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Identification and Management of Turfgrass Diseases
Leaf and sheath spot
Symptoms and signs | Conditions | Management
Rhizoctonia zeae, which forms pink to orange bulbils (resting structures),
received little attention as a turfgrass pathogen until the early 2000s. Since
then the fungus has been observed with increasing frequency. There is ongoing
research and debate on the identification of these isolates. In preliminary DNA
sequencing studies, Tisserat has identified isolates from annual bluegrass from
Kansas and Colorado and an isolate from Kentucky bluegrass from Missouri as R.
zeae. Others have reported that isolates from annual bluegrass are an unknown
Rhizoctonia sp. with a Waitea sp. sexual stage. In this section we are reporting
our experiences with this disease.
Pathogen
Hosts
- Primarily Kentucky bluegrass
- Annual bluegrass
- Creeping bentgrass
- Perennial ryegrass
- Tall fescue
Symptoms can be confused with those of
- Brown patch
- Summer patch
- Fairy ring
- Yellow patch
- White grub damage
Symptoms and signs
In Missouri, leaf and sheath spot was first observed on a mixed Kentucky bluegrass/perennial
ryegrass athletic field in 2000. Symptoms occurred only on the Kentucky bluegrass
in this mixed stand. Since then the disease has been identified on other Kentucky
bluegrass athletic fields and sod fields, on perennial ryegrass from a tee and
in association with tall fescue. Symptoms in these situations occurred from early
to late August under high temperatures.
The symptom on Kentucky bluegrass usually appears as sporadic, dinner-plate-sized
rings of yellowed grass (Figure 1). The grass to the inside of the patch is usually
darker green in color. Sometimes the disease appears to be more of a general
blight (Figure 2). Sheath blight symptoms are commonly visible on individual
plants at the margin of a patch (Figure 3).
The hyphae (individual fungal strands) of Rhizoctonia zeae have the characteristic
right-angled branching pattern typical of other Rhizoctonia species. Additionally,
numerous bulbils have been observed in association with this disease on Kentucky
bluegrass, perennial ryegrass and tall fescue. The pink to orange bulbils are
about the size of BBs and range in shape from round to irregular. Bulbils have
been observed on the surface of decaying grass tissue and embedded in leaf tissue
(Figures 4 and 5 top).
R. zeae has also been reported to cause yellow ring symptoms on creeping
bentgrass and annual bluegrass on putting greens. On mixed greens, the disease
usually occurs on one or the other species and not both. Bright yellow rings
reaching several feet in diameter form in May and June under moderate temperatures
(Figure 6). Leaves and leaf sheaths may not exhibit extensive symptoms, but fungal
mycelium characteristic of Rhizoctonia spp. is usually present on tissue. Bulbils
have rarely been observed in association with the yellow ring symptom on bentgrass
or annual bluegrass in contrast to the disease on Kentucky bluegrass, perennial
ryegrass and tall fescue. Symptoms on putting greens may resemble those of fairy
ring or yellow patch. Laboratory examination may be necessary for confirmation.
Conditions
Little is known about the conditions favoring disease development on Kentucky
bluegrass, except that in Missouri it has been observed under high temperatures
in August. In contrast, ring symptoms on putting greens tend to occur in late
spring or fall during relatively mild temperatures.
As with other diseases caused by Rhizoctonia spp., high relative humidity,
abundant soil moisture, and heavy rainfall or irrigation tend to make leaf and
sheath spot more severe.
Management
Cultural management strategies used to suppress other disease caused by Rhizoctonia spp. should be helpful in managing leaf and sheath spot. For example, core cultivation
in the fall will help improve infiltration rates and prevent saturated conditions
at the surface. Avoid overwatering, especially late in the day. Managing the
thatch layer to avoid excessive buildup should also help to manage leaf and sheath
spot. Avoid large amounts of quick-release nitrogen when the disease is active.
On athletic fields or other sites with a history of leaf and sheath spot
occurring at high temperatures, a preventive fungicide application should be
made just before the historic onset of symptoms. For the most part, fungicides
used to suppress other Rhizoctonia diseases should also be effective in controlling
leaf and sheath blight, although the MBC group has been reported to be ineffective.
Variability in efficacy against R. zeae has also been reported among other groups
of fungicides. Check local recommendations for recommended fungicides. To date,
no products have R. zeae specifically listed on the label.
IPM1029, new June 2007
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