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MYCORRHIZAE 
(Nitrogen producing bacteria)

            

Close up photo of Mycorrhizae inoculated root system. The fine hairs are generated by the bacterium

MYCORRHIZAE 

All species of trees growing under normal soil conditions in the forest are dual organisms-part plant and part root-inhabiting fungi. These highly specialized, root-inhabiting fungi are not pathogenic and, thus, do not cause root disease. They are beneficial to their tree hosts. They infect, symbiotically, the cortical tissues of young feeder roots of trees and form mycorrhizae. The word mycorrhizae means "fungus-roots." Mycorrhizae are the normal feeder roots of all forest trees, as they are for the vast majority of other plant species, including agronomic, horticultural, ornamental, and turf crops.

Classes of Mycorrhizae
The three classes of mycorrhizae are identified by the arrangement of the hyphae of the fungi within root cortical tissues. Ectomycorrhizae.-This class occurs normally on roots of pine, spruce, fir, beech, birch, eucalyptus, alder, oaks, hickories, and certain other trees. Ectomycorrhizae usually can be distinguished from nonmycorrhizal roots because of their swollen appearance and in the case of Pinus, a forking habit.

Not all forked feeder roots of pine are ectomycorrhizal, however, since feeder roots of many species can be stimulated to fork by many factors other than ectomycorrhizal fungus infection. The fungi which form ectomycorrhizae are among those that mainly produce mushrooms and puffballs periodically throughout the year.

Over 2,400 species of ectomycorrhizal fungi are found in North America alone.Under microscopic examination, hyphae of ectomycorrhizal fungi can be observed growing internally only around the cortical cells of the root forming the Hartig net, thus the prefix ecto. The Hartig net hyphal arrangement appears to replace the middle lamella, which is normally composed of pectins, and cements the cortical cells. Hyphae of the fungal symbionts that are present around the outside of the feeder root usually are tightly interwoven, and are designated the fungus mantle. The mantle of ectomycorrhizae may be only one or two hyphal diameters in thickness, or several dozen hyphal diameters. Ectomycorrhizae may be white, brown, yellow, black, blue, or blends of these colors. The color of ectomycorrhizae is apparently determined by  the hyphal color of the specific mycorrhizal fungus encompassing the root.Endomycorrhizae.-This class occurs normally on roots of maple, sycamore, ash, sweetgum, walnut, cypress, cedar, some poplar, and certain other hardwoods. Endomycorrhizae cannot be readily distinguished from nonmycorrhizal roots without the aid of a microscope. Endomycorrhizal infection does not normally cause color or physical changes in the root. The fungi which form endomycorrhizae are different from those of ectomycorrhizae and lack conspicuous aboveground fruiting bodies.Most endomycorrhizal fungi of forest trees apparently belong to the genus Endogone. Dozens of species of endomycorrhizal fungi have been identified; undoubtedly many more exist. Under microscopic examination, hyphae of the endomycorrhizal fungi are observed growing into (thus, prefix endo) the cortical cells of the roots forming arbuscules. Arbuscules are specialized absorbing hyphae or haustoria of these fungal symbionts. Large vesicles, which are swollen hyphae, may also be seen in endomycorrhizal roots. If both vesicles and arbuscules are present, the endomycorrhizae are called "vesicular-arbuscular" (VA) mycorrhizae. The Hartig net and fungus mantle which characterize ectomycorrhizae are not present in endomycorrhizae. Endomycorrhizal fungi will frequently produce large, conspicuous, pearlcolored spores (either zygospores or chlamydospores) on hyphae attached to 
endomycorrhizal roots. The presence of these spores on root surfaces, which are detectable under low magnification, is a reasonably reliable indication of endomycorrhizal fungus infection of the root.

Ectendomycorrhizae.-This class is apparently an intermediate between the other two classes. These fungi grow into the cortical cells of the root with an appearance quite different from the arbuscular formation of endomycorrhizae. These fungi grow around the cortical cells in a Hartig net arrangement. They may or may not develop a fungus mantle over the feeder root surfaces. Ectendomycorrhizae have been observed on roots of certain species of tree seedlings in nurseries, especially in the Pacific Northwest and Northeast, but their significance is unknown. The species of fungi 
which form this class of mycorrhizae are also unknown at present.

 

Benefits of Mycorrhizae to Trees
A considerable amount of research has been done on the benefits of ectomycorrhizae to trees . Only limited research has been done on the importance of endomycorrhizae to plant growth, and research on benefits of ectendomycorrhizae to trees is nearly nil. The following list of benefits of mycorrhizae to tree growth is based almost exclusively on work with ectomycorrhizae ; however certain of these benefits are thought to be derived from endomycorrhizae as well.
1. Tremendous physical increase in absorbing surface of root system; includes both mycorrhizae and hyphae growing from mycorrhizae into soil.
2. More selective ion absorption and accumulation, especially phosphorus.
3. Solubilization of normally nonsoluble minerals and their constituents.
4. Increased longevity of feeder root function; mycorrhizal roots persist longer on root systems than do nonmycorrhizal roots.
5. Resistant to feeder root infections caused by pathogens, such as Phytophthora and Pythium spp., present in many forest and nursery soils. This benefit apparently applies only to ectomycorrhizae.
6. Increased tree tolerance to soil toxins (inorganic and organic), extremes of soil acidity, and high soil temperatures.

General Considerations
1. A given tree species may enter into mycorrhizal association with one or many different species of mycorrhizal fungi at a given time.
2. Some species of mycorrhizal fungi have very broad treehost ranges, whereas, others have very narrow host ranges.
3. Some species of mycorrhizal fungi are more beneficial to tree survival and growth than others.
4. Some trees, in particular Pinus, have an obligate need for mycorrhizae in order to survive. This may not be true for all tree species, even though mycorrhizae occur normally on their roots.
5. Certain mycorrhizal fungi are more ecologically adapted to certain forest sites than are other fungi; trees with adapted fungal symbionts on roots grow better than trees with nonadapted fungal symbionts growing on the same site.
6. Spores of ectomycorrhizal fungi are produced above ground and are readily wind disseminated. Spores of endomycorrhizal fungi are produced underground and are not wind disseminated.
7. Many species of ectomycorrhizal fungi can be grown in pure culture or artificial media. Endomycorrhizal fungi cannot be grown in pure culture in the absence of their plant hosts.
8. Mycorrhizal fungi rarely exist in an active physiological state in soil in the absence of their hosts. However, they may remain in a dormant condition, as spores or resistant hyphae, in soil for many years without a tree host.
9. Not all species of mushrooms or puffballs are ectomycorrhizal. Many are saprophytic in nature and decompose organic matter.

Practical Considerations
Soil fumigation is becoming more and more a routine practice in tree nurseries to control weeds, nematodes, and fungal root pathogens. Most soil fumigants also eradicate the beneficial mycorrhizal fungi. The eradication of ectomycorrhizal fungi from nursery soil is usually not a problem since these fungi produce wind-disseminated spores periodically throughout the year which recolonize the soil. One of the major ectomycorrhizal fungi to colonize fumigated nursery soils, especially in the south, is Thelephora terrestris.

Deficiencies of ectomycorrhizal fungi in previously fumigated nursery soils have been reported. Colonization of soil from airborne spores did not occur in these instances. Deficiencies could be due to unfavorable weather conditions for mushroom production in forests adjacent to the nursery. Spores from these mushrooms serve as inoculum for soil colonization. Also, the nursery may be too far from forests harboring specific mushrooms. Research is currently underway to determine the feasibility and practical value of artificially infesting nursery soils with specific ectomycorrhizal fungi known to be more beneficial to tree survival and growth than those ectomycorrhizal fungi which naturally occur on roots from airborne inoculum.
The consequence of soil fumigation to endomycorrhizal fungi is another problem. Once these symbiotic fungi have been eradicated from soil, reinfestation is very slow because their spores are not normally wind disseminated. Therefore, deficiencies of these fungi in nursery soil following fumigation are not unusual. Endomycorrhizal development following successful soil fumigation is from inoculum of the symbionts (1) still viable in soil depths beyond effective fumigant penetration, (2) washed in from water runoff or heavy rain splash from nonfumigated areas of soil, (3) brought in by 
nonfumigated soil on cultivation equipment, or (4) possibly from windblown nonfumigated soil. Each of these possibilities would result in a slow development of endomycorrhizae.

Considerable research has demonstrated that ectomycorrhizae are not only beneficial to growth of trees, but are actually indispensable for survival and growth of transplant stock, in particular Pinus spp. Thus, to insure survival and good growth of normally ectomycorrhizal trees, seedlings used in reforestation should have abundant ectomycorrhizae. It is not known if this statement can be applied to endomycorrhizae. The significance of endomycorrhizae to survival and growth of several species of hardwood seedlings is currently being investigated.

Selected References
Clark, F. Bryan.
1969. Endotrophic mycorrhizal infection of tree seedlings with Endogone spores. For. 
Sci. 15:134-137.

Hacskaylo, Edward.
1971. Mycorrhizae. U.S. Dep. Agric. Misc. Publ. 1189, 255 p. Jorgensen, J. R., and 

Eugene Shoulders.
1967. Mycorrhizal root development vital to survival of slash pine nursery stock. Tree 
Planters' Notes 18 (2) :7-11.

Kleinschmidt, G. D., and J. W. Gerdemann.
1972. Stunting of citrus seedlings in fumigated nursery soils related to the absence of 
endomycorrhizae. Phytopathology 62:1447-1453.

Marx, Donald H., and W. Craig Bryan.
1971. Influence of ectomycorrhizae on survival and growth of aseptic seedlings of 
loblolly pine at high temperature. For. Sci. 17:37-41. Shoulders, Eugene, and J. R. 

Jorgensen.
1969. Mycorrhizae increase field survival of planted loblolly pine. Tree Planters' Notes 
20 (1) :14-17.