Homeowners install sod lawns and maintain them so they can
have beautiful, lush grass. Unfortunately, factors like insects and fungi try
to destroy that lush greenness of your sod lawn. One of those pests is a
condition called necrotic ring spot (NRS). Cool grasses are especially
susceptible to this condition. Find out if your sod lawn might be susceptible
to this damaging disease.
NRS and Cool Turfgrasses
Not all grass species are sturdy enough to be made into sod.
However, some species prized for their sod durability are Kentucky bluegrass
and fescues. The species are cool-season turfgrasses, which means they can
flourish in cooler temperatures. This fact makes them ideal for shady spots.
What's more, they're prized by lawn experts because they withstand heavy foot
traffic well.
Unfortunately, Kentucky bluegrass and fescues do have a
natural enemy - a fungus named Ophiosphaerella korrae
(O. korrae).
The fungus can infest any cool-season grass, but it's especially damaging to Kentucky
bluegrass
and fescues. The fungus infects both the crown and roots of the
grass, causing round, bald patches of within the grass.
Symptoms of NRS
The main symptom of necrotic ring spot is the bald ring that
gives the disease its name. However, once it's reached that stage, it's at the
full-blown phase of its lifecycle. The disease can start to form two or three
years after initial lawn establishment. It can stay localized, or it can spread
throughout the lawn.
During cool, wet weather, the first symptoms of NRS appear -
small, light green areas in the turf. When a hot, dry period follows that wet
weather, the fungus spreads. The rings can grow to several feet in diameter.
During the progression of the disease, the rings grow and
turn reddish brown before fading to straw yellow. At that point, the grass in
the perimeter of the ring has died and become matted. However, the grass inside
the ring stays green and healthy.
Diagnosis of NRS
Necrotic ring spot is tricky because it resembles other
patch diseases. Therefore, diagnosis isn't always straightforward. Lawn experts
sometimes use the season to distinguish it from a patch disease such as summer
patch.
O. korrae
can produce a dark brown or black substance
on the surface of infected grass. So turf specialists sometimes remove a small
portion of the infected grass and inspect it for signs of the O. korrae
fungus. They also look for blackened roots and rhizomes as well as rotted roots
and crowns on the blades of grass.
O. korrae
is persistent. It hibernates in plant
debris and the thatch layer. So it has a tendency to return after remaining dormant
for part of the year.
Managing NRS
Luckily, necrotic ring spot is not a death sentence for your
sod lawn. Naturally, you have chemical fungicides that can help kill the O.
korrae. The key with NRS is to apply said fungicide when the three-inch
depth of the lawn ranges between 55 to 70 degrees to ensure the fungus is
active. The availability of fungicides varies by state.
A multi-faceted approach is best in solving the NRS
infestation. So if you're having your lawn treated for this disease, you want to
thoroughly water it several days before fungicide application. Likewise, apply
fertilizer at the end of the fungus' life cycle - in the cooler weather.
Likewise, you should engage in good cultural practices to
protect your sod lawn. For one, avoid drought stress by watering your lawn
deeply but infrequently. However, during the hottest times of the year, light
watering every day is appropriate. Additionally, ensure the thatch compaction
doesn't reach over a half-inch, meaning you should thatch your lawn regularly.
The best management for necrotic ring disease is
avoidance. Choose cool-season sod with species resistant to the
O. korrae
fungus. Talk to the lawn experts at
Wright
Turf
about the best cool-season sod for your lawn.
