Department
of Botany1 and Department of Plant Sciences2,
University
of Rhode Island,
Kingston,
Rhode Island
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.
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.
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).
In the greenhouse study, turf without
mycorrhizae began wilting after three days, but mycorrhizal plants were wilted only after
five days (Figure 2).
Mycorrhizal turfs also recovered more rapidly,
producing three times as much leaf matter as the controls (Figure 3).
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.
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.
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.
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.
