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ENDOPHYTIC microorganisms occur in most
species of plants as inhabitants of above- or below-ground organs.
Their presence in the tissues either elicits no apparent effect
in the normal functioning of the infected plants, or the endophytic
may confer various benefits to the host. Grasses are no exception
and present intriguing examples of these associations that can have
application in turf management.
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| Fungi
are the most frequently encountered partners with grasses, and several
species that colonize leaves and stems are now known to confer protection
from herbivores and environmental stresses. These properties are
being exploited for turfgrass species, where resistance to depredation
from surface-feeding insects is a major benefit. Unfortunately,
these fungi do not inhabit root tissues, but, as in most plant roots,
grass roots harbor other endophytic fungi, in particular, many species
of vescular-arbuscular mycorrhizal (VAM) fungi can be found. VAM
endophytes have been extensively documented, and their beneficial
effects on growth and development of a range of plant species have
been demonstrated. However, the species involved and their biology
and impact in the turf environment have received only cursory examination.
In fact, there is a common belief that VAM fungi are of little importance
in highly maintained turf where the extensive fine root system of
the grasses receives ample water and nutrients that eliminate the
requirement for the symbiosis. With the generous support of the
USGA, a research project to investigate the subject of VAM in turfgrasses
commenced at URI in 1990. |
| We
sampled turf throughout New England and performed a variety of greenhouse
and field trials to assess the incidence and importance of VAM fungi
in golf greens. Our efforts were focused on creeping bentgrasses
(Agrostis palustris cv Penncross) and velvet bentgrass (Agrostis
canina cv Kingstown). Initially, we needed to determine how frequently
the fungi occurred in association with these turfs and what species
of fungi were involved. |
| In
our four-year study we found 29 species of VAM fungi occurring with
these bentgrasses, several of which were new species. None of the
species have previously been studied for any particular impact on
bentgrass turf, yet virtually every one of the more than 200 root
zone samples examined contained VAM fungi. |
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We performed numerous growth experiments where bentgrasses were
inoculated with different species of VAM fungi. All experiments
were carried out in a medium meeting USGA Green Section specifications
for sand greens. The fungi were added to the mix before seeding.
The fungus that we used most frequently was
Glomus Intraradices,
the only species for which sufficient inoculum was commercially
available. Results of inoculation were striking. Establishment of
young turf was enhanced by inoculation with mycorrhizal fungi, and
differences were apparent within three weeks after seeding. Turfs
older by several months continued to grow more vigorously with Mycorrhizae.
In addition to improved growth, mycorrhizal turf was greener than
non-mycorrhizal turf and possessed up to 60% more chlorophyll. |
| Phosphorus
fertilization rate affected how well the VAM fungi performed. The
most vigorous mycorrhizal turfs were those that received frequent
applications of a low-P fertilizer solution. When the P concentration
was too high or too low, Mycorrhizae did not enhance growth. |
| Mycorrhizal
fungi are sensitive to a range of pesticides (e.g., Benlate, Aliette,
Phaltan, Diazinon), and the benefits to turf may thus be lost temporarily
if suppressive materials are applied. |
| In both field miniplots and greenhouse
trials in pots, mycorrhizal turf of Penncross survived drought conditions
far better than did non-mycorrhizal turf. After a five-day drought,
mycorrhizal turf in the field study showed 39% less water stress
than did control turf, and after eight days, the difference was
60% (Figure 1). |
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| In the greenhouse study, turf without mycorrhizae
began wilting after three days, but mycorrhizal plants were wilted
only after five days (Figure 2). |
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| Mycorrhizal turfs also recovered more rapidly,
producing three times as much leaf matter as the controls (Figure
3). |
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Preliminary
trials indicated that mycorrhizae may provide some protection against
the take-all fungus Gaeumannomyces graminis. As noted in the growth
trials, however, this benefit was present only when P concentration
was moderately low. At higher levels of P mycorrhizal turfs tended
to be susceptible to take-all.
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| Conclusions |
| The
presence of mycorrhizal fungi in putting greens constructed according
to USGA Green Section specifications offers potential benefits to
the turf. Improved drought tolerance and related rapid recovery
from wilting appear to be the most significant, but increased growth
and establishment rates, greater chlorophyll content, and a lowered
phosphorus requirement are also worthy of note. A probable
result of these benefits may be manifested in an increase in resistance
of mycorrhizal turf to foot traffic (wear), although this was not
measured in our results. |
| During
our four-year investigation of mycorrhizal fungi in greens turf,
we made several discoveries that were not the main object of our
study but have importance to the practical use of Mycorrhizal fungi
in greens. First, mycorrhizal fungi naturally colonize new
greens turf without being added as inoculum. While inoculation
of a new green at the time of seeding is likely to result in a more
rapid establishment of the green, in the longer term it may not
be necessary. We examined a variety of one to four-year-old
greens where VAM fungi had not been intentionally inoculated, and
in most of them the turf roots were already highly mycorrhizal.
It is not clear how the fungi arrived in the root zone of these
greens. Spores of VAM fungi are relatively large and are formed
underground. Thus, they should not move readily into non-mycorrhizal
situations (e.g., sand/peat greens) unless as soil-borne inoculum.
It seems likely that the VAM fungi that were found in these greens
were present in soil that was deliberately added to or contaminated
the sand/peat medium during green construction, or the fungi invaded
the green from the adjacent native soils. The VAM fungi are
ubiquitous in soils but generally are absent from clean sand and
peat. |
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The ease with which the VAM fungi invade new greens may be just
as well because commercially available inoculum is not yet readily
available. Premier Peat, Quebec, Canada does offer a limited
supply of Mycori-Mix, a product that contains
Glomus Intraradices.
As we learn more of the biology of these fungi, it appears that
selected effective species or biotypes may be incorporated into
greens during construction. A protocol may be determined so
that established greens can be managed to obtain the full benefits
of the symbiotic association. More effective VAM species are
likely to be found than the ones that invade by chance, and these
may be matched to particular turfgrass species or cultivars for
specific climate and growing conditions. |
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Ultimately, it may prove to be biologically, environmentally, and
economically feasible to use mycorrhizal fungi in putting greens
to reduce requirements for fertilizer and water while achieving
a greener, more vigorous, disease-resistant turf.
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USGA GREEN SECTION RECORD
November / December
1995
(Reprinted with permission) |
