Anthracnose leaf blight and basal rot are diseases of many plants
including the turfgrasses,
annual bluegrass (Poa annua) and creeping bentgrass (Agrostis
palustris). On grasses, these two
diseases are caused by the same fungus, Colletotrichum graminicola.
This pathogen has long
been known for causing anthracnose blight on annual bluegrass, but has
more recently been
found causing basal rot on creeping bentgrass. Reports of basal rot
appear to be increasing in
central Canada, northern United States and Europe. Basal rot has only
been reported on annual
bluegrass and creeping bentgrass, but leaf blight has also been found
on fine fescues, perennial
ryegrass and various bluegrasses.
Strains of the the anthracnose fungus may be highly specific to host
species. In mixed turf, some
grass species remain unaffected whereas others are severely diseased.
Often, either annual
bluegrass or creeping bentgrass will be affected, but not both at the
same time. A study in the
northeastern U.S.A. showed that the strain from creeping bentgrass
could attack annual
bluegrass, but the strain attacking annual bluegrass was much less able
to attack creeping
bentgrass. However, the study did not show consistent genetic
differences between the strains
from annual bluegrass vs. creeping bentgrass. This lack of
differentiation between strains may
have been due to the relatively small number of infected plants that
were examined from a large
geographical area.
Symptoms
On turf, anthracnose blight can cause extensive damage virtually
overnight. Usually there is
some predisposing factor such as drought stress or heat stress. On
fairways, patches of annual
bluegrass are killed giving irregularly shaped reddish to bleached tan
turf among the surviving
creeping bentgrass (Figure 1). On leaves,
anthracnose blight results in irregular, tan to brown,
dead spots with dark margins, which can enlarge causing the entire leaf
blade to die back. On
leaves killed by anthracnose, small black spots known as acervuli can
be seen (Figure 2). The
acervuli are spore-producing bodies, and contain small dark-brown hairs
known as setae which
can be observed with a hand lens (Figure 3).
The presence of these hairs is a key characteristic to
identify this fungus.
Anthracnose basal rot differs from anthracnose blight in that it
infects the crown of the plant, and
then spreads upward. It almost always results in death of the plant.
The infected tissue usually
becomes blackened, particularly the stolons, as the fungus spreads in
the plant, and the
characteristic dark-brown setae can be found in the infected tissue
along with spores. The
blackened tissue can also extend into the roots. Small black fungal
masses can form in infected
tissue. Eventually, dead reddish or bronze-colored patches of bentgrass
appear (Figure 4), and
this can expand to several square meters as the fungus continues to
grow. Basal rot is more
common in western Europe, coastal British Columbia and western
Washington state; whereas
leaf blight is more common in warmer regions, such as central Canada
and the midwestern USA.
Life Cycle
The fungus overwinters as mycelium or spores in infected tissues.
Basal rot is favored by cool
(15-20C) moist weather during the late spring and early summer. Leaf
anthracnose is favored by
high humidity and much warmer temperatures, and is usually observed in
mid to late summer.
Anthracnose spores are readily spread by rain and splashing water, but
the fungus can also spread
by growth through infected tissues.
Conditions Favouring Disease
Anthracnose basal rot is frequently linked with poor soil conditions
and restricted root growth. It
can be favored by overcompaction, poor drainage and nutrient deficiency
in turf. Reducing
compaction by aerifying and improving soil fertility can reduce the
amount of disease. However,
for annual bluegrass, wounds in crowns created during aeration and
topdressing can also possibly
increase the amount of basal rot. Basal rot has also been linked to
damage caused by parasitic
nematodes feeding on grass roots, but the relationship of nematodes to
the disease is still
uncertain. Controlling nematode populations can reduce the amount of
basal rot, and a
combination of fungicide and nematicide was more effective in control
than fungicide alone.
However, fungicides are often ineffective in controlling basal rot,
especially when the plants
have been weakened, for example, by poor soil conditions. Fungicide
resistant strains of the
pathogen have also been reported from turfgrass.
Anthracnose basal rot is increasing in prevalence across Ontario.
Five years ago, it was virtually
unknown, but in the past two years, turf managers have been confronted
with increasing
incidences of a dieback due to a basal rot that has been very difficult
to manage. Despite
intensive use of fungicides, the grass often does not recover properly.
Even in the U.S.A, there
has been very limited research conducted on this disease. Many
recommendations made for its
control come from our experience with anthracnose foliar blight which
seems to have major
differences in development from anthracnose basal rot.
Because basal anthracnose has been observed more frequently in
Canada in the recent past, and
because of very limited research on this disease even in the U.S.A., we
conducted a survey
on the incidence of anthracnose basal rot, particularly in Ontario, in
order to examine the
relationship between anthracnose isolates which cause basal rot to
those which cause foliar
blight. By studying the differences between the fungal isolates in
terms of their genetic
relatedness and modes of infection, we can gain insights into how to
better manage the disease.
The results from the survey and associated research can be found in the
following presentations and publications:
Presentation at
the GTI Field Day, August 2000 (see #13).
Presentation at the Ontario Turfgrass
Symposium, January 2001.
Publication in the Guelph Turfgrass Institute Annual Reports 1999
(differences between fungal isolates)
Publication in the Guelph Turfgrass Institute Annual Reports 2003
(survey results)
Relevant Literature
Bolton, A.T. and Cordukes, W.E. 1981. Resistance to Colletotrichum graminicola in strains of Poa annua and reaction of other turfgrasses. Canadian Journal of Plant Pathology 3:94-96.
Browning, M., Rowley, L.V., Zeng, P., Chandlee, J.M. and Jackson, N. 1999. Morphological, pathogenic, and genetic comparisons of Colletotrichum graminicola isolates from Poaceae. Plant Disease 83:286-292.
Jackson, N. and Herting, V.J. 1985. Colletotrichum graminicola as an incitant of anthracnose/basal stem rotting of cool season turfgrasses. Proceedings of the International Turfgrass Research Conference. 5: 647-656.
Landschoot, P. and Hoyland, B. 1995. Shedding some light on anthracnose basal rot. Golf Course Management 63(11):52-55.
Shane, W.W. and Danneberger, T.K. 1989. First report of field resistance of Colletotrichum graminicola on turf to benzimidazole fungicides in the United States. Plant Disease 73:775.
Smith, J. D., Jackson, N., and Woolhouse, A. R. 1989. Fungal
diseases of amenity turfgrasses.
3rd ed. E and F. N. spon, New York, NY.
FIGURES
Figure 1: Fairway with morning dew showing
areas killed by anthracnose blight
(photo by T. Hsiang)
Figure 2: On leaves killed by anthracnose,
small black spots known as acervuli can be seen.
Researchers have speculated that fungal spores may be transmitted by
insects, such as the
ladybird larva pictured here above the dead leaves bearing acervuli.
(photo by T. Hsiang)
Figure 3: Acervuli are spore-producing
bodies, and contain small dark-brown hairs known as
setae which can be seen with a hand lens. The presence of these hairs
is a key characteristic to
identify this fungus.
(photo by T. Hsiang)
Figure 4: Anthracnose basal rot on a (cup
cutter-sized) plug of creeping bentgrass (photo by T. Hsiang)
