tubular cells in
long, branched, thread-like structures called hyphae) and the
mushrooms (macroscopic fungi with considerable differentiation of
tissues and hyphae in the mushroom, the sporulating portion of the
thallus, which is fed by a massive underground mycelium). The fungi
like bacteria, are very susceptible to drying and, therefore, are
generally found in very most, if not aquatic habitats. They can resist
desiccation by producing thick, melanized walls which are often seen
in aerial spores, in hyphal strands, and in sclerotia that enable the
fungus to survive suboptimal growing conditions in the soil for
several years. The fungal wall usually consists of layers of chitin, a
linear polysaccharide polymer of N-acetylglucosamine, embedded in and
often cove ed by glucans, branched polymers of glucose and other
sugars. Some species of fungi in the Class Oomycetes such as
Phytophthora and Pythium have cellulose walls rather than chitin. The
melanin components consist of branched polymers of phenolic material
similar to the lignins of plant cell walls.
Return to Top of Document


2. Ecological role of the fungi

The fungi are ubiquitous in nature andconstitute about 85-90 % of a
typical organic soil biomass. Saprophytic fungi are the decomposers
and live on dead organic material. They play a major role in the
re-cycling of nutrients. Decay by fungi is usually considered in a
negative context as the rotting of wood products and textiles or the
mold or mildew on paint or bathroom tiles. But, fungal decomposition
is benefial as can be seen in the thermophilic composting process in
the backyard or the barnyard. Think of the incredible build-up of
organic leaf litter in the forest without the saprophytic activity of
the fungi.

Fungi as parasites cause serious diseases (pathogenic) to animals,
plants and other fungi. As obligate or biotrophic parasites such as
wheat rust, they must obtain nutrients from living cells. They are
very host specific and do not kill their hosts right away. As
faculative or necrotrophic parasites they kill the host by toxins
prior to or at the time of invasion and then continue to live as
saprophytes on the decaying tissues. Some fungi such as the rusts
attack weed species and are fine candidates for biological control of
unwanted plants. Still other fungi attack insect pests and other
pathogenic fungi in the soil or on plant surfaces. They are important
sources of biological control for insect problems and plant disease in
forestry, agricultural and horticultural crops.

We are well aware of the mutualistc association between fungi and
algae or fungi and cyanobacteria in the lichen thallus on rocks, trees
and the forest floor. Fungi can be beneficial also to the growth of
plants by forming mutualistic symbiotic associations with roots called
mycorrhizae. There are numerous kinds of mycorrhizal associations.
Mycorrhizae of different types are ubiquitous on most herbaceous
plants and tree species in a wide variety of habitats including
agricultural systems.
Return to Top of Document


3. Agricultural impact on fungi

Because fungi are everywhere, agricultural practices have profound
effects on their growth, distribution and survival. The impact may be
positive or negative. The amendment of soil with organic material will
enhance the activity of decomposer fungi in the soil. Some of these
fungi may also be antagonistic to fungal pathogens of plants and lead
to suppression of disease. In any event, the community structure of
fungi will be changed through this enrichment process. However,
excessive deep plowing may separate the organic material from the
fungal decomposers which occur in the top few centimeters of the soil.
This tillage will lead to a general decline in fungal biomass. The
addition of pesticides to the soil will also affect the survival of
selected fungi.

Specific fungicides used to destroy specific pathogens may also
destroy close relatives of these fungi that may be beneficial as
saprophytes. This non-target effect is a particular concern in the use
of broad-spectrum biocides such as the fumigant, methyl bromide, which
destroys all biological activity. So, using this fumigant to kill
plant pathogenic nematodes in greenhouse soil will also kill
pathogenic fungi and kill the mycorrhizal fungi that are beneficial to
plants. Use of a specific nematicide on the other hand, will kill
nematodes but not the mycorrhizal fungi. Non-target effects on fungi
in the soil are now being identified for herbicides used to kill weeds
in conservation tillage practices.

Crop rotation can also influence the distribution of fungi in soil
because of the known host specificity of certain species of fungi.
Rotation of susceptible and non-susceptible crops is a
well-established means of managing disease in crop plants. This
technique fails, however, for the control of fungi that produce
sclerotial structures or spores that survive for long periods in the
soil as portentially infective propagules. Moisture manipulation
through irrigation and drainage, or compaction of the soil can affect
the activity of fungi. Poor drainage can encourage the growth of
soil-borne water molds that cause root rot of some crops. Similarly,
dense plantings create humid environments that encourage the growth of
foliar fungi that cause leaf disease and mildews.


4. Mycorrhizae and their significance

Mycorrhizae are mutualistc associations between plant roots and fungi.
These beneficial symbioses are ubiquitous in nature and almost all
plant species have some form of mycorrhizal association with fungi.
Herbaceous and tree species, both deciduous and coniferous, are
receptive to infection by mycorrhizal fungi. A few crop plants in the
cabbage family (Brassicae) and goose-foot families (Chenopodiaceae)
are less likely to be mycorrhizal except in very stressful
environments low in soil fertility.

The types of mycorrhizal fungi and the associations they form with
roots are varied. These associations are classified as endotrophic
(fungus inside roots) or ectotrophic (fungus outside the root forming
a sheath of mycelium over the root surface). Still other associations
in some forest trees are ectendotrophic or combinations of these
types. Other forms exist such as the unique mycorrhizal association
with Rhizoctonia-like fungi inside the roots of orchids.

The endomycorrhizal fungi generally associated with the roots of
agricultural crops are in the Class Zygomycetes to which the common
black bread mold belongs. However, these fungi are obligate symbionts
and cannot be cultivated outside the living roots of plants. Their
colonization is internal to the root and cannot be seen without
staining and microscopy. The common genera are Glomus and Gigaspora
producing large, distinctive azygospores that can be wet sieved from
the soil. These spore germinate in the presence on plant roots and
infect the outer cortical cells. However, the cell is not killed and
although the plant cell wall is penetrated the cell membrane is not
disrupted. The endomycorrhizal fungus produces a highly branched
hyphal structure called an arbuscule within the plant cell by
invaginating its cell membrane. This infection creates an absorptive
structure with a very high surface area of transfer for nutrients
between the plant and the fungus.

The plant usually has few root hairs in this area and the fungus
provides the intimate contact with the soil through fine extraradical
hyphae which extend several millimeters beyond the root. This
extensive hyphal network enhances absorption of water and nutrients,
particularly phosphorus, and promotes growth of the plant. In exchange
for the phosphorus which is transferred to the plant, the fungus
obtains sugars and other organics vital to its growth and
reproduction. These exchanges have been verified by various
histochemical studies and the use of nutrients labelled with
radioisotopes. As the association begins to senesce, the fungus
produces vesiculate storage bodies on and within the root cells and
produces the distinctive asexual spores. Sexual reproduction has
rarely been observed for these fungi.

The ectomycorrhizal fungi are in the Class Basidiomycetes and Class
Ascomycetes, the fungi that we usually identify as wild mushrooms in
various forest environments. Genera such as Russula, Lactarius,
Laccaria, Amanita, Boletus, and Tuber (truffle) or Cenococcum (false
truffle) to name a few, are all ectomycorrhizal. They are facultative
symbionts of the roots of forest trees and their colonization can seen
with the naked eye. These fungi can be cultivated in the laboratory on
special nutrient media. They have limited saprophytic abilities and
prefer to grow in association with plant roots. The mutual benefits
are similar to those described for the endomycorrhizae. The
ectomycorrhizal fungus froms a sheath or mantle of densely packed
hyphae on the surface of tree roots. This mantle is often black or
brightly colored. The mantle is connected to highly branched hyphae
that penetrate the root and grow between but not into the bost cells.
This network of hyphae (hartig net) forms the absorptive structur that
is the site of nutrient exchange. The sheath is connected to
extraradical hyphae that permeate the soil and absorb water and
nutrients for the ectomycorrhizal root.

Other benefits of mycorrhizal associations that are of interest to
plant pathologists include biological control and the various growth
promotion effects that enhance establishment of plants in the field.
Mycorrhizal roots are generally more drought tolerant. The general
vigour of mycorrhizal plants makes them more tolerant of limited root
loss due to diseases. Another mechanism of biocontrol by mycorrhizal
fungi is the competiton for nutrients and space on the root against
the pathogenic soilborne fungi. The ectomycorrhizal fungi have the
added advantage of being able to produce antibiotic substances that
inhibit the fungal pathogens. Prior colonization by mycorrhizal fungi
may also stimulate the root to produce natural defensive wall
structures and chemicals (chitinases and phytoalexins) that protect
the root from attack by pathogens. The mycorrhizosphere (or area on
and around the root of mycorrhizal roots) also contains communities of
helpful microorganisms including fungi and bacteria that are anta
onistic to pathogens and that solubilize nutrients such as rock
phosphate.

There is a great potential for the use of mycorrhizal fungi and
associated microorganisms as inoculants especially in the production
and protection of high-value greenhouse-grown crops or transplanted
vegetable crops such as tomatoes and other bedding plants. The
challenge is to produce the inoculum of endomycorrhizal fungi that
cannot as yet be grown in artificial culture. Various stabilization
and embedding or pelleting methods are being developed for coating
seeds or inoculating potting media with root fragments and spores.
These have been adapted from the technology used to develop Rhizobium
inoculum for leguminous crops. Peat-based potting media with
mycorrhizal inoculum are being developed in Canada by companies such
as Premier Peat Moss (Riviere-du-Loupe, Quebec). The production of
ectomycorrhizal inoculum for forest nurseries based on solid-substrate
fermentation and amendment of potting media is well-underway in The
United States of America and Australia.


5. Biological control of plant disease

Biological control of plant disease is the suppression of disease
symtoms and disease incidence by the application of a biological
agent, usually a microorganism. It is welcomed as a safe and
environmentally acceptable alternative to the use of chemical measures
for disease control. Various mechanisms exist including the use of
microbial antagonists that produce antibiotics or lytic enzymes, that
compete for nutrients with the pathogen, that directly invade and kill
the pathogen as hyperparasites, that invade and transmit viral
avirulence (hypovirulence) factors, or that are non-pathogenic but
trigger or stimulate natural defence mechanisms in the host (induced
ressistance and cross protection). Another biological mechanism is the
use of microbial agents to modify the chemical environment
(allelopathic biocontrol) through the breakdown of organics in the
soil to release antimicrobial compounds such as phenolics or to
enhance the chelation of essential nutrients for the pathogen such as
iron.

Selection of previous or companion crops may affect the availability
of these allelopathic compounds that bring about biological control.
More than one mechanism may function at the same time in any
biocontrol system. The desireable approach is to integrate biological
controls with other control measure including limited chemical
applications, culture practices (crop rotation, tillage, etc) and host
resistance. This approach reduces the rate at which disease resistance
can develop. In general, resistance to biological agents develops very
slowly because of the complex control mechanism involving numerous
biochemical systems and associated genes.
Return to Top of Document


Selected References:

Allen, M.F. (Editor). 1992. Mycorrhizal Functioning: An Integrative
Plant-Fungal Process. Chapman and Hall, London

Altman, J.(Editor) 1993. Pesticide Interactions in Crop Production:
Beneficial and Deleterious Effects. CRC Press, Boca Raton FL

Carlile, M.J., and Watkinson, S.C. 1994. The Fungi. Academic Press,
Inc. New York.

Cook, R.J., and Baker, K.F. (2nd edition). 1983. The Nature and
Practice of Biological Control of Plant Pathogens. The American
Phytopathological Society Press, St Paul MN.

Harley, J.L. and Smith, S.E. 1983. Mycorrhizal Symbiosis. Academic
Press, London.

Jones, D.G. (Editor) 1993. Exploitation of Microorganisms. Chapman and
Hall, London.

Metting, F.B. Jr.(Editor). 1993. Soil Microbial Ecology: Applications
in Agricultural and Environmental Management. Marcel Dekker, Inc. New
York.

Pfleger, F.L., and Linderman, R.G.(Editors) 1994. Mycorrhizae and
Plant Health. American Phytopathological Society Press, St Paul, MN .


OTHER RELATED WEB LINKS


WFCC World Data Center for Microorganisms (WDCM) provides a
comprehensive directory of culture collecions, databases on microbes
and cell lines, and the gateway to biodiversity, molecular biology and
genome projects. Try the STRAINS - fungi Search Interface

California State University Biological Sciences WWW Server. The
purpose of this server is to consolidate existing WWW Biological
Science teaching and research resources and to create and distribute
original multimedia resources for the teaching of biology. Try the
FUNGI-related links

Symptoms of Disease - Alberta Agriculture, Food and Rural Development

Root Biology and Mycorrhiza Research Group, Department of Botany,
University of Guelph, Ontario, Canada

Mycological Resources on the Internet - Cornell University
http://www.wdcm.riken.go.jp
WFCC World Data Center for Microorganisms (WDCM)

http://www.wdcm.riken.go.jp/htbin/STRAINS-fungi.pl

STRAINS - fungi Search Interface

http://130.17.2.215/index.html
California State University Biological Sciences WWW Server
http://arnica.csustan.edu/FU.html
FUNGI-related links
http://www.gov.ab.ca/%7eagric/htmldocs/600/63000101.html
Symptoms of Disease

http://www.uoguelph.ca/CBS/Botany/roots.html

Root Biology and Mycorrhiza Research Group

http://muse.bio.cornell.edu/taxonomy/fungi.html
Mycological Resources on the Internet

_________________________________________________________________


Last Revised: Thursday, August 10, 1995

[unverified links, some may be out of date]



  • [permaculture] FUNGI AND MYCORRHIZAE FREQUENTLY ASKED QUESTIONS: Dr. James Traquair, Lawrence London, 02/12/2013

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