The influence of vesicular arbuscular mycorrhizae on disease development

Material Information

The influence of vesicular arbuscular mycorrhizae on disease development
Series Title:
Bulletin University of Florida. Agricultural Experiment Station
Schenck, N. C ( Norman Carl ), 1928-
Kellam, Mary Katherine, 1953-
Place of Publication:
Gainesville Fla
Agricultural Experiment Stations, Institute of Food and Agricultural Sciences, University of Florida
Publication Date:
Physical Description:
16 p. : ; 23 cm.


Subjects / Keywords:
Mycorrhizas ( lcsh )
Phytopathogenic microorganisms ( lcsh )
Plant diseases ( lcsh )
Plant roots ( jstor )
Mycorrhizal fungi ( jstor )
Fungi ( jstor )
bibliography ( marcgt )
non-fiction ( marcgt )


Bibliography: p. 13-16.
General Note:
Cover title.
Bulletin (University of Florida. Agricultural Experiment Station) ;
Statement of Responsibility:
M.C. Schenck and M.K. Kellam.

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Source Institution:
Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location:
Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
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Resource Identifier:
027017334 ( ALEPH )
18435127 ( OCLC )
AEP0695 ( NOTIS )


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Copyright 2005, Board of Trustees, University
of Florida

RuII~iin 798 I r~chnc~I) Orsober 1978


N. C. Schenck and M. K. Kellam

Agricultural Exoerimeni Salaorns
Institute of Food and Agricultural Sciences
University' of Florida, Gainesville
F. A. Wood, Dean for Research

Bulleitn 798

October 1978

0001100 F

Cover: Spores of the two genera of vesicular abuscular mycor-
rhizal fungi used in interaction studies with plant pathogens.
Upper: Glomus. Lower: Gigaspora.

N. C. Schenck and M. K. Kellam

Professor and Graduate Assistant, respectively
Department of Plant Pathology
University of Florida
Gainesville, Florida

This public document was promulgated at an annual cost of
$1,613.78 or a cost of 40 per copy to provide information
on current work on mycorrhizae and disease development.


Introduction ............................................... 1
Mycorrhizal fungi as plant pathogens? ................... 1
Interactions with Plant Pathogens ........................... 2
Fungal plant pathogens ................................. 2
Plant parasitic nematodes .............................. 6
Viral plant pathogens ................................. 8
Bacterial plant pathogens ............................... 9
Discussion .............................................. 9
Table 1 ................................................. 10
Literature Cited ......................................... 12

Vesicular arbuscular (VA) mycorrhizal fungi were observed
on the roots of plants before the turn of the 20th century (16).
However, research on VA mycorrhizal fungi advanced only after
the development of the pot culture technique for maintaining
these fungi and the wet-sieving and decanting technique for ex-
tracting their spores from soil (15, 17, 31). Most studies in-
volving these fungi have dealt with their effects on plant growth
and nutrient absorption (33). Only within the last ten years has
any deliberate research been reported on the influence of VA
mycorrhizal fungi on plant diseases. The purpose of this bulletin
is to review the literature pertaining to the interaction of VA
mycorrhiza and plant pathogens.

Mycorrhizal fungi as plant pathogens?
Before reviewing interactions of mycorrhizal fungi with
plant pathogens, it should be noted that not all research work-
ers encountering mycorrhizal fungi in the past considered them
beneficial symbionts. Some plant pathologists were concerned
that these fungi actually might be causal agents of disease. Jones
(20) stated that VA mycorrhizal fungi were properly within
the plant pathologists' field of study because of the "unmistak-
able local pathological conditions" he observed on mycorrhizal
plants. Winter (56) suggested that a heavy infestation of my-
corrhizae on a plant usually was associated with poor plant de-
velopment. Wilhelm (54) considered the organism which formed
abundant vesicles and chlamydospores, which he observed in
older strawberry roots, to be the cause of early root senescence.
Maloy (27) noted the regular occurrence of VA mycorrhizal
fungi in roots of bean affected with Fusarium root rot. Deal et al.
(12) investigated the relationship of VA mycorrhizal fungi to
the grape replant disease in New York, and Nemec (34) studied
the involvement of these fungi in strawberry root rot. Deal et al.
(12) suggested that there was some question in the grape re-
plant disease whether or not mycorrhizal fungi were parasitic
or symbiotic. Nemec (34) indicated mycorrhiza were not direct-
ly or indirectly related to strawberry root rot.
Some results clearly indicate that VA mycorrhizal fungi can
cause reduced yields, indicating they may be pathogenic under
certain circumstances. In a greenhouse test, Joseph (21) ob-
tained a significant reduction in shoot and root fresh weight of
maize (Zea mays L.) with Glomus mosseae (Nicol. & Gerd.)

Gerd. & Trappe as compared to nonmycorrhizal maize. In addi-
tion, mycorrhizal maize plants produced more ethylene and had
greater respiration rates and greater glucosidase and peroxidase
activity than nonmycorrhizal maize plants. Mosse (32) reported
large differences in yield between mycorrhizal and nonmycor-
rhizal onion plants in high phosphorus soils. She indicated that
the reduced yields associated with mycorrhizal onions resulted
from phosphorus toxicity caused by the increased ability of my-
corrhizal plants to absorb phosphorus. A similar phenomenon
was observed in the field on soybean (Glycine max (L.) Merr.),
maize and wheat (Triticum aestivum L.) (22, 23, 40). Crush (9)
reported a statistically significant reduction in dry matter pro-
duction by alsike clover (Trifolium hybridum L.) and alfalfa
(Medicago sativa L. 'Wairau') when colonized by the mycor-
rhizal fungus Acaulospora laevis Gerd. & Trappe. He termed
this reduction a "distinct parasitic effect" and supported the
tenet that VA mycorrhizal fungi can be detrimental. Tinker
(52) proposed that yield depressions may result from factors
other than phosphorus toxicity and suggested that demands by
the fungus for photosynthates from the plant might be involved.
In this regard, two separate studies (14, 44) determined that
mycorrhizal plants grew less than nonmycorrhizal plants when
exposed to unfavorably low temperatures. At low temperatures,
the balance between photosynthate production by the plant and
the carbohydrate demand of the fungus may have been tipped
in favor of the fungus.

Although VA mycorrhizal fungi in association with diseased
roots had long been noted, the first reported attempt to specifi-
cally study the interaction of a plant pathogen and a species of
VA mycorrhizal fungus was that of Safir (42). Since his report
there have been over 25 studies of interactions between VA my-
corrhizae and plant pathogens.

Fungal plant pathogens
Most of the studies on the effect of VA mycorrhizae on plant
disease have been with fungal pathogens. Ross (41) was the first
to report a VA mycorrhizal fungus (Glomus macrocarpus var.
geosporus [Nicol. & Gerd.] Gerd. & Trappe) which increased
the severity of a plant disease. Nearly 90% of the susceptible
soybean (Glycine max) plants with mycorrhizae showed inter-

nal stem discoloration symptoms of Phytophthora root rot (P.
megasperma Drechs. var. sojae Hildeb.) race 1, while less than
20% of the nonmycorrhizal plants developed these symptoms.
Ross (41) stated that G. macrocarpus produced vesicles and
chlamydospores in the root cortex which may have affected
penetration or development of the pathogen. He also suggested
that the high phosphorus levels in the plots probably accounted
for the lack of yield response to mycorrhizae. These phosphorus
levels might have been high enough to reduce the incidence and
development of G. macrocarpus.
There are several reports in which VA mycorrhizal fungi
have had no effect on disease severity. Ramirez (36) studied the
effect of three species of VA mycorrhizal fungi, Glomus macro-
carpus var. macrocarpus Tul. & Tul., Gigaspora heterogama
(Nicol. & Gerd.) Gerd. & Trappe, and Gigaspora margarita
Becker & Hall, on root infection by Phytophthora palmivora
Butler on papaya (Carica papaya L.). Although G. macrocarpus
and G. heterogama significantly increased plant height compared
to nonmycorrhizal plants, there was no significant reduction in
the percentage of papaya roots infected by P. palmivora. With
all three species of mycorrhizal fungi, root weights of plants
inoculated with both a mycorrhizal fungus and P. palmivora
were significantly less than root weights of plants inoculated
with P. palmivora alone. These results indicate that VA my-
corrhiza had positive effects (increased plant height) and nega-
tive effects (reduced root weights) on the plant and no effect
on infection by the pathogen.
Menge et al. (30) found that prior root colonization of
Citrus sinensis (L.) Osbeck 'Pineapple' by Glomus fasciculatus
(Thax.) Gerd. and Trappe provided no protection against root
infection by Phytophthora parasitica Dast. Plants colonized by
the mycorrhizal fungus alone had a three-fold increase in plant
height and a 65% increase in dry weight compared to nonmy-
corrhizal plants, but these beneficial effects were negated by in-
fection with the pathogen. Menge et al. (30) further showed that
the roots of mycorrhizal citrus plants preferentially attract a
significantly greater number of P. parasitica zoospores (43%
more) than nonmycorrhizal roots. However, Hall and Finch
(18) did not observe a greater chemotactic response of zoo-
spores of Phytophthora cinnamomi Rands to mycorrhizal roots
of avocado (Persea americana Mill.).
Observations by Sherinkina (49) indicated that no significant
reduction in loose smut (Ustilago nuda (Jens.) Rostr.) of wheat

(Triticum aestivum) occurred between mycorrhizal and nonmy-
corrhizal plants. There was no reduction in the incidence of my-
corrhizae on plants with or without loose smut. Winter (57)
noted no differences in the incidence of Gaeumannomyces grami-
nis (Sacc.) v. Arx & Olivier on the roots of wheat from mycor-
rhizal and nonmycorrhizal plants.
Most reports in the literature indicate that VA mycorrhizal
fungi have decreased disease severity. Schonbeck and Dehne
(48) found that mycorrhizal (G. mosseae) and nonmycorrhizal
cotton plants (Gossypium hirsutum L.) were infected with
Thielaviopsis basicola (Berk. & Br.) Ferraris to the same degree.
Shoot weights of mycorrhizal plants were significantly greater
than nonmycorrhizal plants but the root weight did not differ
significantly. Mycorrhizal plants apparently could withstand
the stress of infection by T. basicola better than nonmycorrhizal
plants. Baltruschat and Sch6nbeck (2) obtained a 10-fold de-
crease in chlamydospore production of T. basicola on mycorrhizal
(G. mosseae) tobacco (Nicotiana tabacum L. 'Havana') plants at
low inoculum levels (10,000 endoconidia/ml). Extracts from roots
of mycorrhizal plants inhibited chlamydospore production of T.
basicola 80-100% on malt extract agar. In later studies, Bal-
truschat and Schonbeck (3) reported that the chlamydospore
production of T. basicola was negatively correlated with my-
corrhizal colonization of the roots of tobacco and alfalfa (Medi-
cago sativa L.). This reduction in chlamydospore production was
attributed to free amino acids in the roots. The free amino acid
content of mycorrhizal roots was 50% higher than nonmycor-
rhizal roots with arginine and citrulline showing the highest
increases. The addition of synthetic arginine to nonmycorrhizal
root extracts inhibited chlamydospore formation on agar, indi-
cating that at least one of these amino acids affected chlamydo-
spore production.
Chou and Schmitthenner (8) conducted a complex study on
the interactions among the VA mycorrhizal fungus Glomus
mosseae, the nodulating bacterium Rhizobium japonicum
(Kirch.) Buch., and the root pathogenic fungi Pythium ultimum
Trow and Phytophthora megasperma var. sojae, races 1 and 3,
on soybean. They reported that the presence of G. mosseae did
not affect Pythium root rot severity but did reduce the number
of plants killed by P. megasperma v. sojae. They observed root
colonization by G. mosseae 21 days after inoculation and noted
the effect of G. mosseae on P. megasperma v. sojae within
35 days, at which time the experiment was terminated. Although

Phytophthora did reduce the number of Rhizobium nodules, ap-
parently it did not interfere with development of G. mosseae in
the roots. Woodhead et al. (58) also studied the effect of P.
megasperma var. sojae race 1 on mycorrhizal and nonmycor-
rhizal soybean seedlings. They found that 3-week old soybean
seedlings colonized with Glomus caledonius (Nicol. & Gerd.)
Gerd. & Trappe had twice the root weight of nonmycorrhizal
seedlings after exposure to P. megasperma var. sojae. Differences
in results among the mycorrhizae-Phytophthora-soybean studies
(8, 41, 58) can be attributed to differences in mycorrhizal species,
soybean cultivars, races of P. megasperma v. sojae, and pro-
Schenck et al. (46) reported that prior root colonization by
Gigaspora margarita or Glomus macrocarpus reduced the dam-
age caused by Phytophthora parasitica to two citrus root stocks,
Carrizo citrange (Citrus sinensis X Poncirus trifoliata (L.)
Raf.) and sour orange (Citrus aurantium L.). Tap root rot was re-
duced by G. macrocarpus on Carrizo and by G. margarita on sour
orange. Plants colonized with either mycorrhizal species had
greater shoot weight, stem diameter, and root weight than non-
mycorrhizal plants. To insure good mycorrhizal establishment
on citrus roots, plants were exposed 110 days to mycorrhizal
fungi before challenging them with P. parasitica. Menge et al.
(30) exposed seedlings to VA mycorrhizal fungi 63 days before
exposure to P. parasitica.
Paget (35) working with Cylindrocarpon destructans (Zins.)
Schol., a mild pathogen of strawberry (Fragaria vesca L.), ob-
tained less plant stunting and a reduced root infection when
roots were colonized by VA mycorrhizae (Mosse's E3 isolate=
Glomus fasciculatus?). Dehne and Schbnbeck (11) indicated that
damage to tomato (Lycopersicon esculentum Mill.) by Fusarium
oxysporum Schlect. f. sp. lycopersicae (Sacc.) Snyd. & Hans.
was reduced by prior root colonization by G. mosseae. The num-
ber of yellow leaves and the amount of electrolyte leakage from
leaf and stem tissues were less in mycorrhizal plants than in
nonmycorrhizal plants.
Stewart and Pfleger (51) obtained stunting of poinsettia
(Euphorbia pulcherrima Willd. 'Annette Hegg Supreme') when
inoculated simultaneously with Glomus mosseae, Pythium ulti-
mum, and Rhizoctonia solani Kuehn. However, when the two
pathogens were added 20 days after G. mosseae, shoot
growth of poinsettia was equivalent to that where no pathogens
had been added. In a greenhouse test, Barnard (5) observed

fewer dead plants and 50% less root infection on mycorrhizal
than on nonmycorrhizal yellow poplar (Liriodendron tulipifera
L.) plants after three months exposure to Cylindrocladium sco-
parium Morgan. Similar results also were obtained in simulated
microplot nurseries in the fiell.
The presence of G. mosseae in the roots of onion (Allium
cepa L.) increased resistance to Pyrenochaeta terrestris (Hans.)
Gorenz, Walker, and Larson (6, 42). Safir (42) found that onion
plants were larger, percent root infection by P. terrestris was
less, and significantly more reducing sugars were present in my-
corrhizal than nonmycorrhizal plants when no nutrients were
added to the potting mix. Mycorrhizal and nonmycorrhizal onion
plants supplied with added nutrients on alternate days showed
no such differences. In a field survey by McGraw and Zalewski
(29), the incidence of mycorrhizae was higher in onion fields
with low levels of P. terrestris ("new" fields) than in "old" fields
with a high level of P. terrestris. Becker (6) challenged indi-
vidual mycorrhizal and nonmycorrhizal roots on the same plant
with P. terrestris and found that the inward invasion of the
pathogen was restricted more on mycorrhizal roots. Becker (6)
attributed the differences in penetration he observed to the cell
wall thickenings callositiess or lignitubers) of the host at the
point of penetration by the pathogen. These callosities were more
developed and more numerous on mycorrhizal than nonmycor-
rhizal roots. This was a clear example of disease resistance in-
duced directly by the presence of the mycorrhizal fungus rather
than induced indirectly by changes in nutrition of the mycor-
rhizal plant.

Plant parasitic nematodes
The first report of an interaction study between a nematode
and VA mycorrhizal fungus was that of Fox and Spasoff (13).
They found a mutual suppression in reproduction of both Hetero-
dera solanacearum Miller & Gray and Gigaspora gigantea (Nicol.
& Gerd.) Gerd. & Trappe on nematode susceptible and resistant
cultivars of tobacco. Fewer nematodes and spores were recovered
from plants colonized with both organisms than from plants
colonized with either alone. In both the nematode susceptible
and resistant cultivars, the dry weights of plants inoculated with
G. gigantea and H. solanacearum were less than the dry weights
of plants inoculated with H. solanacearum alone.
Atilano et al. (1) obtained increased shoot length, shoot dry

weight, and root dry weight of grape (Vitis vinifera L.) when
G. fasciculatus was present. The presence of Meloidogyne arenaria
(Neal) Chitwood on mycorrhizal grape plants completely negated
the beneficial effects of G. fasciculatus. The nematode did not
significantly affect the production of fungal chlamydospores,
but the number of M. arenaria larvae recovered from mycor-
rhizal plants was greater than from nonmycorrhizal plants.
In 1973, Baltruschat et al. (4) reported that 75% fewer
larvae of Meloidogyne incognita (Kofoid & White) Chitwood
developed into adults in mycorrhizal tobacco plants. Sikora and
Schbnbeck (50) also obtained a significant reduction in the initial
number of M. incognita larvae that developed into adults on
mycorrhizal tobacco, tomato, and oat (Avena sativa L.) plants.
The population of M. hapla remained suppressed on mycorrhizal
carrot (Daucus carota L.) roots for up to 18 weeks. The authors
consistently observed mycelia of G. mosseae in the galls and sug-
gested that the presence of the mycorrhizal fungus plus possible
physiological changes caused the depressed nematode popula-
tions. Root extracts from mycorrhizal roots had no effect on
larval mobility.
Schenck et al. (45) compared the interaction of three species
of VA mycorrhizal fungi-Gigaspora heterogama, Glomus mac-
rocarpus, and Gigaspora margarita-with Meloidogyne incog-
nita on root-knot resistant and susceptible soybean cultivars.
Generally an increase in root weight resulted in increased nema-
tode populations. However, on the susceptible cultivar (Pickett),
root weights and seed yields were greatest when plants were
inoculated with G. macrocarpus, but nematode populations re-
mained low. A similar effect was obtained on the resistant culti-
var (Forrest) when it was inoculated with G. heterogama. These
results indicate that the antagonistic effects of VA mycorrhizae
on root-knot nematodes may be observed only with specific my-
corrhiza-host-pathogen combinations.
Kellam and Schenck (25) found that prior inoculation by
either Meloidogyne incognita or Glomus macrocarpus did not
significantly affect subsequent colonization by either organism
on roots of the same soybean plant. Eight weeks after planting,
shoot weight, number of galls, number of chlamydospores, and
the percentage of mycorrhizal roots were not significantly dif-
ferent among plants with both organisms present and those with
only one organism present. However, plants which were inocu-
lated with both the fungus and nematode at planting had con-
sistently greater root weights, more mycorrhizal roots, and more

galls per plant than those plants which were inoculated with one
organism at planting and the other organism 14 days later.
In other experiments, Kellam and Schenck (26) studied the
development of mycorrhizae and a root-knot nematode on soy-
bean plants maintained to maturity. At the end of the growing
season, the presence of G. macrocarpus on soybean roots signifi-
cantly increased the seed yield and significantly reduced the
number of M. incognita galls. Atypical mycelia, vesicles, and
arbuscles were observed in galls of M. incognita. Most galls (60%)
developed on roots without mycorrhizal fungi present, but 40%
of the galls had mycorrhizal hyphae contiguous to the hyper-
trophied area. Although mycorrhizal fungi did not persist in
gall tissue, there was no significant reduction in the mean per-
cent colonization of soybean roots by G. macrocarpus.
Roncadori and Hussey (39) compared the interactions of
Gigaspora margarita and M. incognita on cotton (Gossypium
hirsutum) at the recommended and half the recommended
fertility levels. In general, mycorrhizal colonization of a plant
offset the reduced shoot growth obtained with M. incognita,
while M. incognita had little effect on the sporulation of G. mar-
garita. No mycelium of the mycorrhizal fungus was associated
with gall tissue 12 weeks after inoculation. Nematode egg pro-
duction per gram of root was not affected by G. margarita, but
the total number of eggs per plant was greater on mycorrhizal
roots than on nonmycorrhizal roots of the nematode susceptible
cultivar (Stoneville 213).
In a similar study by Hussey and Roncadori (19), Gigaspora
margarita significantly reduced the numbers (per gram of root)
of a migratory endoparasitic nematode, Pratylenchus brachyurus
(Godfrey) Filipj. & Schuur.-Stekh., at both a high and low fer-
tility level. The nematode did not affect the number of mycor-
rhizal spores recovered.
Viral plant pathogens
There have been two reports on the effect of VA mycorrhizae
on viral pathogens. When Sch6nbeck and Schinzer (48) inoculated
tobacco plants (Nicotiana tabacum L. 'Xanthi-nc') with tobacco
mosaic virus, they obtained more lesions on leaves of mycor-
rhizal (Glomus mosseae) than nonmycorrhizal plants. They sug-
gested that this effect was due to the increased nutrition asso-
ciated with the mycorrhizal plants. Daft and Okusanya (10)
clearly demonstrated that the increase in virus titer associ-
ated with mycorrhizal (Glomus macrocarpus var. geosporus)

plants was due to increased phosphate levels. Tomato aucuba
mosaic virus and potato virus X on tomato (Lycopersicon es-
culentum 'Eurocross A') and arabis mosaic virus on petunia
(Petunia violacea Lindl. 'Rose of Heaven') or strawberry (Fra-
garia chiloensis Duch. var. ananassa Bail. 'Talisman') were ob-
served to have increased titer on their mycorrhizal hosts. With
tomato aucuba mosaic, an increase in virus titer in tomato oc-
curred in nonmycorrhizal plants when increasing amounts of
soluble phosphate salts were added to the soil (10).

Bacterial plant pathogens
Only one report occurred in which plant susceptibility to a
bacterial pathogen was studied in relationship to the incidence
of VA mycorrhizal fungi in roots. Weaver and Wehunt (53)
found no relationship between the percentage of mycorrhizal
roots of peach seedlings (Prunus persica (L.) Batsch) and their
susceptibility to bacterial canker caused by Pseudomonas sy-
ringae von Hall.

In the research thus far, it is apparent that disease severity
can be increased, decreased, or not affected by the presence of
VA mycorrhizal fungi. There have been 29 reports in the litera-
ture that include 38 comparisons of mycorrhizal fungi and plant
pathogens (Table 1). By far, most of the comparisons have been
with the VA mycorrhizal fungus Glomus mosseae. Most com-
parisons have been with fungal plant pathogens or parasitic
nematodes. Of the total 38 comparisons, seven resulted in in-
creased disease severity, 22 in decreased disease severity, and
nine in no change in disease severity. A total of eight species of
mycorrhizal fungi plus unidentified species were evaluated with
four viral pathogens, 12 fungal pathogens, four species of
nematodes, and one bacterial pathogen.
The nature of the effect exerted by the VA mycorrhizal
fungus on the pathogen is not known in most instances. How-
ever, in a few studies the differences in chemical constituents
between mycorrhizal and nonmycorrhizal plants were observed.
The increase in virus titer in mycorrhizal plants was shown to
be related to increased phosphorus levels in the plant (10).
Baltruschat and Sch6nbeck (3) demonstrated that mycorrhizal
plants contained higher levels of arginine, and that this amino
acid, when added to extracts of nonmycorrhizal roots, could in-

TABLE 1. Effect of va
ported in th

Species of VA
mycorrhizal fungi
Glomus macrocarpus
var. geosporus
Glomus macrocarpus
var. macrocarpus
SGlomus fasciculatus
Glomus mosseae

Glomus caledonius
Gigaspora margarita
Gigaspora gigantea
Gigaspora heterogama
Unidentified mycorrhiza


rious species of vesicular arbuscular
e literature.
Kinds of plant pathogens evaluated
Virus Fungus Nematode Bacterium
3 1 -

(VA) mycorrhizal fungi on disease severity as re-

Effect on disease seven
Increase Decrease No

- 2

- 1
- 3

- 1
- "4

4 23"

- 1 9

- 1
- 1 4
-- 1 -
- 1
1 2

-rity Literature cited
change reference number
- 9,38

1 23,24,33,42,43

2, 3, 8,10, 39, 44,
45, 47, 48
33, 42

7 22

"Note: Numbers are larger than the actual number of pathogen species
than one species of mycorrhizal fungus.

evaluated since some pathogens were evaluated with more

hibit chlamydospore formation of Thielaviopsis basicola. Safir
(42) found larger amounts of reducing sugars in mycorrhizal
plants and suggested this could explain the decreased root in-
fection in onion by Pyrenochaeta terrestris.
Becker (6)found an increased morphogenic response of onion
cells walls to penetration by P. terrestris in mycorrhizal plants.
More callosities developed in mycorrhizal than nonmycorrhizal
roots, and this appeared to delay the spread of P. terrestris. Safir
(42) observed the mycelia of P. terrestris and Glomus mosseae
in the same root, which suggested the two organisms were com-
patible. Sikora and Schinbeck (50) reported the frequent oc-
currence of mycelia of G. mosseae in galls produced by Meloi-
dogyne incognita. In contrast, others (24, 38) have found that
VA mycorrhizal fungi and root-knot nematodes were not com-
patible in the same tissue. In most instances reported, the patho-
gen reduced the incidence of the mycorrhizal fungus in the roots,
thus partially reducing the beneficial effects of VA mycorrhizae.
This situation has been noted on several crop plants in the field
Zak (59) suggested four means by which ectomycorrhizae
could afford protection to roots: i) by using surplus carbohy-
drates, ii) by providing a physical barrier, iii) by secreting anti-
biotics, or iv) by favoring protective rhizosphere organisms. All
these phenomena could be operative with VA mycorrhizal fungi,
except that of providing a physical barrier since no hyphal
mantles are formed over the root surface. In addition, VA my-
corrhizal fungi in some instances can affect plant pathogens by
inducing morphogenic and biochemical changes in host tissue
that are unfavorable to pathogen development (4, 6). The prior
colonization phenomenon as suggested by Wilhelm (55) could
also be operative in antagonistic interactions.
Mosse (33) discussed the specificity of species of VA mycor-
rhizal fungi and indicated that there is ample evidence to indicate
that one mycorrhizal fungus can be more beneficial to a specific
plant species than to another. Interactions between VA mycor-
rhizal fungi, plant pathogens, and host plants may also vary with
specific combinations. One species of mycorrhizal fungus which
provides protection from a pathogen on one plant species may
not be as effective on another plant species, although the same
pathogen is involved. A comparable situation may also occur be-
tween different cultivars on the same plant species. Schenck et al.
(45) found Glomus macrocarpus effective in reducing nematode
populations on root-knot susceptible soybean cultivars, but this

same mycorrhizal fungus was not as effective on a root-knot
resistant cultivar.
Wilhelm (55) stated that the evaluation of VA mycorrhizae
as biocontrol agents is one of the most challenging areas in
plant pathology. To continue the evaluation of the potential of
these fungi in biological control, it is necessary to compare one
interaction study with another. Thus far, interactions among
symbiont-pathogen-host combinations appear complex and seem
to vary with each combination. To facilitate comparisons of
interaction studies, researchers should specifically determine
and report such information as: the type and amount of inoculum
of the pathogen; the manner in which the challenge organism
is introduced; the percentage of root colonization by the mycor-
rhizal fungus when the pathogen is introduced; the type, pH and
nutritional status of the soil; and the environmental conditions
during the evaluation period. Pure culture studies such as those
conducted by Marx (28) on ectomycorrhizae provide information
concerning mycorrhizal-pathogen interactions, but the usefulness
of mycorrhizae in biological control can be determined only by
conducting interaction studies in the field. Thus far these studies
have been largely lacking.
It is important that studies on the effect of VA mycorrhizal
fungi on disease development be continued and expanded. In
some instances VA mycorrhizal fungi may be very effective as
biocontrols for specific pathogens, and further investigations of
this type should be encouraged. The fact that VA mycorrhizal
fungi can increase disease severity should also direct more at-
tention to mycorrhizal-pathogen studies. The symbiotic nature
of these fungi with their beneficial effects on plant growth has
stimulated interest in their widespread use in agriculture as
"biological fertilizers". However, their use in soils containing
crops and pathogens on which they enhance disease could result
in considerable losses. To minimize such losses, considerably
more studies involving VA mycorrhizae and plant pathogenic
organisms are necessary.

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