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Identification and Management of Turfgrass Diseases
Dead spot
Symptoms and signs | Conditions | Management
Dead spot is a disease of new sand-based bentgrass greens or renovated greens
where methyl bromide was used in the renovation. The disease first occurs one
to four years after construction or renovation, gradually disappearing one to
three years after its first occurrence. Dead spot has also been reported in Texas
and Florida on hybrid bermudagrass greens that were overseeded with rough bluegrass.
Pathogen
Hosts
- Primarily bentgrass
- Hybrid bermudagrass
Symptoms can be confused with those of
- Copper spot
- Dollar spot
- Ball marks
- Cutworms
Symptoms and signs
The first symptoms of dead spot on bentgrass appear as reddish brown patches
about 1/2 inch in diameter (Figure 1). The patches are most common on mounds,
ridges, slopes and other historically dry areas on the putting surface. The diameter
of the patches never gets much larger than 3 inches, and the patches do not coalesce.
The patches are often sunken, interfering with putting quality. In the early
stages of disease development, the small patches may be mistaken for ball marks
or copper spot. As the disease progresses, the inner area of the patch turns
light tan and at this stage can resemble dollar spot (Figure 2). When the fungus
is active, the outer margins of the patch are red-brown to orange.
Disease generally starts in May, continuing throughout the summer until the
first hard frost. Disease incidence varies among locations of greens, and the
number of infection sites per green can range from a few to several hundred.
New growth into the patch is inhibited, so recovery is slow.
All plant parts become infected, including leaves, leaf sheaths, roots and
stolons. Unlike the dollar spot fungus, Ophiosphaerella agrostis generally does
not produce visible mycelium (mass of fungal vegetative growth) in the field.
One can sometimes observe a pink to white mycelium when plugs with symptoms are
incubated in a moist chamber for several days.
Pseudothecia (fruiting bodies) develop in abundance in leaves, leaf sheaths
and stolons (Figure 3). The pseudothecia are darkly pigmented, flask-shaped structures
detectable with a hand lens (Figure 4). Ascopores are produced in pseudothecia
in saclike structures (asci) (Figure 5). Mature ascospores are forcibly discharged
or exude from the fruiting bodies under conditions of high relative humidity.
Ascospores are easily spread by rainfall and irrigation water to initiate new
infection sites.
Conditions
Dead spot most commonly occurs on new greens or greens treated with methyl
bromide during a renovation project. The disease has not been observed on fairways
with underlying native soil. The disease gradually disappears, presumably in
response to the buildup of competitive microorganisms in the new or fumigated
root zone.
Dead spot is a high-temperature (77 to 86 degrees F), low-moisture disease
in which symptoms most commonly occur on areas of the green most prone to drying.
Management
Dead spot has been reported on nearly all commonly used creeping bentgrass
cultivars. Although there is some variability in cultivar susceptibility, dead
spot can probably be expected to occur under any new construction, irrespective
of the cultivar.
When only a few patches are observed, remove the infection center by plugging
out and disposing of the plugs.
When disease incidence is high, the use of fungicides at weekly intervals
may be necessary to protect putting quality. Refer to Table 5 for a list of fungicides
labeled for control of dead spot. Fungicide application will suppress the activity
of O. agrostis, but generally will not speed recovery of infection sites. Tank
mixing a low rate of nitrogen with the fungicide has been shown to promote recovery.
There is some evidence that dead spot severity is lower when ammonium sulfate
is used as the primary nitrogen source, whereas nitrate forms of nitrogen may
result in more severe disease. This is probably related to changes in pH at the
rhizosphere level. Amonium sulfate generally lowers rhizosphere pH in contrast
to nitrate forms, which generally result in higher pH at the rhizosphere.
IPM1029, new June 2007
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