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Identification and Management of Turfgrass Diseases
Dollar spot
Symptoms and signs | Conditions | Management
Dollar spot is a common disease on creeping bentgrass, Kentucky bluegrass
and perennial ryegrass, but it is rare on tall fescue. It also occurs on zoysiagrass,
bermudagrass, buffalograss and annual bluegrass . The disease can occur irrespective
of management program or soil fertility status, although damage is usually most
severe when the turfgrass is nitrogen-deficient.
Pathogen
Hosts
- Creeping bentgrass
- Kentucky bluegrass
- Perennial ryegrass
- Zoysiagrass
- Bermudagrass
- Buffalograss
Symptoms can be confused with those of
- Ascochyta leaf blight
- Dead spot
- Brown patch
- Cutworms
Symptoms and signs
In higher cut turfgrass, Sclerotinia homoeocarpa infection results in the
formation of small, roughly circular tan patches. Patch size varies with mowing
height and environmental conditions and can range from a few inches to 6 inches
or more in diameter (Figure 1). During extended periods of disease-favorable
weather, patches coalesce into large, irregular, dead areas. Infected plants
within the diseased spots wilt and eventually turn tan or brown because of girdling
lesions on the leaf blades. This can result in substantial damage to the turfgrass.
On putting greens, dollar spot infection sites rarely exceed 2 inches in
diameter (Figure 2). During weather that is favorable to dollar spot, hundreds
of spots can coalesce to blight large areas of the putting surface. If left untreated,
the spots become sunken, leaving a low-quality, pockmarked putting surface.
Foliar symptoms of dollar spot can be seen on plants at the transition zone
between healthy and infected turfgrass. On Kentucky bluegrass, the lesions appear
as bleached areas with a constricted or hourglass appearance, usually with reddish
brown borders (Figure 3). The lesions are often located near the middle of the
leaf and extend completely across the leaf blade. On creeping bentgrass, individual
infected leaves exhibit light tan lesions that may or may not be surrounded by
a dark, reddish-brown border (Figure 4). The lesions are difficult to see on
closely mowed bentgrass without a hand lens. On warm-season grasses (bermudagrass,
buffalograss and zoysiagrass) the symptoms occur as discrete lesions bordered
by brown bands (Figure 5).
In the early morning when dew is still present on the turfgrass, small cottony
webs of fungal mycelium can sometimes be seen growing from the diseased leaf
blades (Figure 6 top). This growth can be confused with spider webs, "cotton" from
plant seeds, or other fungi, and therefore is not a reliable diagnostic feature.
When viewed under a compound microscope, the fungal hyphae have a characteristic
Y-shaped branching, in contrast to hyphae of Rhizoctonia solani, which are predominantly
right-angled (Figure 6 bottom).
Conditions
Dollar spot is more prevalent in nitrogen-deficient turfgrass. S. homoeocarpa survives indefinitely in thatch and soil. In the presence of a thin film of moisture
on leaves and moderate temperatures (65 to 85 degrees F), the fungus infects
leaves. S. homoeocarpa rarely infects turfgrass roots, although fungal toxins
may adversely affect root formation. Dollar spot is most severe in late spring
and early summer and again in early fall; however, it can occur throughout the
summer months. The patches are more numerous in areas where air circulation or
soil drainage is poor.
Management
On higher cut turfgrass
The first step to reduce the negative impact of dollar spot on any cool-season
turfgrass is to apply enough nitrogen at the right time to meet the needs of
the turfgrass species under management.
Dollar spot is rarely serious on home lawns, but irrigation practices that
extend the duration of leaf wetness can promote disease development. Irrigation
of any turf, whether home lawn, fairway or green is best applied from midnight
to early morning when the grass is normally wet.
A preventive fungicide application is generally not needed on home lawns.
Nevertheless, dollar spot can reduce the quality of the turf for short periods.
On higher cut, cool-season turfgrass on golf courses, the need for fungicides
to suppress dollar spot depends on the susceptibility of the turfgrass species
being managed and on the facility's expectations and budget.
On bentgrass or bentgrass/annual bluegrass greens
The first line of defense is to use a bentgrass cultivar with resistance to dollar
spot. Check the National Turfgrass Evaluation Program Web
site, ntep.org, for the most up-to-date information.
Minimize the duration of leaf wetness by physically removing dew (poling,
mowing, syringing) in early morning. Do not allow the grass to go under drought
stress. Maintain adequate nitrogen fertility. Biological control of dollar spot
by fertilization with composted turkey litter, bovine wastes and other organic
amendments has been achieved by some superintendents. However, organic amendments
have not consistently reduced dollar spot incidence and severity in university
research trials. Similarly, results with the addition of microbial biological
control agents (fungi and bacteria) and plant defense activators have not been
consistent.
Preventive fungicide applications at 7- to 21-day intervals are needed to
suppress dollar spot. The timing and frequency of application depends on weather
conditions and on the type of fungicide that is applied. Continuous use of xylem
mobile fungicides, including thiophanate methyl and the DMI fungicides (fenarimol,
myclobutanil, propiconazole and triadimefon) may result in the selection and
increase of fungicide-resistant strains of S. homoeocarpa. Superintendents should
consider limiting the number of applications of these fungicides during the growing
season and tank mixing or alternating these products with contact fungicides
not prone to resistance problems.
Many superintendents have reported that dollar spot on bentgrass or bentgrass/annual
bluegrass greens is becoming more difficult to control, raising concerns about
fungicide-resistant strains of S. homeoeocarpa. Careful study has shown that
the lack of control is often related to fungicide application errors. One such
error is application of lower fungicide rates than recommended on the product
label, either intentionally to save money or accidently because of sprayer calibration
issues. Another error is the attempt to extend the interval of protection beyond
label recommendations. Although fungicides can suppress S. homoeocarpa, bentgrass
recovery may not occur quickly. This is especially true when the bentgrass is
under growth regulation and generation of new tissue is delayed.
IPM1029, new June 2007
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