Gray Snow Mold
Pink Snow Mold
Leaf Spot/Melting Out
Red Thread
Dollar Spot
Brown Patch
Gray Leaf Spot
Anthracnose
Pythium Blight
Leaf Rust
Powdery Mildew
Slime Mold
Fairy Ring
Take All Patch
Summer Patch
Necrotic Ring Spot
Rhizoctonia Large Patch
Yellow Patch
Turfgrass Disease Profiles
PURDUE EXTENSION
BP-103-W
Leaf Spot/Melting Out
Richard Latin, Professor of Plant Pathology
Turfgrass Disease Profiles
Until recently, leaf spot and melting out were
classified as a single disease and referred to as
Helminthosporium leaf spot. Leaf spot and melting
out actually are different diseases with similar
symptoms and pathogen characteristics.
They both attack Kentucky bluegrass, perennial
ryegrass, and tall fescue, and appear to cause the
most damage to golf course roughs, sports turf,
and residential lawns. Both pathogens can affect
all parts of the plant (leaves, shoots, and roots).
The major apparent difference between the two is
that the melting out pathogen is active during cool,
wet weather, while the leaf spot pathogen infects
during the heat of the summer.
Symptoms and Disease
Characteristics
From a distance, the symptoms of both diseases
resemble a diffuse pattern of off-color turf (Figure
1). Close inspection of leaves from affected plants
reveals decaying leaf blades with purple/brown
lesions (Figure 2). These diseases can damage
large areas of turf if favorable conditions persist
(Figure 3).
The symptoms of leaf spot closely resemble
gray leaf spot disease on perennial ryegrass (for
more on gray leaf spot, see Purdue Extension
publication BP-107-W,
Gray Leaf Spot, www.
extension.purdue.edu/extmedia/BP/BP-107-W.pdf).
Melting out (caused by
Bipolaris sorokiniana)
typically appears in April and May during cold,
rainy weather. High nitrogen fertilization during the
middle weeks of spring also favor the disease.
Leaf spot (caused by
Drechslera poae) is
a summer disease that requires long dew
periods, warm evening temperatures, and ample
precipitation to establish and spread. Close
mowing can aggravate disease outbreaks by
increasing stress on plants and allowing spores
greater access to crowns and roots.
Neither pathogen results in gray or white
mycelium growing on leaf surfaces. Instead, the
disease is spread by wind-blown and rain-splashed
spores. Both pathogens survive among infested
debris in thatch and the turf canopy.
Figure 1
Figure 2
Figure 3
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4/08
Turfgrass Disease Profiles
Leaf Spot/Melting Out
BP-103-W
PURDUE EXTENSION
PURDUE AGRICULTURE
Cultural Control Options
There appears to be good, reliable genetic resistance to leaf
spot and melting out infection in Kentucky bluegrass varieties.
Over-seeding damaged areas with resistant varieties (for
Kentucky bluegrass stands) offers the least expensive option for
long-term disease control. Resistant varieties and their ratings
are listed annually by the National Turfgrass Evaluation Program
(NTEP) at www.ntep.org.
Avoiding excess nitrogen fertility in early spring and
midsummer will limit the severity of disease outbreaks. Raising
the mowing height and relieving other stresses (like redirecting
traffic away from affected turf) also will reduce disease severity
and help hasten turf recovery. Also, avoiding irrigation during
the late afternoon and early evening during the heat of the
summer helps restrict the development and spread of leaf spot.
Fungicides for Disease Control
In situations where rather severe outbreaks of either of
these diseases occur in the same location in successive years,
fungicide application may be necessary to suppress disease
development to tolerable levels. QoI fungicides (stobilurins)
such as Compass®, Disarm®, Heritage®, and Insignia® are very
effective against these diseases. Iprodione (Chipco 26GT®) and
chlorothalonil also will help suppress disease development.
Application timing is critical for satisfactory fungicide
performance. Apply sprays at the first sign of the leaf spot stage
of the diseases, or apply them preventatively when disease-
favorable weather arrives. Once large areas are blighted to the
point of crown infection and plant death, the disease pressure
will be too great to be confident that a single application will
provide adequate control. Even repeated fungicide applications
may be only marginally effective in such cases.
Before selecting a fungicide, double check that your diagnosis
is accurate. Leaf spot resembles the gray leaf spot disease of
perennial ryegrass, which is caused by an unrelated pathogen.
Only the QoI fungicides are effective against gray leaf spot,
melting out, and the leaf spot disease. Fungicides that are highly
effective against leaf spot may have little or no effect on gray
leaf spot. Also, an application for melting out control in April will
have no effect on leaf spot development in July and August.
Home Lawn Help
Leaf spot and melting out can cause serious problems on
residential lawns. Mowing at a height less than 2 inches and
applying the bulk of nitrogen fertilizer in spring will increase the
vulnerability to disease outbreaks. Therefore, raising the mowing
height and applying most of the nitrogen fertilizer in the fall will
contribute to a healthier lawn that is less prone to these two
diseases.
Avoiding early evening irrigation during July and August also
will reduce the chance for leaf spot establishment and spread.
If a problem persists, the best approach is to over-seed with
resistant varieties.
Fungicides are expensive and can have especially variable
results if not applied at the most appropriate times. Note that
chlorothalonil may not be applied to residential turf. Consult
a professional lawn care contractor regarding fungicide
application for leaf spot and melting out control.
Other turf-related publications are available on the Purdue
Turfgrass Management Program Web site: www.agry.purdue.
edu/turf/pubs/index.htm.
