Quick Facts...
- Necrotic ringspot (NRS) is a perennial disease of Kentucky bluegrass.
- NRS results in circular or doughnut-shaped patches of dead grass.
- Symptoms often develop in late summer.
- NRS can be controlled by the use of resistant varieties, good turf
management practices and fungicide applications.
Necrotic ringspot (NRS) is the most destructive disease of Kentucky bluegrass
in Colorado. The disease also damages red fescue and annual bluegrass.
Necrotic ringspot is particularly damaging to bluegrass because it is
a perennial problem and the fungus attacks and kills the roots and crowns.
Recovery from a disease outbreak is often very slow.
Symptoms
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| Figure 1. Necrotic ringspot symptoms
may initially develop as light green to straw-colored rings or frog
eyes in lawn. |
Necrotic ringspot often appears two to three years after lawn establishment,
although in some cases the disease may not develop for a decade or more.
Initially light green to straw colored patches from several inches to
several feet in diameter develop in the lawn. The patches may be localized
to one part of the yard or widely scattered. As the disease progresses,
patches develop more of a doughnut or frog eye pattern. The ring becomes
matted and crater-like whereas the grass inside the ring remains healthy
and green. Rings may eventually unite to create larger arcs of dead turf.
Symptoms may persist through the growing season and increase in size and
severity insuccessive years.
Bluegrass affected by NRS exhibit extensive root and crown discoloration
and rot. 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 threads may be visible with a hand lens on the root surface. The
presence of large frog eyes and root discoloration are usually diagnostic
for NRS. However, NRS can be confused with another disease called summer
patch. Plant disease diagnostic labs can verify the presence of NRS.
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| Figure 2. NRS symptoms may be localized,
as shown here, or scattered throughout the lawn. |
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Figure 3. Advanced necrotic ringspot
symptoms result in cratered rings and arcs of dead grass. |
About the Disease
Necrotic ringspot is caused by a soil-borne fungus called Ophiosphaerella
korrae. The fungus 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 and 80
degrees F. These temperatures occur from mid-May through mid-September
along the Front Range, but may vary somewhat on the west slope and at
higher elevations. The fungus 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, NRS symptoms often don’t
show up until late summer or early fall even though root colonization
occurs throughout the summer. Symptoms may persist through the winter.
Disease Management
Necrotic ringspot 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 NRS. Consider using these when establishing
a new lawn from seed or overseeding extensively damage areas. Also perennial
ryegrass and tall fescue are immune to NRS and can be used to overseed
damaged areas. Mixing perennial ryegrass seed with resistant bluegrass
seed (20:80 ratio based on weight) can also suppress NRS. However, some
people object to the reduced turf quality associated with this grass mixture.
Cultural Control
Necrotic ringspot tends to be more severe in lawns established from sod
and on sites with compacted soils. Excessive irrigation and fertilization
may also exacerbate the disease. Follow these cultural practices:
- Core aerate the lawn 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 water stress.
- Maintain a mowing height of 2 ½ to 3 inches.
- Avoid excessive amounts of nitrogen fertilizer (more than 4 lb N/
1000 square feet). Follow the recommended application times for Kentucky
bluegrass. 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 NRS develops, water the grass lightly at midday during periods
of high temperatures. This will help cool the turfgrass and may prevent
additional turfgrass death.
Chemical Control
For many years the fungicide fenarimol (trade name Rubigan) was used
to suppress NRS. However, this fungicide is no longer labeled for NRS
control. Several other fungicides including azoxystrobin (Heritage), myclobutanil
(Eagle), propiconazole (Banner MAXX, Propiconazole Pro, Fertilome Liquid
Systemic Fungicide), azoxystrobin + propiconazole (Headway) thiophanate
methyl (Cleary’s 3336 and 3336 Plus) and triadimefon (Bayleton) are
labeled for NRS. Most of these products are not packaged for sale at retail
outlets. Therefore, these fungicides are usually applied by professional
lawn care operators.
Timing of fungicide applications is critical for disease control. Products
must be applied in spring before root colonization by the fungus occurs.
Make an application in May when soil temperatures reach 65 degrees F at
a depth of 2 inches. Consider making a second application in 30 to 60
days to maximize disease control. Lightly water the fungicide into the
turf (less than ¼ inch) but do not drench the lawn. Fungicide applications
only suppress disease development; they do not eradicate the NRS fungus.
Therefore applications over several years may be necessary to manage NRS
on severely damaged lawns.
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