permaculture

[Lawrence London <lfljvenaura@gmail.com>]

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]