General Information
There are somewhere in the neighborhood of 100 different diseases
that affect turfgrasses. Just like human illnesses, each turf
disease has a specific prescription for its cure and prevention.
Some diseases can be suppressed by the application of nitrogen
fertilizer, whereas others are encouraged by more nitrogen. Some
diseases are suppressed by high soil pH, whereas others are
encouraged by low pH. A long list of other turf management practices
can be used to control disease, but the most effective prescription
is specific for each disease.
As the older, broad-spectrum fungicides are removed from the market
due to environmental concerns, they are being replaced with a new
generation of products with narrow control-spectrums. These new
products are very effective, and safe to the environment, but they
are effective against a small number of diseases. Because of this
change in the turfgrass industry, accurate diagnosis of turfgrass
diseases is becoming even more important.
How do I diagnose a disease?
Turfgrass diseases are very difficult to identify. Grass plants are
very small, and most diseases are caused my microorganisms that can't
be seen without a microscope. However, with some basic knowledge and
a lot of practice, you can vastly improve your diagnostic skills. By
learning how to diagnose just a few common diseases, you will be able
to diagnose most of the disease problems that you encounter during
the season. If you cannot diagnose a problem with reasonable
certainty, then it may be worthwhile to submit a sample to a plant
disease clinic.
The first thing to remember is that disease diagnosis is much like
detective work. A detective rarely looks at a situation and solves
the mystery on the spot. Rather, a detective develops a list of
possibilities, gathers evidence by careful observation, and then
arrives at a conclusion by a process of elimination. When you see a
disease on your turf, first develop a list of possible
diseases. This list can be developed based on the turfgrass species
that is being affected and the time of year the disease developed.
Then, closely observe and make note of the symptoms and signs
that you see on the turf. Also, make note of recent weather
conditions and major cultural practices that were
performed recently.
What are symptoms and signs?
Figure 1: Dollar spot of creeping
bentgrass develops in small spots from 1 to 2 inches in diameter. | |
Most diseases leave a unique and reliable pattern on the turf. These
patterns are expressed as either symptoms or signs, and are very
useful for disease diagnosis. Symptoms are the evidence of
disease on the turf plants. Two types of symptoms are produced in
turf: stand symptoms and plant symptoms. |
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 Figure 2:
Gray snow mold is a
disease that appears in perfect circles up to 1 foot in diameter. |
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 Figure 3: Brown patch appears in
a patch, or an irregular-shaped area greater than 4 inches in diameter. Note
that the infection centers are not perfectly circular.
Figure 5:
Some diseases develop in an irregular pattern
across the turf stand. The irregular yellowing on this tall fescue landscape
is caused by rust.
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Stand symptoms are most easily observed by standing and
looking across the turf area. There are several different types of
stand symptoms, which basically describe the pattern of the disease
on the lawn or landscape. Diseases may produce spots,
circles, patches, rings, or an irregular
stand symptom. A spot is an area of diseased turf less than 4
inches in diameter (Figure 1).
A circle is a perfectly circular area of diseased turf
greater than 4 inches in diameter (Figure
2). A patch is an irregularly shaped area of diseased
turf greater than 4 inches in diameter (Figure
3). As the name implies, a ring is circular area of
diseased turf with healthy turf to the inside and outside, leaving a
ring-like pattern on the turf stand (Figure
4). Rather than producing a regular pattern of symptoms on
the turf, some diseases produce an irregular or non-patterned
symptom across the turf stand (Figure
5). Most diseases produce a very specific stand symptom, but
some diseases can produce more than one of the above symptoms.
Nevertheless, the stand symptoms are very useful for narrowing the
list of possible diseases. |
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 Figure 4: Fairy rings commonly produce a
ring, or a halo of diseased turf with healthy turf to the inside and outside.
Figure 6:
A leaf spot is a round or oval area with a distinct border that appears on the
leaf blade.
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 Figure 7: A lesion is an
irregularly shaped area with a distinct border that appears on the leaf blade.
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Far too many turfgrass managers
attempt to diagnose diseases just by looking at the problem from a
distance. However, there is another important set of clues available on
the individual plants, called the plant symptoms. Plant symptoms
describe the location and pattern of symptoms on the individual plants.
It is important, however, to look for plant symptoms in the right spot.
Generally, the best plant symptoms can be observed along the border between
healthy and diseased turf. The plants in severely affected areas that are
already dead are not very useful for diagnosis of turfgrass diseases.
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 Figure 8: Some diseases,
such as large patch, produce lesions on the leaf sheath instead of on the leaves.
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 Figure 9: Foliar blights
cause the rapid dieback of entire leaves or tillers.
Figure 11:
Root rots cause a noticeable reduction in root depth. The roots
of affected plants are typically dark and rotten.
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Symptoms observed on individual plants include leaf spots,
foliar lesions, stem lesions, foliar blight,
foliar dieback, crown rot, and root rot. A leaf
spot is a round or oval area on the leaf with a distinct border,
which is usually a different color than the center of the spot
(Figure 6). A foliar
lesion is irregular in shape and is typically larger than a leaf
spot, but still has a distinct border that is usually a different
color (Figure 7). A stem
lesion is very similar to a foliar lesion, but is present on the
stem or leaf sheath of the grass plant rather than on the leaves
(Figure 8). Foliar
blight and foliar dieback produce symptoms on whole leaves
or entire plants (Figure 9);
the two are distinguished in that a foliar blight produces a distinct
border between healthy and diseased turf, whereas a foliar dieback
does not. Crown rot is observed as a dark and rotten area at
the base of the turfgrass plant (Figure
10). Root rots produce a visibly dark and rotten root
system, and also a noticeable reduction in root depth in affected
areas (Figure 11). Crown
rots and root rots often occur together, and may also include rotting
of stolons and rhizomes if present (Figure 12).
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 Figure 10: Crown rots
cause a dark rotting evident on the base of turfgrass plants.
Figure 12:
Crown and root rots often occur together, and also cause rotting
of stolons or rhizomes if present. The crown, root, and stolon rot symptoms on this
zoysiagrass plant are caused by spring dead spot.
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 Figure 13: Large masses of hyphae visible to the naked eye
are called mycelium. Mycelium is usually most evident in the early morning hours when
the turf is wet from dew and guttation.
Figure 15:
The pathogen that causes anthracnose, Colletotrichum graminicola, produces spores
inside of black saucer-shaped sporophores caller acervuli. The small black hairs protruding
from the acervuli are called setae.
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Signs are the visible evidence of the presence of a pathogen.
Most turfgrass diseases are caused by fungi, and even though fungi
are microscopic organisms, some produce larger structures at certain
times in their life cycle that can be seen with the naked eye.
Mycelium is a cottony or spider-web-like mass of fungal growth
that certain fungi produce when the turf is wet or humidity is high
(Figure 13). Spore
masses are fuzzy or jelly-like growths produced on the diseased
tissue by certain fungi, again usually when the turf is wet or
humidity is high (Figure 14).
Sporophores are enclosed structures that contain fungal
spores. If present, sporophores are often seen as small, dark specks
on the diseased tissue (Figure 15).
Sclerotia are small, round, hard structures produced on the
diseased turf or in the thatch layer by certain fungi (Figure
16). Sclerotia are actually survival structures that some
fungi use to survive through periods of unfavorable weather
conditions. Most people are familiar with mushrooms, which
are the large spore-producing structures produced above-ground by
Basidiomycete fungi. Some turfgrass pathogens, most notably the fairy
ring fungi, produce mushrooms as a sign of their presence
(Figure 17).
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 Figure 14: The gray, fuzzy growth evident on these leaf spots are
masses of fungal spores. Spore masses produced by some fungi may appear jelly-like rather
than fuzzy.
Figure 16:
Sclerotia are small, round survival structures produced on diseased plants by certain pathogens.
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What other clues can be used to diagnose diseases?
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Is the disease limited to or more severe in a particular microclimate
on the turf stand? Some diseases are more severe in low-lying, wet
areas, whereas others are worse in high and dry locations. Some
diseases are encouraged by shade, whereas others are more severe in
open areas. Take a few minutes to carefully look at the distribution
of the disease and see if you can come up with any consistent
relationships - the power of observation is very valuable when
diagnosing a turfgrass disease.
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 Figure 17: Mushrooms are typically a sign of fairy ring
development in turfgrasses. |
The development of disease is highly dependent on weather conditions,
so recent weather conditions are important clues for disease
diagnosis. In order to grow and cause disease, fungi need moisture,
either in the form of rain, irrigation, or atmospheric humidity.
Fungi are also very sensitive to changes in temperature; each fungus
has a narrow range of temperatures in which it is able to grow and
cause disease. Since most disease development occurs at night when
the turf is wet from dew, nighttime low temperatures are the most
important parameter influencing the development of foliar diseases.
The development of root diseases is determined by soil temperature
rather than air temperatures.
Consider any cultural practices that were (or were not) conducted on
the turf in the last month. If a pound of nitrogen fertilizer per
1000 sq ft was applied two weeks ago, then your turf probably has a
disease that is encouraged by high nitrogen levels. If you recently
aerified and topdressed the turf, then it is possible that the turf
is affected with a stress-related disease like anthracnose basal rot.
Distinguishing diseases from other problems
There are many other problems that occur in turfgrasses that can be
confused with disease. These include cultural problems, environmental
problems, nutrient deficiency or toxicity, chemical spills, or a
myriad of other problems. There are a few rules of thumb that can be
used to distinguish these problems from disease. The first rule of
thumb is that diseases progress. Disease symptoms are
initially mild and localized in a small area, then the symptoms
gradually become more severe and widespread over time. The rate at
which a disease progresses depends on the disease, the host, weather
conditions, and cultural factors, but this generally occurs over a
period of several days to several weeks. In addition, there is
usually not a sharp, well-defined line between healthy and affected
turf, but this transition occurs very gradually. If large areas of
turf die overnight or during the course of one day, or if there is a
sharp line between healthy and diseased turf, then the damage was
probably not caused by a disease (Figure
18). One simple way to determine if a problem is actively
spreading is to mark the edge of the affected area with a surveying
flag or stake. Monitor the area over a course of several days and
determine if the problem is spreading in relation to the flag or
stake.
 Figure 18: This problem appeared overnight, and
there is a very distinct line between then healthy and dead turf. These
observations lead to the conclusion that disease is not responsible for
the damage. | |
Another rule of thumb is that diseases never occur in straight
lines or other regular patterns. Instead, disease symptoms are
randomly distributed across a turf stand. If the symptoms of a
problem are in straight lines or some other recognizable pattern,
then this damage was not likely caused by a disease. In most cases,
management-related errors, such as misapplication of a fertilizer or
chemical, are responsible for these types of symptoms
(Figure 19). |
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Finally, diseases typically do not kill large areas of turf and leave
adjacent areas of the same turfgrass species untouched. A disease may
be more severe in a certain microclimate, such as a compacted area,
but some symptoms of the disease will also be evident in adjacent,
less compacted areas. Environmental, cultural, or management-related
issues should be investigated when large areas of severe damage are
surrounded by large areas of completely healthy turf.
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 Figure 19: Diseases never develop in straight
lines. Problems that appear in straight lines or other recognizable patterns
are usually caused by misapplication of a fertilizer or pesticide. |
Review
Disease diagnosis is a process of elimination. First, start with a
list of possible diseases based on time of year and the species of
grass that is being affected. Second, look for stand symptoms, plant
symptoms, signs, and other clues. Based on the evidence that you
collect, narrow the list of possibilities. Repeat these two steps if
necessary. If you can narrow the list down to one disease, then you
have diagnosed the problem and can now develop a management program
for its control. If you can only narrow the list down to two or three
diseases, then you have two choices. First, you can design a
management program that will control all of the possible diseases.
This is a reasonable choice when the diseases can be controlled by
similar cultural practices or fungicides. Second, you can send a
sample of the affected turf to a plant disease clinic. This is
recommended when different cultural practices or fungicides are
needed to control the possible diseases.
I still can't diagnose the disease, what do I do?
If you can't narrow the list of possible diseases to a
reasonable number, then it may be time to send a sample of the turf
to a plant disease clinic. The Plant Disease and Insect Clinic at
North Carolina State University is one of the few clinics in the
United States that specializes in turfgrass disease diagnosis. For
more information on how to submit a sample to the PDIC, please
contact your county extension office or visit the following website:
http://www.ces.ncsu.edu/depts/ent/clinic/.
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Published by the North Carolina Cooperative Extension Service Distributed
in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment
and program opportunities are offered to all people regardless of race,
color, national origin, sex, age, or disability. North Carolina State
University at Raleigh, North Carolina A&T State University, U.S. Department
of Agriculture, and local governments cooperating.
Last update to information: April 2005
Last checked by author: April 2005
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