MYCORRHIZAE: FACTS AND FALLACIES - Arboriculture & Urban Forestry

Journal of Arboriculture 6(8): August 1980 213

MYCORRHIZAE: FACTS AND FALLACIESby J.G. Iyer, R.B. Corey and S.A. Wilde

Abstract. This paper points out the ephemeral nature ofmycorrhizal short roots, influenced in their development byradiation, soil drainage, and eradicants. Emphasis is placed onrhizospheric and extramatrical mycelia as agents enrichingsoils in enzymes, chelates, and available nutrients. Inocula-tions of soil, seed, or stock with mycorrhiza-forming fungiwithout ascertaining the supply of major and minor nutrientsand the soil's freedom from toxic substances will inflict finan-cial losses and undermine the program. Inoculation of biocide-impaired nursery soils usually necessitates addition of suitableenergy material and adjustment of soil fertility. Suggestions aremade for clarification of conceptual discrepancies relevant totree-fungus symbiosis.

The symbiosis of tree and fungi is one of themost important relationships in the realm of plantphysiology, especially in tree nutrition. As hasbeen repeatedly demonstrated, seed of nearly alltree species planted in grassland soils, includingthose of very high fertility, usually yield single-whorl seedlings which in one or two years die ex-hibiting signs of starvation. However, addition tothe grassland soil of a minute amount of forest soilcontaining fungal symbionts of trees induces amost spectacular change: the starving, nearlydead seedlings recover from their lethargy and at-tain normal, often vigorous growth (Fig. 1).

The symbiosis of trees and fungi has been in-vestigated for over one hundred years. Yet, thesestudies have left many misinterpretations. This isparticularly true of investigations conducted in ar-tificial environments with little attention to soils andvegetative cover, especially its density. A very un-fortunate disadvantage for many researchers ofmycorrhizae was and still is their remoteness fromsoils of indigenous grasslands, the medium whichinfallibly reveals the presence or the absence ofsymbionts essential for the vast majority of treesand other lignophytes. Many years' studies of thesilvicultural importance of mycorrhizae, withWisconsin prairie soils as a test medium, permit usto suggest essential corrections and simplifica-tions of some commonly held concepts. All ofthese were established on the basis of repeatedexperiments and all of them can be verified by sim-ple trials. We stress this point because some of

our findings are in discord with credos expressedin recent literature.Distribution of mycorrhiza-forming fungi

The fungal symbionts essential for the large ma-jority of tree species, i.e., fungi of ectocellular andepirhizal development, are absent in prairie andother grassland soils which never supported trees(Wilde, 1958). Such soils are often located in theimmediate neighborhood of forest stands andreceive wind-blown spores of mycorrhizal fungi;yet they remain treeless because, in the absenceof tree roots, mycorrhizal spores undergo the fateof unfertilized eggs. However, once established,mycorrhizal fungi preserve their symbiotic poten-tial for a very long, indefinite period. Grasslandsestablished on previously forested soils are notdeficient in tree symbionts.

All forested or previously forested soils in allgeographic regions, including soils subjected tosevere fires, grazing, or cultivation, harbor fungi-forming mycorrhizae with all tree species. Our

Fig. 1. Effect of inoculation of a prairie soil withmycorrhiza-forming fungi on the growth of 6-month-oldseedlings of Monterey pine, Pinus radiata. A-Carrington(Piano) prairie silt loam; B-The same soil with addition of0.2% by volume of humus-enriched horizon of Plainfieldsand, a forest soil supporting jack pine and harboringmycorrhiza-forming fungi Cenococcum gran/forme andBoletus luteus; C-Plainfield sand (After Wilde, 1968).

214 lyeretal: Mycorrhizae

studies revealed them in Wisconsin bogmossswamps, sand dunes, and soils which producedfarm crops for a period as long as 60 years(Rosendahl and Wilde, 1942). According to thelate Eric Bjorkman (1950), mycorrhizal fungi arepresent in Swedish soils which were under cultiva-tion longer than a century. With the exception ofsoils subjected to prolonged inundation, such asthose of abandoned beaver flowages (Wilde et al.,1950), there is probably not a single square footof forested or previously forested soil in the worldfree from endocellular, ectocellular and epirhizalmycorrhiza-forming fungi essential for trees andother lignophytes. By and large, the statement,once forest soil, forever forest soil, i.e., soil har-boring mycorrhiza-forming fungi (Wilde, 1954),preserves its validity. And outside of hydroponicsand nutrient-enriched greenhouse sand cultures,there are no "nonmycorrhizal" or "autotrophic"trees.

The almost incredible perseverence of fungiessential for tree growth is best illustrated by thepresent distribution of vegetation in Wisconsinand Minnesota. In spite of clearcut logging,severe fires, and nutrient-depleted lands of aban-doned farms, all previously forested soils thatescaped man's interference within the last 70years now support naturally established foreststands. On the other hand, neighboring prairiesoils remain treeless as they were during themany past centuries. The encroachment of forestupon prairie is largely limited to the narrow borderstrip penetrated by lateral roots of trees.

Inoculations with mycorrhiza-forming fungiIntroduction of mycorrhiza-forming fungi into

grassland soils or soils scalped by grading doesnot present any particular difficulties. The firstsuccessful, large scale inoculation of chernozemsoils, free from the essential tree symbionts, wasaccomplished about 80 years ago by Wissotsky(1902) who sprayed oak acorns with a thicksuspension of forest topsoil. The same method isstill the simplest and least expensive procedurefor afforestation of grasslands by direct seeding.And it can be accomplished by the use of anyforest soil. We achieved normal growth of manyseed-planted coniferous and deciduous trees on

prairie soils of Wisconsin by addition of a fractionof one percent of surface soil obtained from NewZealand, India, Italy, even Alaska permafrost andWisconsin weathered limestone outcrops, or byspraying the seed with suspensions of these soils(Wilde, 1968).

Inoculation of grassland soils for production oftree nursery stock can be achieved by seedsprays, application of leafmold and humus-enriched surface soil from healthy forest stands,or by using the area during one or two years fortransplanting seedlings obtained from a nurseryproducing healthy stock of a similar either ec-tocellular or endocellular mycotrophic make-up(Iyer, 1978b). Inoculation of hydroponic trees in-tended for planting on grasslands is accomplishedby hilling them in any forest topsoil for a period of2 or 3 weeks (Fig. 2). Planting of nursery-produced trees on grassland soils requires no in-oculation with mycorrhizal fungi because there isno nursery stock of plantable size free frommycorrhizae of either ectocellular, endocellular, orepirhizal morphology.

A

Fig. 2. Nine-month-old seedlings of white pine, Pinusstrobus, raised for 2 months in a sterile nutrient solutionand then transplanted to Carrington (Piano) prairie siltloam: A-seedlings transplanted directed from hydroponicsto prairie soil; B-seedlings inoculated with mycorrhizalfungi by hilling them for 2 weeks in Plainfield forest sandprior to their transplanting to prairie soil.

In recent years, successful inoculations in theU.S.A. have been achieved with pure cultures of

Journal of Arboriculture 6(8): August 1 980 215

mycorrhiza-forming fungi, such as Pisolithus tine-torius and Thelephora terrestris (Marx et al.,1978). However, the success of pure culture in-oculations is dependent upon climatic conditionsand compatibility of the exotic symbiont withnative microorganisms, including mycorrhizaformers. In some instances, according to Marx etal. (lit. cit.), the pure culture inoculations are moresuccessful following destruction of nativemicroflora by fumigation. Moreover, successful in-troduction of a particularly effective symbiont, orany mycorrhiza-former into biocide-impairednursery soil often necessitates addition of suitableenergy material and adjustment of soil fertility re-quired not only by the trees but also by the sym-biont being introduced. In some cases, restorationof mycotrophy in eradicant-inhibited nursery soilswas accomplished by application of fibrous paper-mill sludge and a nutrient-enriched suspension ofleafmold (Iyer and Oilschlager, 1977).

An important detail pertinent to pure culture in-oculations is that not all fungal symbionts of treesare wholly beneficial organisms. Thelephora ter-restris, a fungus used recently for inoculation ofnursery soils, forms large superficial blankets inforests eliminating all other plants by either toxicor antibiotic excretions. Thelephora laciniata, aclose relative of Th. terrestris, is well known toEuropean foresters as the most noxious of"smothering" fungi damaging and even killingnursery stock by enveloping stems of seedlings(Kavina, 1923).

Of equal importance is that mycorrhiza-formingfungi are not nitrogen-fixing organisms and cannotaugment soil fertility. Hence, the introduction of asymbiont, regardless of its inherent effectiveness,can only promote utilization of the soil productivepotential, but not increase it. No inoculation willpromote the growth of trees on purely quartziticsand, acid muskegs, soils lacking any essentialmajor or minor nutrient, or soil containing toxicsubstances. The latter are of particular importancein industrial waste beds. Without a precedingthorough soil analysis, promiscuous inoculationwith tree symbionts may result in waste of fundsand undermine the entire program.

Specificity of fungal symbiontsUnder natural conditions, existence of trees and

other lignophytes is similar to that of lichens; itdepends upon their inseparable union with sym-biotic fungi. This union is foreign to segregation; atree of any origin finds an acceptable fungalassociate in any forested or previously forestedsoil not subjected to a prolonged inundation. Anda distance of several thousand miles between bir-thplaces of the two symbionts presents noobstacle to their union. As was stated, we achiev-ed normal growth of many seed-originated con-iferous and deciduous trees on prairie soils by ad-dition of a minute amount of surface soils obtainedfrom distant parts of the world and thusdemonstrated that specificity of mycorrhizal sym-bionts is a chimera. New Zealand provides thebest illustration of the validity of this thesis. Bothof its islands are dotted with exotic trees fromEurope grown by the settlers from seed on soilswhich previously supported Nothofagus and othertrees unknown in the rest of the world. The in-troduction of mycorrhizal symbionts was essentialin New Zealand only for soils of grasslands.

Mycorrhizal short rootsIn one form of tree-fungus symbiosis, fungal

hyphae penetrate cortical tissues and produce, onthe surface of roots, wartlike mycelial offshoots,named mycorrhizae or mycelial short roots. Thesemycelial appendages present an obvious displayof the existing root-fungus union and they receiv-ed a great deal of attention by the investigators ofmycorrhizae. This concentrated effort may well bequestioned because of the ephemeral, merely in-cidental nature of short roots.

Short roots are largely accessories of treeswhich receive a large supply of radiant energy,and their abundance is confined to trees ofnurseries, greenhouses, and open stands.Shading by a dense canopy drastically reducesand even completely suppresses the develop-ment of mycorrhizal short roots (Fassi, 1967).

To demonstrate to our students the imaginaryimportance of the short roots in the growth ofnatural stands, we conducted at different timesduring the growing season, repeated, careful ex-cavations of 3 to 10-year-old seedlings of redpine, Pinus resinosa,and white pine, P. strobus.Most of these excavations were performed in thewell-stocked, virgin stand of red and white pines,

216 Iyer et al: Mycorrhizae

located in the University of Wisconsin Finnerudstudy area in Oneida County. With practically noexceptions, excavated trees revealed no mycor-rhizal short roots, but only superficial mycelia andmycelia-agglutinated terminal clusters (Fig. 3).Identical root morphology was disclosed innaturally reproduced seedlings of dense stands ofponderosa pine, Pinus ponderosa, and Douglasfir, Pseudotsuga menziesii, of Idaho and Oregon(Benson and Iyer, 1978).

Fig. 3. Roots of naturally reproduced seedlings of redpine, Pinus resinosa, and white pine, Pinus strobus, ex-hibiting grumose, mycelia-agglutinated clusters, but nomycorrhizal short roots. Randomly excavated trees from asoil supporting a dense, virgin stand of red and white pinesin the Finnerud study area of the University of Wisconsin inOneida County, Wl.

The influence of radiation on the development ofshort roots is also revealed conspicuously by theirrelative abundance in nursery stock of differentages and corresponding density. The maximumoccurrence of short roots is usually in 1 -year-oldseedlings exposed to radiation, and the minimumin fully-stocked 3-year-old seedlings, the foliageof which receives only a small fraction of fullsunlight (Iyer, 1978a). Particularly great dif-ferences in the abundance of short roots are alsoobserved in nursery stock located on the borderand in the center of nursery beds (Fig. 4).

Disappearance or radical reduction of mycor-rhizal short roots is also prevalent in dense forestplantations. In the course of the Wisconsin

statewide survey of coniferous plantations (Wildeet al., 1965), jet excavations of many red andwhite pine sample trees revealed no, or nearabsence of short roots (Wilde and Iyer, 1962;Wilde, 1967). All our observations left the impres-sion that mycorrhizal short roots are symptoms ofa measles-like disease, inflicted by exposure tofull sunlight.

Fig. 4. Effect of radiation on the development of mycor-rhizal short roots of 2-year-old seedlings of red and whitepines. A and C: mycorrhiza-endowed roots of trees liftedfrom the borders of nursery beds; B and D: mycorrhiza-deficient roots of trees lifted from the center of nurserybeds. Wilson state nursery of Wisconsin (After Iyer, 1978).

Epirhizal mycelia and terminalmycelial clusters

While the past investigations of mycorrhizaedevoted the lion's share of attention to mycor-rhizal short roots, they have largely neglected thecomposition and effects of mycelia which coverthe roots with mantles, mycochlenes (Peyronel,1922), and pellicles, mycochlamides, and myceliawhich form agglutinated, terminal clusters,mycoplasts (Wilde and Lafond, 1967). The sym-biotic or mycotrophic nature of superficial and ter-minal mycelia cannot be doubted. Their verygrowth on the surface and at the tips of rootstestifies that they are beneficiaries of trees,receiving from them absolutely essential andotherwise unprocurable carbohydrates. In turn,these mycelia are benefactors of their host plantsfor they enrich the soil in enzymatic and chelatingcompounds effecting the solubilization of availablenutrients (Leaf, 1957; Spyridakis et al., 1967;

Journal of Arboriculture 6(8): August 1 980 217

Voigt, 1969; Wilde et al., 1978). The sym-biotrophic effect of epirhizal and terminal myceliais convincingly revealed by trees of closedcanopy deprived of direct sunlight and producingno mycorrhizal short roots. In this connection, itshould not be forgotten that some, perhaps agreat many, "contact" fungi such as Trichodermaviride produce, by their mere presence in theproximity of tree roots, the same growth effectsas do root-penetrating fungi (Wilde et al., 1956).Trichoderma also provides a very instructive il-lustration of the antiseptic role of epirhizal mycelia;its excretions of antibiotic substances eliminatenearly all other soil microorganisms (Yatazawa etal., 1960).

Among the variants of epigenous mycelia, ter-minal clusters, or mycoplasts, provide the bestopportunity for detailed investigations. This formof rhizospheric union of fungi and tree roots per-mits its easy isolation, microscopic examination,and analysis. The previous neglect of theseessential components of tree roots has beenlargely due to their fragile nature and very weakconnection with the roots. In lifting trees, theclusters are usually broken from the fine rootletsand left in the ground, or detached from the rootsby shaking or washing in an effort to removeadhering soil particles.

Depending on the nature of the soil, the roundor oval mycelia-agglutinated clusters vary in sizefrom a few mm to about 3 cm (Fig. 5). They con-sist of an intimate aggregation of soil particles andfine rootlets enmeshed in a network of fungalmycelia. Mycroscopic examination often revealsfragments of silicate minerals in a state of ad-vanced weathering. The clusters formed withinthe soil layers enriched in humus include finelydispersed organic matter (Fig. 6).

Analysis of roots and their terminal clusters of3-to 7-year-old seedlings of white and red pinesincluded determinations of catalytic potential,reaction, exchange capacity, and exchangeablebases (Wilde et al., 1979). These analyses show-ed that the enzymatic content of the active frac-tion of clusters is about 20 times as great as thatof their parent rootlets. The clusters also revealedabout four times as great a cation exchangecapacity and a much higher content of ex-

changeable cations in comparison with surround-ing soils (Wilde et al., 1978).

These results suggest the following conclu-dions. The high concentration of enzymaticsubstances in the terminal clusters testifies to ahigh biological activity. The growth ofmicroorganisms, predominantly mycelia of non-parasitic fungi, in these root appendages isdependent on the availability of otherwise un-

Fig. 5. Mycelia-agglutinated rhizospheric root clusters ormycoplasts of a naturally reproduced 5-year-old white pineseedling.

Fig. 6. Cross-section of a mycelial root cluster compris-ing an aggregate of rootlets, fungal mycelia, soil particles,and dispersed organic matter (10 x).

218 lyeretal: Mycorrhizae

procurable carbohydrates from the host plant. Theobserved production of organic colloids and thesubsequent concentration in the vicinity of theroots of exchangeable cations indicates agradual release of these nutrients in accordancewith requirements of the host plant. Under condi-tions of dense forest cover, mycelial clusters mostlikely perform the function that mycorrhizal shortroots perform under high light intensity. It isprobable that mycelia of the clusters exert anaseptic influence by release of antibioticsubstances.

The other important factor in the production ofenzymes, chelates, and available nutrients is theactivity of extramatrical mycelia. Their enormousnetwork extends through the entire area of rootdistribution and, at times, produces so-called"fairy rings" of mushrooms. Recent investigationshave disclosed that external mycelia of graftedroots of stumps continue their delivery of nutrientsto trees spared in partial cuttings (Dosen and Iyer,1979).

A tree lifted from the ground is usually only apart of an organism because an essential compo-nent, the nourishing mycelium is left in the soil.

Mycorrhizal short roots and thequality of nursery stock

The notion that mycotropic capacity and thevalue of nursery stock are expressed by therelative abundance of mycorrhizal short roots iserroneous. Only in some instances does the quan-tity of short roots serve as an indicator of thenursery stock quality; in other instances, theabundance of short roots is of questionable or nosignificance. Nursery stock with superabundantshort roots may have a very low performancepotential; vice-versa, stock with sparse shortroots may be excellent planting material.

During recent years, suggestions have beenmade that high fertility of nursery soils depressesthe development of mycorrhizal short roots andlowers the survival of outplanted trees. In conse-quence, attempts have been made to record theroot morphology of nursery stock by countingshort roots and expressing their abundance aspercentages of feeder roots.

As was reported by Benson and Iyer (1978),

under conditions of an accepted density, 2-year-old nursery seedlings of ponderosa and lodgepolepine, Douglas fir, and Engelmann spruce exhibitedadequate development of short roots in spite of anexceptionally high level of soil fertility including400 lbs/a of available phosphorus pentoxide and500 lbs/a of available potassium. By and large, areduction in the abundance of short roots is not ef-fected by high soil fertility, but by the density ofnursery beds, correspondingly diminished radia-tion of the foliage, and by phytotoxicity of potenteradicants.

Counting short roots, especially if preceded bystaining, is a tedious and costly procedure and thepercentages are not acceptable values in this typeof analysis; they are likely to provide a totallymisconstrued picture of root morphology andmisleading information on the performance poten-tial of planting stock.

In appraisals of nursery stock quality, therelative abundance of short roots may havesignificance only as a supplement to more impor-tant attributes of trees, such as color of thefoliage, root-top and height-diameter ratios,specific gravity of stems, and foliar composition.In rapid surveys, the development of short rootscan be recorded in accordance with the followingscale: superabundant-3, abundant-2, sparse-1,absent-0. In more exacting appraisals, the relativeabundance of short roots along with other formsof mycorrhizal fungi can be rapidly estimated onthe basis of the enzymatic content of rootletssmaller than 2 mm in diameter by manometricanalyses (Wilde et al., 1979). These analysesreflect the relative abundance of short roots andalso provide information on the biological activityof the nutritionally most effective part of the rootsystems of trees.

According to the results obtained in a survey ofstock produced in five large nurseries of the LakeStates region (Iyer, 1978a), the catalytic poten-tials in fully-stocked jack, red and white pines, andwhite spruce were expressed by the followingaverages: one-year-old seedlings, 55± 9.1 mmHg/g; two-year-old seedlings, 42 ± 8.5 mmHg/g; three-year-old seedlings, 26 ± 7.2 mmHg/g. These averages indicate a linear relation-ship between catalytic potential and age with a

Journal of Arboriculture 6(8): August 1980 219

standard error of estimate in the proximity of 8 mmHg/g. A similar relationship was noted for in-cidence of short roots. Figure 7 illustrates thesignificance of the catalytic potential by featuringthe extremes encountered in the development ofroots of nursery stock. Experience in determina-tion of catalytic potential may help nurserymen ap-praise the direct and mycotrophic capabities ofroots by ocular estimates.

Fig. 7. Catalytic potential of roots of 3-year-old white pineseedlings. A-seedlings with inadequately developed roots;catalytic potential of 12 mm Hg; B-seedlings with abundantdevelopment of feeder rootlets and mycorrhizal short root;Cp of 63 mm Hg.

Mycorrhiza and eradicantsThe great concern about the effect of high fer-

tility of nursery soils on the development ofmycorrhizae is rather out of place at this timewhen nearly all nursery soils are treated with po-tent, often destructive biocides (Iyer, 1963; Iyeret al., 1969). Some of these chemicals at largeconcentration, or partial detoxification, eradicteboth ectocellular and rhizospheric symbionts anddepress the growth of trees (Fig. 8). Such effectsmay be particularly expected in soils with impededdrainage (Morby et al., 1978).

On the other hand, when eradicants are appliedat reduced concentration, they may still annihilatemycorrhizal short roots and yet induce an ex-uberant, highly harmful growth stimulation of treecrowns and impart to nursery stock calamitoustop-root ratios (Fig. 9) and succulent tissues (Iyerand Wilde, 1965).

The destruction of mycorrhizae, or the entire

root system of young seedlings, may at times becaused not only by toxic chemicals, but also bybiotic eradicants, such as green manure ofsorghum-Sudan hybrids, containing hydrocyanicacid (Fig. 10).

Under present conditions, nursery stock pro-duction must be concerned with the developmentof all roots, be they short, long, or intermediate,and also with a number of other external and inter-

Fig. 8. Red pine seedlings, in the second year of theirgrowth, raised in depressions with highly concentratedMylone (DMTT) herbicide (A); normally developed seed-lings on elevated parts of the same nursery bed (B), ex-hibiting initial development of mycorrhizal short roots (Grif-fith state nursery of Wisconsin).

Fig. 9. Three-year-old red pine seedlings raised ineradicant-free soil (A) and in soil that received 60 lbs/a ofVapam (SMDC) fumigant (B). Note abnormally large crowns,but drastically reduced roots (Hayward state nursery ofWisconsin).

220 Iyer et al: Micorrhizae

nal attributes of trees (Wilde et al., 1 979).

B

Fig. 10. Effect of sorghum-Sudan green manure on thegrowth of 9-week-old white pine seedlings. A-normallydeveloped seedlings raised in untreated sandy soil;B-seedlings raised in a similar soil treated with greenmanure of sorghum-Sudan hybrids and exhibiting "burn-ing" of roots similar to that produced by lead arsenate.

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AcknowledgmentResearch supported by the College of Agricultural and Life

Sciences, University of Wisconsin-Madison, and the Wiscon-sin Department of Natural Resources.

Lecturer, Professor, and Professor Emeritus,Department of Soil Science,University of WisconsinMadison, Wisconsin 53 706