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SCIENCE FOR THE GOLF COURSE
dedicated to enriching the environment of golf
Managing gray leaf spot
in the Midwest
Because disease outbreaks are sporadic, Midwestern superintendents must decide whether
to continue managing gray leaf spot with fungicides or to convert fairways to turf species
that are not susceptible to the disease.
Despite confirmed outbreaks of gray leaf
spot in the Mid-Atlantic region in the early to
mid-1990s (3,7), superintendents were unprepared for its arrival in the lower Midwest (Ohio
Valley states) a few years later (Figure 1). In
many cases, they were overwhelmed in late
summer by three important factors that set the
stage for severe gray leaf spot epidemics and significant turf loss. First, early symptoms of gray
leaf spot can be overlooked because similar disease symptoms caused by less virulent leaf spot
fungi (Curvularia species and Bipolaris species)
also occur during August and September.
Second, overseeding fairways with perennial
ryegrass (Lolium perenne L.) normally begins
during this time period, and because juvenile
perennial ryegrass is most susceptible to infection, extensive damage can result in a matter of
days. Third, fungicide budgets, especially for
treatment of perennial ryegrass fairways, are
already wearing thin by late summer. The concurrence of these factors has forced superintendents to confront issues regarding chemical
control and conversion to other turf species.
Perennial ryegrass gained favor in the mid1970s because it is easy to establish, has natural resistance to summer patch and is
tolerant of lower mowing heights (0.75 inches
[1.9 centimeters] or less). Conventional wisdom holds that the gray leaf spot pathogen
(Magnaporthe oryzae) trailed perennial ryegrass
EDITOR’S
note:
Related information on gray leaf spot
prediction appears in this month’s Turf
Talk on p. 24.
Photo courtesy of R. Latin
R. Latin, Ph.D., and P. Harmon, Ph.D.
Figure 1. Outbreaks of gray leaf spot can disturb the appearance and playability of perennial ryegrass turf.
as its use became widespread for golf course
fairways in the northeastern quadrant of the
United States. Although the use of perennial
ryegrass for fairways in the northeastern
United States has leveled off, the pathogen has
become well established (it is capable of overwintering in infested residue).
Gray leaf spot appears to be a chronic
problem in the Middle Atlantic region and
south of the Ohio River (2). The disease has
not been reported or confirmed in northerntier Midwestern states (Michigan, Wisconsin
and Minnesota) and only occasionally in
Iowa, Nebraska and Kansas. Outbreaks appear
to be more sporadic across Ohio, Indiana,
Illinois and Missouri, where superintendents
must manage their perennial ryegrass mindful
that, although the likelihood of a gray leaf
spot outbreak may not be great, the amount
of damage the disease can cause is significant.
Each year superintendents are faced with the
dilemma of managing the turf with fungicides
or making a convincing argument for avoiding the disease by changing to a turfgrass
species that is not susceptible (for example,
creeping bentgrass or bermudagrass).
Managing gray leaf spot with fungicides
In many cases, gray leaf spot has been
managed successfully with systemic or localized penetrant fungicides. Strobilurins
(Heritage 50WG, Compass 50WG and
Insignia 20WG) and thiophanate-methyl
fungicides (Cleary’s 3336, Fungo Flo, TOctober 2004
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Storm Flowable) are most effective (8), but they
also are costly. A single application will exceed
$300 per acre. Less-expensive products (for
example, DMI fungicides) are available, but
they are less effective in suppressing gray leaf
spot.
The chemical approach to gray leaf spot
control is hampered by a high risk of developing fungicide-resistant pathogen populations.
If fungicide-resistant populations become predominant, then the fungicides will be useless
against gray leaf spot. A strobilurin-resistant
population already has become established in
at least one location in Kentucky, and resistant
strains have been identified in Indiana.
Adding a contact fungicide into the spray
mixture may reduce the risk of resistance, but
it will increase expenses. Furthermore, the only
effective contact tank-mix partner (chlorothalonil) has federal limitations on the amount
that can be used on fairways during a single season.
Using fungicides to manage gray leaf spot
appears to have more disadvantages than
advantages. However, if the disease occurs sporadically, the fungicide approach represents an
economical option when gray leaf spot is
detected early and when an effective product
is applied at recommended rates and scheduled at appropriate times.
Kentucky bluegrass
Kentucky bluegrass (Poa pratensis L.) is
one option. New varieties with some tolerance
to low mowing heights have been released, but
they suffer a competitive disadvantage at
mowing heights below 1 inch (2.5 centimeters) and they remain susceptible to summer
patch and necrotic ring spot. In addition,
many turf scientists believe that after four or
five years, Kentucky bluegrass maintained at
fairway height (0.5-0.75 inches [1.3-1.9 centimeters]) will be transformed into a mosaic
that includes large proportions of annual bluegrass (Poa annua). Kentucky bluegrass is a
viable option in some circumstances, but golf
course fairways with moderate to heavy play
may not be one of them.
Bermudagrass
For fairway conversion in the Ohio River
Valley, bermudagrass (Cynodon species)
appears to be the species of choice.
Bermudagrass is not susceptible to gray leaf
spot and can tolerate low mowing heights.
However, superintendents must contend
with spring dead spot, a serious root disease.
An obvious disadvantage of conversion to a
warm-season turf species north of the Ohio
River Valley is the relatively short season of
Illustration by Kelly Neis
Alternative turfgrass species
The alternative to fungicides for gray leaf
spot control is to convert stands of perennial
ryegrass to a different turfgrass species that is
not susceptible to the disease.
Figure 2. A gray leaf spot transition zone (purple area) occurs in the lower Midwest. Outbreaks are more
common south of the zone (red area), very rare north of the zone (blue area) and sporadic within.
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October 2004
green and vibrant turf. Bermudagrass normally emerges from dormancy during May
and retreats to a dormant state with the first
frost, providing at best about three full
months of green fairways.
Creeping bentgrass
Conversion to creeping bentgrass (Agrostis
stolonifera L.) has been the most popular
option for golf courses. Although creeping
bentgrass provides certain agronomic advantages, the threat to perennial ryegrass posed by
gray leaf spot has factored into conversion
decisions. Creeping bentgrass can tolerate low
mowing heights and is not susceptible to summer patch or gray leaf spot, but it is not disease-free. Dollar spot can be an active threat
for the entire growing season on creeping
bentgrass fairways. Although dollar spot outbreaks do not result in large areas of dead turf,
they certainly affect appearance and playability. The same worries that accompany perennial ryegrass and gray leaf spot (turf damage,
fungicide expense, limitations on the use of
chlorothalonil and evolution of fungicide resistance) also are considerations for managing
dollar spot on creeping bentgrass.
The superintendent’s dilemma
There appears to be a gray leaf spot transition zone that is defined by a geographic area
where outbreaks are sporadic (Figure 2).
Superintendents working in the gray leaf spot
transition zone could make the best-possible
management decisions if they could predict
the frequency and severity of the gray leaf spot
epidemics. Unfortunately, the level of confidence in gray leaf spot predictions is quite low
because we still do not have a thorough understanding of this complex problem despite all
of the recent research.
We do know that the frequency of gray leaf
spot outbreaks (disease incidence) and the
amount of damage an outbreak causes (disease
severity) depend on two factors: the presence
of the pathogen (inoculum) and the environmental conditions that prevail during the
growing season. An understanding of how
these factors influence gray leaf spot epidemics
will help superintendents improve their disease
management skills.
The pathogen factor
The pathogen factor is a function of its
mechanisms for survival and spread. Because
the disease is relatively new to the Midwest and
because it is spread by means of airborne coni-
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Environmental factors
The environment represents the second
disease-determining factor. Scientists have
made several attempts to define environmental factors that favor gray leaf spot development. Early research on ryegrass blast (caused
by a similar fungus on annual ryegrass)
described conditions favorable for infection in
the range of 64 F-90 F (17.8 C-32.2 C) during wet periods lasting six to 16 hours (4).
A recently published predictive model
attempted to describe the relationship
between disease development and temperature and leaf wetness (6). (The predictive
model was published in the February 2003
issue of GCM as “Temperature and moisture:
Predictors of gray leaf spot” by Wakkar
Uddin, Ph.D. The article is available on the
Web at: www.gcsaa.org/gcm/2003/feb03/
PDFs/02Temperature.pdf.) The polynomial
equation was intended to predict disease
incidence (percentage) given the hours
of leaf wetness and the temperature during the
wet period. However, running the model
results in values for disease incidence that cannot be interpreted, even when favorable values are entered for temperature and hours of
leaf wetness.
The growth chamber research (6) led to
conclusions that disease incidence was greatest at temperatures ranging between 82 F
(27.8 C) and 90 F (32.2C). In the field, the
likelihood for more than a few hours of leaf
wetness at temperatures greater than 80 F
(26.7 C) is remote. Dew periods occur at
night, and sustained evening temperatures
above 72 F (22.2 C) are rare in the central and
northern portions of the area described in
Figure 2.
In lieu of a usable quantitative model
to define environmental conditions that
favor gray leaf spot development, we are
left with the reasonable assumption that the
disease is favored by extended periods of
warm, wet weather.
Inoculum: the limiting factor
Gray leaf spot incidence was not consistent
in Indiana during the summers of 2000-2003
(1). Confirmed reports were fairly widespread
in 2000 and 2002, rare in 2001, and no reports
(confirmed or otherwise) were made in 2003.
Inspection of records of environmental conditions in West Lafayette, Ind., during the
months of July, August and September revealed
little difference among years in terms of environmental conditions that favor disease development (temperature and moisture) (5). If any
pattern could be discerned, it was that the wetter summers (2001 and 2003) sustained the
least amount of disease.
If environmental conditions were favorable for disease development in all four summers, but outbreaks occurred only in 2000
and 2002, then the inoculum (the presence of
the pathogen) must have been the limiting
factor in 2001 and 2003. Therefore, the
pathogen either did not survive the winter
locally, and/or airborne inoculum was not
introduced from sources south of the Ohio
River Valley. Our results suggest that
pathogen populations were diminished during the colder winters preceding the summers
of 2001 and 2003 (Figure 4). Winter temperatures appear to influence pathogen survival and can help explain the sporadic nature
PATHOGEN SURVIVAL
60
Conidia per gram residue (× 1000)
dia (inoculum) (7), we can conclude that the
initial outbreaks were due to an influx of inoculum from infected perennial ryegrass (or tall
fescue) from locations south of the Ohio River.
In Indiana, this may have occurred in 1999,
when the disease was reported and confirmed
throughout much of the state. The general pattern of spread was consistent with a pathogen
that is disseminated by airborne inoculum.
In subsequent years, however, outbreaks
appeared to be clustered around the site of initial establishment. For example, on golf
courses in Bloomington, Lebanon, and
Rensselaer, Ind., the first symptoms in 2000
appeared precisely where they had appeared
the year before. Other anecdotal evidence tells
of gray leaf spot outbreaks occurring in the
same areas of the same fairways each year.
This cluster pattern is typical of a pathogen
that can overwinter locally on infested residue
and produces spores on the residue as warm
weather returns.
Research at Purdue University has established that winter conditions drastically
reduce the magnitude of the pathogen population (1). Attempts to recover the pathogen
from infested residue over three successive
winters showed that the likelihood of winter
survival in north-central Indiana is very low
(Figure 3). It appears that unless summertime
outbreaks are especially severe, the pathogen
population at that latitude will not survive the
winter in quantities sufficient to initiate new
outbreaks the following year. The result is a
pattern of sporadic disease outbreaks across
the gray leaf spot transition zone.
50
40
30
20
10
0
0
50
100
150
200
Soil EC (dS/m)
Figure 3. Decline of the gray leaf spot pathogen from fall through spring over three years in Lafayette, Ind.
Infested residue was placed in the turf canopy in autumn, 2000, 2001 and 2002. Residue was recovered
periodically through winter months to determine the levels of pathogen survival over time.
October 2004
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of gray leaf spot outbreaks.
This information will help superintendents
make better decisions to deal with the threat of
gray leaf spot. Superintendents at courses
located close to the Ohio River are more likely
to be justified in converting perennial ryegrass
to creeping bentgrass or bermudagrass because
outbreaks of gray leaf spot will be more consistent in their area. In the red zone in Figure
2, the chance of local winter survival of the
pathogen is greater, and those courses are closer
to a more consistent supply of inoculum from
south of the Ohio River. Outbreaks still may be
sporadic, especially after very cold winters, so
it is possible to get by with using fungicides to
manage gray leaf spot.
Gray leaf spot outbreaks are expected
to be much more sporadic for golf courses
located toward the blue area in Figure 2. In
those cases, adequate disease control can be
achieved with the use of effective fungicides.
Converting perennial ryegrass fairways to
other turf species because of the perceived
threat of gray leaf spot in these areas is
ill-advised.
THE RESEARCH
says . . .
➤ Gray leaf spot, a disease of perennial ryegrass, occurs sporadically in the Midwest and
sometimes results in significant turf damage.
➤ Two options for managing the disease are applying fungicides or converting fairways to
alternative turf species (creeping bentgrass or bermudagrass). Both approaches are expensive.
➤ There appears to be a gray leaf spot transition zone (purple area of the map) where outbreaks are sporadic. Research suggests that gray leaf spot is less likely to occur in this area
during summers following cold winters.
➤ Given this information, superintendents should be better able to make decisions to deal
with the threat of gray leaf spot.
➤ In areas close to the red zone of the map, gray leaf spot will be more of a consistent threat.
Superintendents at those courses are more likely to be justified in converting to another
species, but still may get by with using fungicides, especially after a cold winter.
➤ Courses located closer to the blue area of the map can achieve adequate gray leaf spot
control with fungicides because outbreaks will be more sporadic and probably less severe.
Acknowledgments
Funding for this research was provided by Purdue
University and the Midwest Regional Turf Foundation.
2.
Literature cited
1. Harmon, P.F. 2003. Winter survival and rapid PCR
3.
COLD WINTERS
6000
Cumulative heating degree days (HDD)
5000
■ 2002/2003
■ 2001/2002
■ 2000/2001
■ 1999/2000
4.
5.
4000
6.
3000
7.
2000
8.
detection of Magnaporthe oryzae, the gray leaf spot
pathogen on perennial ryegrass. Ph. D. dissertation.
Purdue University, West Lafayette, Ind.
Harmon, P.F., and R. Latin. 2003. Gray leaf spot of
perennial ryegrass. Plant Health Progress Online.
www.plantmanagementnetwork.org/php/default.asp
doi:10.1094/PHP-2003-1223-01-DG. (Verified April
27, 2004.)
Landschoot, P.J., and B.F. Hoyland. 1992. Gray leaf
spot of perennial ryegrass turf in Pennsylvania. Plant
Disease 76:1280-1282.
Moss, M.A., and L.E.Trevathan. 1987. Environmental
conditions conducive to infection of ryegrass by
Pyricularia grisea. Phytopathology 77:863-866.
Purdue University, Department of Agronomy, Applied
Meteorology Group. Indiana Climate Page. [Internet
database]. West Lafayette, Ind. [updated Aug. 26,
2003; cited Dec. 22, 2003] Available from:
http://shadow.agry.purdue.edu/sc.index.html.
(Verified April 27, 2004.)
Uddin, W., K. Serlemitsos and G. Viji. 2003. A temperature and leaf wetness duration-based model for
prediction of gray leaf spot of perennial ryegrass turf.
Phytopathology 93:336-343.
Uddin, W., G. Viji and P. Vincelli. 2003. Gary leaf
spot (blast) of perennial ryegrass turf: An emerging
problem for the turfgrass industry. Plant Disease
87:880-889.
Vincelli, P. 1999. Gray leaf spot, an emerging disease
of perennial ryegrass. Turfgrass Trends 7(6):1-8.
1000
0
t 1 10 20 30 10 20 30 10 20 30 10 20 30 10 20 r 1 10 20 30
Oc Oct Oct Oct Nov Nov Nov Dec Dec Dec Jan Jan Jan Feb Feb Ma Mar Mar Mar
Figure 4. Cumulative heating-degree-day (HDD) values for the six-month period from Oct. 1 through March 31
over four years. HDD values are calculated by summing the difference between 65 F (18.3 C) and the average
air temperature (in degrees Fahrenheit). HDD values provide a measure of coldness for each six-month period.
This figure shows that the fall/winter months of 2000/2001 and 2002/2003 were not as mild as those of
1999/2000 and 2001/2002.
92
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October 2004
R. Latin, Ph.D. ([email protected]), is a professor of
plant pathology in the department of botany and plant
pathology at Purdue University, West Lafayette, Ind. P.
Harmon, Ph.D., is an assistant professor of plant
pathology in the department of plant pathology at the
University of Florida, Gainesville.