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Diseases
By Chris Harrison
As with pests,
there are a number of diseases that can cause damage to your lawns...
Dollar Spot
Dollar Spot is
a disease that is caused by the fungus Sclerotinia
homeocarpa. It’s known more as a southern lawn disease,
but in reality can be found in all species of grass across the
country. Some of the more susceptible grasses to the disease are
Bentgrass, bermudagrass, fescue, ryegrass, bluegrass and zoysia, while
showing more tolerance are St. Augustine and centipede.
The disease
itself leaves a sort of silver dollar shaped circular area within a
lawn, usually in the range from 1 to 5 inches in diameter. Gradually
these spots may merge together to form larger, more irregular areas.
The activity
level of Dollar Spot is at its high point throughout the spring and
fall when warm, moist days are followed by cool nights. The fungus is
capable of growth in a wide range of temperatures, but the greatest
risk seems to be anywhere from the low 70’s to the low 80’s during the
day followed by the 60’s at night. Along with the range of
temperatures, long periods of leaf wetness from dew, rain or
sprinklers favor the growth of the fungus.
Other
contributing factors like low fertility levels with nitrogen and
potassium can increase the severity, but an excess amount of nitrogen
can also escalate the disease. Dry soil conditions, where the soil
moisture level is low, but moisture from dew or a light watering are
present on the leaf can also enhance the disease. Basically, the
length of time that foliage remains wet is the key factor as the
disease is more of a water stress disease. During times when the
temperature exceeds 90 degrees, the growth of the fungus may be
inhibited.
Spreads by way
of wind, water, mowers, shoes, etc… all the forms we’ve seen before,
and survives in unfavorable conditions by hanging out in plant tissue,
thatch, grass clippings, and in the soil as a dormant, compact mass of
mycelium, almost in a resting phase, as a stromata.
Fairy Rings
Many years ago
there was a folk-tale that went around that talked about “fairies”
that would dance around a ring shaped pattern during the night. Come
morning, this ringed area would be surrounded by a cluster of new
mushrooms and an outer edge of dark green grass that were attributed
to the “dancing fairies”…hence the name “Fairy Rings”. Today we know
that Fairy Rings aren’t caused by a group of dancing fairies, but are
in fact a disease that’s attributed to a number of soil inhabiting
fungi, primarily the Marasmius oreades
fungi.
Development
starts with either a spore that begins to germinate or a strand of
mycelium that has become active. The central body of the fungus, the
mycelium, will be underground where it will feed on dead vegetation
and other organic matter below the surface.
As the fungus
continues to grow, it will grow outward in all directions, in a
somewhat circular, or ring-like, formation. The rate of growth from
the central point can vary from a few inches to several feet per
year. Throughout this time, the mycelium will continue to spread
throughout the soil to depths up to a foot. It continues to feed off
the organic matter robbing the soil of nutrients that are beneficial
to your lawn.
The edges of the
mycelium continue growing outward, while the area within the center
eventually exhausts the soil nutrients and dies. This results in a
brown or dead area of grass encompassed by the growing edges. If you
were to try and dig into this dead area, you would find a dense growth
of white mycelium within the soil. In fact, the mycelium will be so
thick, that water will not be able to penetrate the area. As the
edges continue to grow, they secrete chemicals into the ground ahead
of them. These chemicals are used to break down the organic matter
releasing a rush of nutrients for the mycelium to feed on when they
reach that point. But for the time being, this rush of nutrients will
make the grass darker, taller, and thicker than the rest of your
lawn. A cluster of mushrooms, or the fruiting body of the fungus,
will also begin to appear in this nutrient rich area. Eventually, the
spurt of energy will be cut off as the mycelium reaches the area and
once again steals the nutrients.
After some time,
with the continued depletion of nutrients, the central mycelium will
die and the nutrients will be returned to the soil and the grass will
begin to grow again. However, during their growing ordeal, the sky’s
the limit for fairy rings, or “free” rings. If no barriers are to be
found, they will grow and grow and grow, sometimes up to 8 inches a
year and sometimes reaching a diameter of over 30 feet. If several
distinct rings were to converge together, this would present a barrier
for each ring. The fungus activity will stop at the area where the
nutrients have been depleted.
Three types of
fairy rings can be found in a lawn. The first type will have the
stimulated grass growth at the outer edges but will result in no
mushroom activity. The second type will have both the stimulated
grass growth and mushroom activity. Finally, the third type will only
contain mushroom activity but no simulated grass growth.
Fairy Rings
appear during spring and summer months and are most common in sandy
soils that are low in water and fertility. During times of
unfavorable conditions such as cold temperatures and a dry summer
drought, the development of fairy rings may halt. But, don’t be
alarmed they will come back!
Leaf Spot
Leaf Spot is one
of the most common turf grass diseases in the country. The disease
can affect all varieties of grasses (though Kentucky bluegrass seems
to be the most susceptible), and in fact, the fungi that cause left
spot are present in every established lawn. Now, don’t worry too much
as leaf spot itself isn’t a huge threat, as it causes no permanent
damage, but beware it can set the stage for a more serious “melting
out” phase of the disease if extreme weather conditions involving lots
of rain are met with improper lawn care practices.
The leaf spot
fungi can be active in a variety of conditions, but tends to favor
warm, humid conditions, where there is frequent moisture on the leaf
blades. During the spring, when temperatures reach anywhere in the
mid to upper 50’s, and moisture is in abundance with spring showers,
the fungus begins to grow and spread its spores by way of wind and
water. These types of weather conditions set the stage for grass
plants to begin to grow rapidly which the leaf spot fungi thrives
upon. During this 1st phase of Leaf Spot, the disease may
cause the lawn to thin a bit. Later, as spring turns to summer, and
the weather begins to warm, the disease moves to its 2nd
phase of “melting out”. During “melting out”, the severe stage, the
fungus will invade the crowns and roots of the grass, leaving the
grass weakened, rotted, and can result in the loss of the grass blade
and sometimes even the entire plant.
The life cycle of
the fungi that cause leaf spot and “melting out” is very simplistic.
As described above, when spores are exposed to moisture on a leaf
blade for a period of time, they begin to germinate and infect the
grass. The fungus will penetrate the leaf tissue at random or will
enter by way of a mowing wound. That’s why it’s best not to allow
your grass to grow too tall, only to mow the lawn too short.
Remember, proper lawn care practices are essential! So, the disease
penetrates the tissue, killing the cells and absorbing the nutrients.
Spring is when
the Drechslera poae and Drechslera
siccans fungi first become active. Infection may occur,
and expand to a thinning out that can further lead to “melting out”.
Summer, another species of the fungus, Bipolaris
sorokiniana, can attack the lawn during high temperatures and
relative humidity. Fall, the same fungi that were active in the
spring may crop back up as rainy conditions become more common again.
Usually not as severe as during the spring, but will increase the
potential of disease come spring. Finally, winter, the spores and
mycelium of the fungi will survive in the disease turf tissue waiting
for spring conditions to arrive.
Necrotic Ring Spot
Necrotic Ring
Spot is one of several forms of diseases that is caused by the
Leptosphaeria korrae fungus. Primarily effecting Kentucky bluegrass,
it can also have effects on red fescues and Bermudagrass. When
Bermudagrass is the host, the disease is referred to as spring dead
spot.
Favorable
conditions for the disease are during the spring and fall months,
March to May and September to November, when the temperature ranges
between 50 to 70 degrees. Throughout these months, the activity level
will rise during wet periods that are followed by dry, hot spells.
Other environmental conditions such as heat and moisture (or lack of)
can also have weakening effects on the grass structure making it more
susceptible to the disease.
Lawns that are
laid with sod seem to encounter more problems with ring spot then
lawns that are planted by other methods (such as seed). Sod that is
laid on hard, compacted clay soil can produce a lawn with shallow
roots, poor drainage and thatch buildup. This leaves the grass facing
the environmental conditions we just discussed which can cause the
favorable conditions for the disease. Even though it may seem
expensive at first to prepare and plant sod the right way, in the long
run it pays off as overall maintenance costs will be lower.
During
unfavorable conditions, such as the summer and winter, the disease
survives in infected plants, thatch layers, and dead plant debris as
mycelia (fungal threads) waiting for the favorable spring and fall
conditions to arrive.
As we saw with
red thread disease, ring spot fungus can be spread to non-infected
areas by way of foot and mechanical equipment.
Powdery Mildew
Powdery Mildew
is a disease that is commonly found in areas with poor air circulation
and shady conditions. The disease is the result of the fungus
Erysiphe graminis that thrives when nights are cool and damp
and days are warm and humid. The spores of the fungus can be spread
by rain and wind when infected grass blades come into contact with
non-infected blades. The spores will infect the new grass leaf at a
rapid pace, usually less than 2 hours, and will produce an abundance
of new spores in about a week’s time. During the winter, the fungus
will survive by taking shelter in infected plant material waiting for
the spring season to arrive where it will again go about producing
spores throughout the growing season.
So, to sum up,
disease is more of a shade disease that is most severe in the spring
and autumn when temperatures are cool and humidity is high.
Red Thread
Red thread
disease, or pink patch as it’s also known, is caused by the fungus
Laetisaria fuciformis. Deriving its name from the pink to red
thread-like strands that can protrude upward from the grass blade
tips, the disease is most commonly found on cool season lawns with a
fescue and ryegrass blend.
The disease
strives in the spring or fall when the temperature reaches a range of
60 to 70 degrees with an extended period of humid, wet conditions.
Other then the condition of the weather, red thread disease takes off
in lawns that are slow growing, stressed by soil compaction or
drought, and where the nitrogen and calcium levels are at low levels.
The fungus is
primarily dispersed by spreading the infected tissues to healthy areas
of your lawn. This can be done by water, feet, or mowing. Just
think, if you walk over an affected area onto a healthy area, you’re
contributing to the spread of the disease. But don’t put too much
blame on yourself; the wind can also be your enemy. The fungus can
produce spores that can be dispersed by the wind and the red threads
that are produced can be moved by the wind to touch nearby blades to
spread the infection. Invasion is quick, and takes little time for
the healthy areas to become infected. Usually leaves will begin to
die within 2 days.
Finally, during
unfavorable conditions, such as during the winter or during the dry
summer conditions, the fungal hyphae (any of the
threadlike filaments forming the vegetative part (mycelium)
of a fungus) and dried pieces of the “red thread” enable the fungus to
survive.
Rust
I assume most
of you have seen some sort of rust in your lifetime. Be it on your
car or some other appliance that’s been left outdoors for a period of
time, but I bet most of you didn’t know it was a disease that could
affect your lawn as well.
Rust is a disease
that typically effects slow growing lawns and those that are not,
you’ll never guess, cared for properly. What a surprise? Are we
seeing a common theme by now? The fungus, Puccinia,
which is the cause of the disease, shows most activity during moist,
warm conditions
Night
temperatures that range from the low 70s along with day temperatures
ranging around the 90s, heavy morning dew that lasts hours after
sunrise, and frequent amounts of rainfall all contribute to the
favorable conditions for rust development. However, on the other side
of the spectrum, warm, humid, cloudy days that are followed by hot,
sunny days can also lead to rust development. You can’t win!
Whatever the case, be it an excessive amount of rainfall, or none at
all, these scenarios lead to the ultimate conditions that the disease
looks for. Conditions where either the fertility levels of the soil
are low because of a lack of nitrogen (thanks to an abundance of
rainfall) or a slow down of grass growth (thanks to the hot, dry
conditions).
Kentucky
bluegrass, perennial ryegrass, and tall fescue can all be affected,
depending on the cultivar. Rust that has affected a lawn can weaken
the lawn making it more susceptible to other problems later on and
possibly result in death during the cold unforgiving winter months.
Besides the
weather conditions which are out of our control, rust favors shady
areas, grass that’s been cut too low, and newly planted sod. As with
other diseases, it spreads thru the air, water, shoes, lawn mowers,
etc…
Snow Mold
What a joyous
and relaxing feeling it is to watch the first snow fall of the
season. As the snow begins to pile up, you’re eventually left with a
beautiful white landscape. As picturesque as it all may seem,
remember there’s still a lawn full of grass underneath this blanket of
snow.
Where most fungi
thrive in warm, dry conditions, the snow mold fungi is most active in
conditions where the temperature is just above freezing and there is
moisture in the air. In fact, the perfect conditions for snow mold to
become active would be after a nice wet snow suddenly falls and covers
an unfrozen ground surface for an extended period of time. Since the
soil temperatures are relatively high, the grass plants are still
hardy and an excellent food base for the fungi. The snow mold works
under the cover of snow to damage your lawn, and even after the snow
has melted, as long as the grass remains cool and wet the fungi will
stay active. In the early spring damage will become visible. Like
all living organisms, the fungus needs moisture to survive. It will
be inactive during the warm summer months and during the winter if the
conditions are cold and dry.
There are two
types of snow fungi that affect your lawn, gray snow mold
and pink snow mold. The gray mold becomes active after
a prolonged snow cover. It will continue with its activity level
until the temperature exceeds 45 degrees or the surface dries out.
During the summer, the gray mold will become dormant and will survive
in thatch or in the soil as sclerotia, or a dense mass of branched
fungi that has the ability to store food and is capable of remaining
dormant for long periods of time. The pink mold becomes active in
cool, wet conditions from late fall through early spring, even in the
absence of snow. Underneath the cover of snow, there is greater
potential that the pink mold will become a damaging disease. It will
continue to stay active during wet fall and spring weather, and as
long as there is a temperature between 32 and 60 degrees. During the
summer, the pink mold survives as a spore in thatch and on plants.
Summer Patch
Grasses such as
bluegrass and fescue can be affected by a host of environmental
conditions during the summer, which can lead to the root and crown
structure of the grass to rot by the fungus Manaporthe
poae, also known as Summer Patch.
Sometimes
referred to as frogeye, summer patch begins to take shape late in the
spring when the soil temperatures reach the lower 50’s. The warming
spring weather, along with wet conditions will make the symptoms of
the disease more evident. The fungus will first colonize on the grass
roots and crown, penetrating the tissue and invading the inner root
tissue. Throughout this process, the grass plant should still be able
to absorb water and maintain growth, so everything may seem fine.
Come the hot,
humid summer months, the effects become much more noticeable as the
infected roots begin to die. The high soil temperatures inhibit the
ability for the roots to grow, and the grass plant is unable to
replace the diseased roots. With the roots becoming instable, an
adequate amount of water is not able to be collected and the grass
plant will die.
The fungus can
spread by way of infected equipment such as a mower, but is more
likely to spread between the grass plant roots. Like we saw with
Fairy Disease, the fungus will spread from a central infection point,
which results in the symptoms appearing in a circular patch.
Not really a new
lawn disease, but more of one that appears two to three years after
the grass has had time to establish. But once it appears, the fungus
can survive many years in the soil and the symptoms will develop year
after year in the same location. During unfavorable conditions, the
mycelium will take shelter in the infected plant tissue and plant
debris waiting for more favorable conditions to occur.
Click here to learn more about the
different types of
lawn diseases
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