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Identification and Management of Turfgrass Diseases
Anthracnose basal rot
Symptoms and signs | Conditions | Management
Anthracnose basal rot is a destructive crown rotting disease of creeping bentgrass
and annual bluegrass on putting greens. On mixed bentgrass/annual bluegrass putting
greens, the causal fungus infects one species or the other but rarely both (Figure
1). In the Midwest, the disease has been observed more often on creeping bentgrass
than on annual bluegrass.
Colletotrichum cereale is commonly observed on senescing tissue of many grass
species, including those grown as forage crops. When diagnosing anthracnose basal
rot on putting greens, it is important to differentiate between an active infection
and a secondary infection on damaged, senescing or dead tissue.
Pathogen
Hosts
- Creeping bentgrass
- Annual bluegrass
Symptoms can be confused with those of
- Copper spot
- Red leaf spot
- Moisture stress
- Heat stress
Symptoms and signs
From a distance, infected annual bluegrass appears unthrifty and has a yellow
or bronze cast (Figure 2). Infected bentgrass first appears purple to reddish
brown, resembling a dry spot (Figure 3). The symptoms on the green often appear
as a patch, because the fungus differentially infects annual bluegrass or susceptible
selections (clones) of creeping bentgrass on the putting surface. Bentgrass within
the affected patches is thinned.
Colletotrichum cereale produces acervuli (fruiting structures) on leaves,
stems and crowns (Figure 4 top). Acervuli are surrounded by sterile setae (black,
hairlike structures) (Figure 4 middle). Setae are easily visible with a 20X
hand lens, so turfgrass managers and other turfgrass professionals often use
the observation of setae to diagnose the disease. To determine if the disease
is active, however, it is necessary to examine the acervuli for viable conidia
(spores), which are produced in a gelatinous matrix on the surface of the acervuli.
The conidia are single celled and crescent shaped and can only be seen under
high magnification (Figure 4 bottom). This usually requires the assistance
of a turfgrass pathologist. Another indication that the disease is active is
the observation of sporulation in green tissue (Figure 5).
Conidia germinate to form darkly pigmented appressoria (infection structures).
The presence of appressoria under the leaf sheaths on green tissue is another
indication that the disease is active. Appressoria are abundant on lower leaf
sheaths and appear through a hand lens as small pepperlike dots (Figure 6). A
close-up reveals the germ pore (Figure 7) through which C.
cereale penetrates
host tissue.
Crowns, old stems and roots of bentgrass or annual bluegrass appear black
due to the presence of mycelial mats (aggregate hyphae) and other fungal structures
in this tissue (Figure 8).
Conditions
Anthracnose basal rot most often develops during the warm, humid weather
of midsummer, but it may occasionally occur during cool weather. Disease development
is usually associated with predisposing factors such as high temperature and
humidity, excessive soil moisture, low mowing heights and soil compaction, all
of which stress the turfgrass. The disease also tends to be more severe in situations
where the greens have been allowed to dry out. The fungus has been observed to
overwinter in crowns and roots. Infected but symptomless plants may survive until
they are exposed to stress conditions.
Management
Anthracnose basal rot is one of the more difficult diseases to control, especially
after symptoms appear. The best strategy to prevent outbreak of this disease
is to combine cultural practices that reduce turfgrass stress with a preventive
fungicide program.
Turfgrass grown at optimal nutritional levels is less likely to be damaged.
Therefore maintain balanced fertility. Do not starve the turf, especially of
nitrogen, during the summer months. Light nitrogen application (0.1 pound per
1,000 square feet) during the summer may help the turfgrass withstand stresses
and recover quickly from anthracnose basal rot damage.
Low, frequent mowing can enhance disease development. Recent studies indicate
that raising the mowing height, even to 5/32 inch, can help reduce turfgrass
stress, which in turn can decrease anthracnose basal rot severity. Avoid double
cutting on greens with a history of the disease. Mow the perimeter of the green
every other day to prevent compaction. Don't mow when the greens are wet
if C. cereale is active. Also suspend topdressing activities, because physical
damage can result in more severe infection. Some research results suggest that
use of the growth regulator trinexapac-ethyl during the season can reduce disease
severity on annual bluegrass. Under heavy disease pressure, however, it is best
to suspend growth regulator programs to promote recovery.
Anthracnose basal rot tends to be more severe on putting greens with poor
air movement and slow infiltration rate. On greens with a history of the disease,
consider a rigorous core cultivation program in fall and spring. Compacted greens
may be periodically cultivated by spiking, slicing, or hydro-jecting, but avoid
excessive injury to the turf during periods of stress. Don't overwater
putting greens either by irrigation or by supplemental hand watering. Excessive
soil moisture can damage roots and decrease the rate of photosynthesis.
This puts the grass into a decline and predisposes it to infection. Monitor greens
regularly for dry spots, especially in the early spring. Anthracnose basal rot
often begins in areas that have undergone moisture stress. Application of a wetting
agent can help with moisture management.
On greens with a history of anthracnose basal rot, a preventive fungicide
program is necessary. In most regions, it is advisable to begin this program
in mid-April. Refer to Table 5 for a list of fungicides labeled for anthracnose
control. All products work best when applied on a preventive or early curative
schedule. Applications after development of severe damage to putting greens are
not very effective. The most rapid improvement from anthracnose damage occurs
following significant (cooler) weather changes, which promote turfgrass recovery.
Some strains of C. cereale resistant to MBC, DMI and QoI fungicides have been
reported. Avoid sequential applications of these products and tank mix with a
contact fungicide to reduce the selection of resistant populations.
Reports suggest that the preemergent herbicides bensulide and dithiopyr increase
disease severity.
IPM1029, new June 2007
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