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Identification and Management of Turfgrass Diseases
Necrotic ring spot
Symptoms and signs | Conditions | Management
Necrotic ring spot is a destructive disease of Kentucky bluegrass, but may
also occur on red fescue and annual bluegrass. The disease is particularly damaging
to bluegrass, because Ophiosphaerella korrae infects and kills the roots and
crowns, resulting in slow recovery. The disease is also likely to occur in the
same location year after year.
Pathogen
Hosts
- Primarily Kentucky bluegrass
- Red fescue
- Annual bluegrass
Symptoms can be confused with those of
- Leaf and sheath spot
- Microdochium patch
- Summer patch
- Yellow patch
Symptoms and signs
The first symptoms usually appear in early fall or late spring. Initially,
light green to straw-colored patches from several inches to several feet in diameter
develop in the turf (Figure 1). As the disease progresses, patches develop more
of a doughnut or frogeye pattern. The ring becomes matted and craterlike, whereas
the grass inside the ring remains healthy and green (Figure 2). The patches may
be localized to one part of the turf area (Figure 3) or widely scattered throughout.
Rings may eventually coalesce to create larger arcs of dead turf (Figure 4).
Symptoms may persist throughout the growing season and increase in size and severity
in successive years.
Bluegrass infected by Ophiosphaerella korrae exhibits extensive root and
crown discoloration and rot (Figure 5). This can be observed by cutting a small
sod piece from the lawn and carefully washing off soil and organic matter from
roots. Darkly pigmented fungal hyphae may be visible on the root surface with
the aid of a hand lens. The presence of large frogeye patches and root discoloration
are usually diagnostic; however, necrotic ring spot can be confused with summer
patch. Summer patch symptoms usually do not appear until late June or July and
then symptoms tend to disappear with cooler weather in the fall. Also, frogeye
patches associated with summer patch tend to be smaller than those associated
with necrotic ring spot. Plant disease diagnostic labs can verify the presence
of necrotic ring spot.
Small pear-shaped pseudothecia (fruiting bodies) may form on decaying roots
and crowns, but this rarely occurs in the field (Figure 6 top). Darkly pigmented
multicelled ascopores are produced in asci (sac-like structures) within the pseudothecia
(Figure 6 bottom).
Conditions
O. korrae survives from year to year on dead, colonized bluegrass roots and
crowns or on the surface of living roots. The fungus actively colonizes the outside
of roots at soil temperatures between 65 degrees and 80 degrees F. In the Midwest,
these temperatures occur in the spring and fall, but along the Front Range of
the Rockies they occur from mid-May through mid-September, with some variation
on the west slope and at higher elevations. O. korrae eventually penetrates roots
and colonizes the root cortex. This either debilitates or kills roots and leads
to a decrease in water and nutrient uptake by the plant. Plants with extensively
damaged root systems are more prone to injury or death, particularly during periods
of high temperature or drought stress. Thus, symptoms often do not show up until
late summer or early fall even though root colonization occurs earlier. Symptoms
may persist through the winter.
Symptoms generally appear two to three years after turf establishment, although
in some cases the disease may not develop for a decade or more. The disease tends
to be more severe in areas established from sod and on sites with compacted soils.
Excessive irrigation and fertilization may also exacerbate the severity of the
disease.
Management
Necrotic ring spot can be a difficult disease to manage, but there are several
genetic, cultural, and chemical options that can help minimize damage. Several
varieties of Kentucky bluegrass, including 'Adelphi', 'Apex', 'Eclipse', 'Kelly', 'Midnight', 'NuBlue', 'Nugget', 'NewStar' and 'Wabash' have
moderate to good resistance to O. korrae. Consider using these when establishing
a new area from seed or overseeding extensively damaged areas. Necrotic ring
spot is not a problem on perennial ryegrass or tall fescue, so depending on the
region, these grasses can also be used to overseed damaged areas. Mixing perennial
ryegrass seed with resistant bluegrass seed (20:80 ratio based on weight) can
also suppress necrotic ring spot. However, some people object to the reduced
turf quality associated with this grass mixture.
Core cultivate the turf area at least once a year (spring or fall) to help
reduce thatch buildup and improve soil drainage. Do not overwater. Water the
lawn to a depth of 6 to 8 inches as infrequently as possible without creating
moisture stress. Maintain a mowing height of 2.5 to 3 inches.
Avoid excessive amounts of nitrogen fertilizer (more than 4 pounds of nitrogen
per 1,000 square feet), especially when most of this nitrogen is applied in the
fall. Consider the use of slow-release formulations of nitrogen. This allows
for more uniform release of nitrogen through the growing season and avoids "bursts" of
growth following fertilizer application.
If necrotic ring spot develops, water the grass lightly at midday during
periods of high temperatures. This will help cool the turfgrass and may prevent
additional turfgrass death.
Preventive fungicide applications may be effective in suppressing necrotic
ring spot. Refer to Table 5 for a list of fungicides labeled to control this
disease.
Timing of fungicide applications is critical for effective disease management.
Products must be applied in the spring before root colonization by the fungus
occurs. Apply fungicides in May when soil temperatures reach 65 degrees F at
a depth of 2 inches. Make a second application in 30 to 60 days to maximize disease
control. Fungicide applications suppress disease development but do not eradicate
O. korrae. Applications over several years may be necessary to manage necrotic
ring spot on severely damaged turf.
IPM1029, new June 2007
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