Turfgrass Diseases: Anthracnose (Causal fungus: Colletotrichum cereale)

Anthracnose is a foliar and crown/root rotting disease of golf course turf, caused by the fungus Colletotrichum cereale. It is typically associated with stress factors that weaken disease resistance in annual bluegrass, such as excessive shading, poor drainage, soil compaction, temperature extremes, nitrogen deficiencies, and low mowing heights.

Figure 1. Anthracnose symptoms on a golf course putting green. Photo: Peter Landschoot, Penn State

Symptoms and Signs

Symptoms of anthracnose may vary depending on the grass species affected and the time of year the disease occurs. On annual bluegrass putting greens, symptoms typically appear as bright yellowing of turf in irregular patterns. Patches can be an inch or two in diameter to more than a foot across. In rare instances, creeping bentgrass in putting greens may be affected by anthracnose and appear as irregular yellow, red, or bronze patches of various sizes.

Under cool temperatures, anthracnose-affected annual bluegrass may remain yellow for days or weeks before succumbing to the disease. When the disease occurs during warm and humid portions of the growing season, affected annual bluegrass turns yellow then dies quickly, leaving irregular patches of dead turf that may not recover for weeks. On greens composed of mixtures of annual bluegrass and bentgrass, the bentgrass is usually not affected and fills in diseased areas almost as quickly as the annual bluegrass dies out.

The most reliable diagnostic features of anthracnose are the dark coloration that occurs at the base of the plant and the presence of fungal spore-producing structures called acervuli on stems and leaves. On newly infected plants the crown region begins to turn brown. As the disease progresses, the crowns, roots, and lower stem bases become black. This is accompanied by yellowing of leaves, starting at the tips of older leaves and gradually progressing to sheathes, younger leaves, and shoots. As crowns and stems turn black, shoots can be easily separated from crowns.

Acervuli of C. cereale are often found on the leaves and stems of diseased plants and can be observed with a microscope or a powerful hand lens. These structures look like small pin cushions and contain hundreds of tiny spores, which can incite new infections. Although acervuli are good diagnostic features, they usually develop in the later stages of the disease and are not always present on newly infected plants.

Disease Development

Anthracnose can occur during periods of cool temperatures in early spring and warm, humid weather in summer. Optimum temperatures for growth of the pathogen are between 70 and 82°F, but the disease often occurs when soil and air temperatures are significantly lower or higher than this range. The role of temperature in anthracnose development probably relates more to the increased susceptibility of the host plant than with favoring the growth of the pathogen. As annual bluegrass growth is slowed and plants are stressed during very cool (early spring) or very warm temperatures (midsummer), plants are less able to defend against disease-causing activities of C. cereale. When low and high temperatures are combined with other stresses such as traffic and extremely low cutting heights, the disease pressure on plants is compounded and severe outbreaks can occur.

Anthracnose is often associated with poor soil conditions. In many cases, soil-related problems such as compaction, layering of different soil textures, use of inferior root zone mixes, and/or improper construction practices cause restricted drainage and poor root development, resulting in favorable conditions for the development of anthracnose. Wet soils are frequently associated with outbreaks of anthracnose; thus, the disease is most likely to occur following periods of excessive rainfall and high humidity. Any condition that slows the drying of turf and soil surfaces, including overcast periods, shade, poor air circulation, and poor drainage, tends to exacerbate anthracnose.

Research at Penn State and other universities has shown that nitrogen deficiency in annual bluegrass is an important predisposing factor in anthracnose development. Recent research at Rutgers University has demonstrated that soils deficient in potassium can enhance anthracnose disease symptoms.

Disease Cycle

The causal fungus of anthracnose overwinters as quiescent mycelium in dead and living plants. The mycelium can resume disease-causing activities in spring or summer. Spores produced in acervuli on diseased plants can be carried to non-diseased plants via water or on turf equipment and shoes. If conditions are conducive for spore germination, hyphal strands emerging from spores will form dark-brown microscopic structures called appressoria, which attach to plant surfaces and produce infection pegs that penetrate tissues and result in disease.

Cultural Management

Because anthracnose is often associated with poor drainage, compaction, and wet surfaces of putting greens, any management practice that alleviates these conditions, such as aeration, redirecting traffic, reduced watering, and tree pruning or removal, will generally help reduce disease severity. Where anthracnose is associated with soil textural interface problems or poor putting green construction, corrective measures may be required before the disease can be adequately controlled.

Nitrogen fertility is the single most important factor in managing anthracnose on annual bluegrass putting greens. Light, frequent applications of nitrogen fertilizer will reduce disease severity and aid in recovery from disease-related injury provided turf is not overstimulated during periods of heat stress.

Other management practices that may help reduce the severity of anthracnose on putting greens include proper adjustment of mowers to avoid scalping turf, light and frequent sand topdressing, lightweight rolling with smooth rollers, and soil testing to avoid potassium deficiencies in annual bluegrass.

Chemical Control

Control of anthracnose with fungicides can vary from one location to another depending on the product(s) used, the timing of applications, weather conditions, and other factors. Generally, preventative fungicide programs (beginning 2 to 4 weeks before symptoms appear) using penetrant fungicides or combinations of penetrant and contact fungicides at label recommended rates and timings, provide better control of anthracnose than post-symptom applications. Since disease outbreaks can occur at different times of the growing season, even within a limited geographical area, keeping records of when the disease occurs at your golf course can serve as a guide for application timing in subsequent years.

Because resistance to some penetrant fungicides with narrow modes of action has occurred with C. cereale, alternating penetrant fungicides in different classes and mixing contact fungicides with penetrant fungicides in a control program is suggested to delay the onset of reduced sensitivity or complete resistance. The best way to lessen the chance of resistance development is to employ a broad-based cultural management program that reduces the number of fungicide applications over the growing season.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide's ability to penetrate plant surfaces or remain on plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. Chemical control of turfgrass diseases 2020[2024]. University of Kentucky, Rutgers University, and University of Wisconsin.

Inguagiato, J.C. 2009. Mowing and rolling strategies to manage anthracnose on annual bluegrass. Golf Course Management. May, 111-116.

Latin, R. 2011. A practical guide to turfgrass fungicides. American Phytopathological Society Press, St. Paul, MN.

Murphy, J.A., C. Schmid, and B. Clarke. 2014. Annual bluegrass response to potassium and calcium fertilization and soil pH. USGA Turfgrass and Environmental Research Online. 14(2):35-38.

Murphy, J., F. Wong, L. Tredway, J.A. Crouch, J. Inguagiato, B. Clarke, T. Hsiang, F. Rossi. 2008. Best management practices for anthracnose on annual bluegrass turf. Golf Course Management. 76(8): 93-104.

Tredway, L. and F. Wong. 2012. Managing anthracnose with fungicides. Golf Course Management. June, p. 90-98.