Quick Facts...
- Ascochyta leaf blight has become a common problem on Kentucky bluegrass lawns in Colorado.
- Large uniform areas of affected turf will turn straw-colored.
- Leaves usually start dying back from the tips.
- Ascochyta can occur throughout the growing season, but is more prevalent in the spring when there are extended wet periods.
- The first line of defense against Ascochyta leaf blight is to manage the turf properly.
Ascochyta leaf blight is a common disease of Kentucky bluegrass
lawns in Colorado. It may also occur on tall fescue and perennial ryegrass.
The disease results in rapid development of large, straw-colored blighted
areas of the lawn during the summer. The environmental conditions that
trigger Ascochyta leaf blight are poorly understood.
Symptoms
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| Figure 1: Bleached leaf tips and banding are characteristic
of Ascochyta leaf blight. |
Ascochyta leaf spot symptoms may develop throughout the growing
season but are more common during hot, droughty periods that were preceded
by cool, rainy conditions. Large irregular patches of turf rapidly turn
a straw-color and appear dead. The overall appearance of the disease may
resemble drought stress, except that the symptoms of Asochyta blight
appear quickly (i.e. sometimes overnight). Although the blighting within
an area appears complete from a distance, healthy leaves are interspersed
within the patch. Blighting is usually restricted to the leaves; bluegrass
crowns and roots typically are not killed.
Leaves infected with the Ascochyta fungus often exhibit a bleached
tip dieback that extends approximately a third to halfway down the leaf
blade. The margin between healthy and diseased tissue is abrupt and slightly
pinched, but doesn’t have the dark brown to purple banding that is
characteristic of another disease called dollar spot. In other cases leaves
may exhibit white banding or entirely collapse and shrivel. These leaf
symptoms resemble heat or drought stress.
The Ascochyta fungus produces minute yellow to dark brown, flask-shaped
fungal fruiting bodies called pycnidia in diseased leaf tissue. These
fruiting bodies, which are easier to view with the aid of a hand lens,
are peppered throughout the dead leaves and can be very useful as a diagnostic
feature.
Conditions
Ascochyta species can be found on senescing or dead leaves of
several turfgrass species, however the disease appears to be most serious
on Kentucky bluegrass.
The Ascochyta fungus likely survives as spores in pycnidia on
dead leaves or clippings remaining in the thatch. These pycnidia are highly
resistant to drought and extreme temperatures. Thousands of spores may
ooze from a single pycnidium during wet weather and be dispersed by splashing
rain, irrigation, mowing or other management activities.
Conditions that favor Ascochyta blight are poorly understood.
The disease occurs in late spring and summer and appears to be enhanced
by soil moisture fluctuations, especially drought stress caused by watering
restrictions and poor irrigation system coverage. However, the disease
may also develop during periods of hot weather preceded by unusually wet
soil conditions caused by excessive rain or over-irrigation. Frequent
mowing and dull mower blades may contribute to disease severity by creating
more infection sites (wounds).
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| Figure 2. Ascochyta
leaf blight on Kentucky Bluegrass |
Figure 3. Ascochyta
leaf blight. Note that not all of the leaves within a diseased area
are blighted. |
Management
Ascochyta leaf blight can be managed by following good cultural
practices that minimize stress in the lawn.
- Reduce thatch and promote water penetration through the soil by yearly
aerification.
- Maintain grass height between 2 ½ and 3 inches. Minimize wounding
of the leaf blades by maintaining sharp mower blades. Avoid mowing during
wet weather, especially when Ascochyta blight is active. Reduce
mowing frequency and increase mowing height during Ascochyta
outbreaks. Although the fungus can be spread from one location to another
on grass clippings it is unlikely to contribute significantly to disease
development because the fungus is already present throughout every lawn.
Similarly collecting clippings while mowing to reduce the amount of
fungus is unlikely to reduce disease severity. The disease is more likely
intensified by environmental factors and not the quantity of fungus
present.
- Maintain a balanced fertilization program. Follow Colorado State University
recommendations. Avoid excessive applications of nitrogen fertilizer,
especially in spring. This promotes rapid, succulent leaf growth that
requires more frequent mowing (hence wounds).
- Avoid drought stress. Check your irrigation system to make sure all
irrigation heads are working properly and that water is being distributed
uniformly. On the other hand excessive irrigation and poorly drained
soils may also promote disease development. Try to maintain a uniform
soil moisture.
- Do not use fungicides for disease control. Although several fungicides
will inhibit Ascochyta growth, they can be expensive and difficult
to apply. Furthermore Ascochyta leaf blight development is sporadic
and rapid, making timing of preventive fungicide applications difficult.
Similarly, curative (rescue) applications tend to be ineffective because
turf damage occurs rapidly. The turf is already damaged by the time
the fungicide is applied.
Remember that Ascochyta blight is primarily a foliage and not
a root or crown disease. Therefore individual bluegrass plants are usually
not killed. Given enough time, usually several weeks to a month depending
on weather, new leaves will emerge from the surviving shoots. Be patient
following a disease outbreak and maintain normal management practices.
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