A version of this article appeared in GreenMaster, December 2006, Vol. 41, No. 6, pages 8-9.

Anthracnose of turfgrass: what is it, and how does it get into the plant?

by Dr. Tom Hsiang, University of Guelph

Anthracnose of turfgrass is caused by a fungus known as Colletotrichum graminicola. Earlier in 2006, a group of scientists at Rutgers University in New Jersey proposed renaming this fungus as Colletotrichum cereale based on some data that this organism is quite different than the one which attacks corn, and because the corn pathogen has priority for the name C. graminicola (Crouch et al. 2006). Both scientific names will probably appear in the popular and scientific literature for years to come, since scientific name changes take time to become accepted.

The name anthracnose is also somewhat puzzling and perhaps frightening, since it sounds like the word anthrax, which was involved in the bio-terrorism incidents a week after the September 11, 2001 attacks. Anthrax is caused by the soil-dwelling bacterium Bacillus anthracnis, and is a serious disease of cattle and sheep, which can be transmitted to humans to cause symptoms ranging from painful sores, fever to death. Fortunately these two diseases are very different, and anthracnose of turfgrass does not infect humans.

The word anthracnose comes from the Greek words "anthrax" meaning carbuncle (= an abscess larger than a boil) and "nosos" which means disease. Anthracnose diseases are very common on foliage of many different plants, ranging from deciduous trees species to grasses. Each fungus is generally specific to the host plants or group of related plants it affects. Anthracnose is defined by the production of a spore-bearing body called an acervulus (plural is acervuli), which usually look like tiny black spots on the leaf surface.

Anthracnose can be found on all northern turfgrasses, and causes a severe disease of annual bluegrass (Poa annua) called anthracnose foliar blight (AFB). The following conditions are associated with severe outbreaks of AFB: several hot days over 25C, severe stresses (drought, heat, compacted soils), and unbalanced fertility (too much or too little).

Disease cycle

Spores (Figure 1) are produced in acervuli (Figure 2), and these are water-splashed (rain or irrigation) onto crowns and foliage. The spores germinate, produce appressoria (Figure 1) and directly penetrate into the plant epidermal cells. The fungus feeds initially without visible damage, but then plants cells start to die and collapse as the fungus grows from cell to cell (Figure 3). After that, the fungus will produce acervuli on the surface of the infected area (Figure 4) from which spores are released ((Figure 5) to start the cycle again. For more details on the infection process, please see Khan & Hsiang (2003).



 

 

Figure 1. Spores (top) of the anthracnose fungus are generally cresent shaped and have more than one cell (cell wall visible across the center of each spore). The spores take up water and germinate to produce a tube (middle) which grows over the surface of the plant looking for a suitable entry point. The germ tube produces a dark bulbous structure called an appressorium (bottom) which glues itself to the plant surface and forces a peg into the plant for infection.

Figure 2. Acervuli (spore producing body) of the anthracnose fungus are found here on the stem of a creeping bentgrass plant, looking like eyelashes, and each one producing thousands of spores.

Figure 3. Fungal growth (thread-like hyphae) in epidermal cells of annual bluegrass stained with a blue dye, and growing from cell to cell. Although the fungus does not penetrate through the stomates (which look like little swimming pools in this image), the hyphae often emerge or erupt from stomates to produce acervuli on the surface of the plant.




 

Figure 4. An acervulus of the anthracnose fungi seen through a microscope with dark hairs called setae which give it the spiked look, and lots of spores between the setae.

Figure 5. Spores are being released from the surface of a leaf blade to initiate new infections and cause more anthracnose disease.


This infection cycle is a complicated process, and the fungus requires free water during the pre-penetration phase in order for the spore to germinate until it forms an appressorium. At this stage, the fungus is most vulnerable as it moves its living cytoplasm from the spore into the appressorium and into vesicles in the host plant cell. But no symptoms on the plants are observed at this stage, so turf managers would not know that infection is occurring. When disease breaks out, it can occur over wide area very quickly, seemingly overnight. But this only happens when the plant is under other severe stresses such as cyclical (e.g. daily) drought stress, heat stress, and wear stress, and unable to respond quickly to infection and to outgrow the fungus.

The fungus is present in the grass all year around, and is able to survive on dead plant tissues (saprophytically). There is probably a low level of infection all the time (chronic infection), but the disease does not break out unless there are severe plant stresses involved. This fungus is also able to cause a basal rot (ABR) which attacks the crowns and even roots of turfgrasses, particularly creeping bentgrass (Agrostis stolonifera). ABR occurs over a much wider range of climatic conditions from cool wet springs, hot humid summers to, warm wet autumns. Commonalities to these ABR outbreaks are long periods of wetness associated with stressful or injurious conditions to the plants such as topdressing, core aeration and other such cultural operations. For more information on anthracnose basal rot, please see an earlier edition of Green Master, Volume 34, Issue 3, Pages 38-42 (Hsiang & Goodwin 1999).

Disease Management

Recommendations for the control of AFB include stress reduction (heat & drought especially), low doses of nitrogen at frequent intervals (very infrequent large doses), dethatching to promote vigor, and avoiding night watering during periods conducive to outbreaks. Fungicides registered in Canada for the control of anthracnose contain the active ingredients azoxystrobin, propiconazole, and chlorothalonil. Please consult local provincial publications for details on fungicide usage.

References

Crouch JA, Clarke BB and Hillman BI. 2006. Unraveling evolutionary relationships among the divergent lineages of Colletotrichum causing anthracnose disease in turfgrass and corn. Phytopathology 96:46-60.

Hsiang T and Goodwin P. 1999. Anthracnose basal rot of creeping bentgrass. GreenMaster 34(3):38-42. http://www.uoguelph.ca/~thsiang/turf/survey99/basalrot.htm

Khan A and Hsiang T. 2003. The infection process of Colletotrichum graminicola and relative aggressiveness on four turfgrass species. Canadian Journal of Microbiology 49:433-442. http://www.uoguelph.ca/~thsiang/pubs/pdf/03cgram_cjm.pdf