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.
patch (photo from the Maryland Cooperative Extension)
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
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.
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.
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
from the UMass Extension, 2000