An earlier version of this article appeared in summer 1999 in 

"Green is Beautiful" (June 1999, page 20-21), and

"GreenMaster" (Volume 34, Issue 3, Pages 38-42)

.

Anthracnose basal rot of creeping bentgrass

by Drs. Tom Hsiang and Paul Goodwin, Plant Pathologists, University of Guelph

Funding provided by the Ontario Turfgrass Research Foundation.

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.

Survey

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.

Research publication in the Canadian Journal of Microbiology showing infection structures of Colletotrichum graminicola

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)