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University
of Rhode Island GreenShare Factsheets
Turf
Diseases: Necrotic Ring Spot & Summer Patch
Despite the different names, these
two diseases have essentially identical symptoms. As the names
suggest, symptoms of necrotic
ring spot and summer patch include rings and circular patches
of dead or dying turfgrass. Sometimes there is a center of relatively
healthy grass in the "doughnut" or "frog-eye" patch. In addition,
weeds or non-susceptible turfgrass species may colonize the centers
of patches where the diseased turfgrass has died. Both diseases
are most severe on Kentucky bluegrass lawns that were seeded
or sodded two to four years previously. Fine-leaf fescues are
also susceptible. Bentgrass and perennial ryegrass lawns are
not susceptible to these diseases. Symptoms often become obvious
in warm to hot, dry weather when the lawn becomes stressed. Areas
along driveways, walls, sidewalks and other places that are heat-
and water-stressed will often show symptoms first. When the disease
first begins, rings or patches of blue-green wilting turfgrass
will develop in the heat of the day, but this early stage is
not commonly observed. With time, the patches may coalesce, obscuring
the circular pattern of the damage. In lawns containing mixed
turfgrass species, symptoms are not as distinct, but circular
patterns of diseased turfgrass are generally present. Both diseases
are caused by root-infecting fungi. Above-ground symptoms may
be confused with other diseases such as Rhizoctonia
brown patch. However, the roots and crowns of plants with
necrotic ring spot or summer patch will be brown to black and
rotted.
Leptosphaeria korrae (necrotic ring spot) infects
turfgrass in spring and fall in wet weather, but symptoms become
severe when environmental stress conditions impact the diseased
plants from early summer on.
Magnaporthe poae is the fungus which causes summer patch.
It is more active in hotter weather, so summer patch symptoms
are most severe in mid-summer, especially following a heavy rainfall.
The times of occurrence of these diseases overlap, so it is not
always possible to separate the diseases strictly by season or
field symptoms.
Both fungi produce very similar dark
brown, microscopic "runner
hyphae" that are found along the roots, crowns, and rhizomes
of turf-grass plants (it is not possible to separate the diseases
by looking only at these hyphae). In both diseases, infection
occurs before symptoms appear. The fungi spread from plant to
plant with the runner hyphae and are spread longer distances
when feet and mowers spread spores, soil, or infected plant parts.
Severe damage is most common on sodded Kentucky bluegrass
lawns that have been planted on poorly prepared sites with poor
drainage and compacted soil. It is important to properly prepare
a lawn site with well-drained topsoil, correct pH, and balanced
fertility. When possible, plant Kentucky bluegrass cultivars
with resistance to these diseases or consider using sod with
perennial ryegrass in the mix. This will help mask areas of infection.
Similarly, overseeding with resistant cultivars and perennial
ryegrass will improve already damaged areas. Once an area has
been seeded or sodded, avoid stressful growing conditions. Mow
at a recommended height and frequency. Water deeply, especially
in areas of heat and moisture stress. Some research indicates
that light mid-day irrigation in addition to routine irrigation
will relieve plant stress and improve natural microbial activity
against the fungus responsible for necrotic ring spot. Maintain
balanced fertility according to a soil test and avoid high rates
of nitrogen before hot weather. There is evidence that slow-release
nitrogen and some organic fertilizers reduce disease severity.
Ammonium-based nitrogen sources lower root zone pH which has
been shown to reduce summer patch. Remove excess thatch and aerate
compacted areas.
Penetrant, systemic fungicides can help prevent these diseases
but are not generally cost-effective for use on lawns and grounds.
Considerable damage to the root system has usually already occurred
by the time symptoms are noticed, so curative treatments with
fungicides may offer disappointing results. Because the infection
occurs in roots, foliar applications of contact fungicides are
not effective. Because of the expense and difficulty in fungicide
applications to turfgrass roots, the cultural controls described
above are strongly recommended for lawns.
Adapted from the UMass Extension,
2000
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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