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BMPs for anthracnose on annual bluegrass putting greens

Researchers at Rutgers University have developed best management practices for anthracnose. In this photo, leaves have turned yellow to brown and are sprinkled with black spore-bearing structures. (Photo: James Hempfling)

Researchers at Rutgers University have developed best management practices for anthracnose. In this photo, leaves have turned yellow to brown and are sprinkled with black spore-bearing structures. (Photo: James Hempfling)

Anthracnose (Colletotrichum cereale) is a destructive fungal disease that occurs on cool-season turf throughout the world (7, 30). During the mid-1990s, outbreaks of anthracnose on annual bluegrass (Poa annua) putting greens increased in frequency and intensity (8, 9, 21, 22). However, scientists within the Northeast U.S. made progress in developing best management practices (BMPs) for the suppression of anthracnose disease on golf course putting greens (23, 24).

Research has repeatedly demonstrated that anthracnose can be very destructive on weakened turf. Plant nutrient deficiencies, ultralow mowing and excessively wet or dry irrigation programs are all critical stress factors that significantly increase the risk of severe damage from anthracnose. As a result, BMPs include practices that address these plant health factors.

The research demonstrated that wounding caused by practices such as topdressing, light verticutting, coring and solid tining does not intensify anthracnose (10, 19, 26). Well-designed topdressing and cultivation programs that strengthen healthy turf will help reduce anthracnose symptoms.

Nitrogen

Superintendents need to evaluate whether nitrogen fertility is limiting turf vigor when attempting to manage anthracnose.

Frequent, light-rate nitrogen fertilization during the summer is highly effective at suppressing anthracnose (15). On research plots without fungicides, fertilization at 0.1 to 0.2 pound of nitrogen per 1,000 square feet per week greatly reduced disease severity (15, 28). In fact, in some years, 0.1 pound of nitrogen per 1,000 square feet per week, along with the correct combination of mowing height and sand topdressing, suppressed the disease to levels that were effectively managed with early curative fungicide applications (13).

Although some superintendents might consider 0.1 pound of nitrogen per 1,000 square feet a rather high rate, it is not excessive for managing anthracnose. Under high disease pressure, up to 0.2 pound of nitrogen per 1,000 square feet per week provides optimal disease suppression. Anthracnose did not intensify on putting greens until applications of 0.3 pound per 1,000 square feet per week (28).

The key to anthracnose suppression is maintaining sufficient nitrogen in the plant (about 3.35 percent nitrogen in leaf tissue) through frequent, light-rate applications of soluble nitrogen (14).

Plant growth regulator applications are an effective tactic for maintaining turf quality at low mowing heights and for eliminating excess vertical shoot growth caused by frequent, light-rate nitrogen fertilization. Embark (mefluidide) or Primo MAXX (trinexapac-ethyl) applied alone or in combination with Proxy (ethephon) have not intensified anthracnose severity (4, 15, 17, 18).

Sand topdressing had the greatest impact on reducing anthracnose disease severity under lower heights of cut (HOCs) and reduced nitrogen fertility. (Photos: James Hempfling)

Sand topdressing had the greatest impact on reducing anthracnose disease severity under lower heights of cut (HOCs) and reduced nitrogen fertility. (Photos: James Hempfling)

Double-cutting and lightweight rolling are other practices that are useful to maintain acceptable green speed under a light-rate, frequent nitrogen fertilization program without intensifying anthracnose (13, 16).

Spring granular nitrogen applications suppress anthracnose severity better than fall applications (4, 28). Superintendents who struggle to control anthracnose may want to redistribute a greater portion of their fall granular fertilization to the spring.

Potassium

When soil potassium is very low or low (< 50 parts per million (ppm)), anthracnose will be more severe (28). An accurate sampling of the root-zone profile is necessary for a correct assessment of the soil potassium status. Sample the surface 2 inches, which is composed mainly of sand from topdressing mixed with thatch (organic matter).

Any inclusion of a nutrient-rich soil layer from below the turfgrass root zone will bias the soil test. The result is an inaccurate interpretation that there is more available potassium than the turfgrass root system can access.

Our research, which used the Mehlich-3 extractant method, identified a soil potassium level of 40 to 50 ppm as being critically low. Levels below 50 ppm are likely to result in greater anthracnose severity if potassium fertilization is not applied. Remember that interpretation of soil test data varies for the extractant used by the laboratory conducting the test (3).

Approximately 1 pound of potassium per 1,000 square feet was needed to offset the effect of low potassium in the mat layer (28). Also, tissue concentrations of potassium below 1.9 percent were considered deficient, and turf suffered extensive damage from anthracnose (28). Annual potassium fertilization rates ranging from 1.1 to 4.4 pounds per 1,000 square feet increased tissue potassium into the range of 2.4 to 3.0 percent.

Table 1

Table 1

Root-zone pH

Although root-zone pH did not have as striking an effect on anthracnose as nitrogen or potassium fertilization, plots with moderately acidic soil (pH < 5.5) exhibit greater anthracnose severity compared with plots with a slightly acidic to neutral soil pH (28). The sampling depth for testing pH was the same as described for our potassium research.

The pH of root zones composed of silica sand and thatch can be adjusted relatively easily compared with the root zones of finer-textured or calcareous soils. Putting greens that received liming (calcium carbonate) treatments that maintained the root zone pH between 6.0 and 6.5 had less anthracnose, but they also had higher levels of extractable soil calcium.

Mowing and rolling

Mowing, rolling, topdressing and irrigation can produce a high-quality playing surface but can also impact the agronomic health of the grass. Creating excellent playability and healthy turfgrass at the same time is a challenge, especially when anthracnose is a constant threat.

Anthracnose is more severe at lower cutting heights (2, 30). Fortunately, other practices that improve green speed, such as double-cutting and lightweight rolling (16, 27), do not increase the severity of this disease (16).

Thus, a practical approach to improving anthracnose management is to identify a combination of lightweight rolling and/or double-cutting that provides the desired green speed at the highest-feasible cutting height.

Irrigation

Both excessively wet and dry soil conditions can intensify anthracnose (25, 30, 31, 32). Admittedly, some drying of the turf and root zone is necessary to produce firm and fast playing conditions. However, the ability to anticipate and prevent wilt stress on putting greens is key to minimizing the risk of intensifying anthracnose under a very dry irrigation program (25).

Topdressing

Topdressing with sand throughout the growing season is a beneficial practice for maintaining a firm and fast playing surface and, fortunately, is also effective at reducing anthracnose severity.

Before our research, initial speculation frequently suggested that sand topdressing would contribute to anthracnose epidemics on putting greens. However, topdressing has consistently reduced anthracnose in numerous research trials.

Any increase or continuation of a routine sand topdressing program may eventually require subtle adjustments in existing fertilization and irrigation programs to compensate for the increasingly infertile and droughty mat (sand + thatch) layer that develops over time.

The beneficial effect of sand topdressing is useful under conditions of heavy foot traffic (that is, the equivalent of 200 rounds per day with golf shoes fitted with soft spikes; 26). A study of sand incorporation techniques (stiff- and soft-bristled brushes, vibratory rolling and light irrigation) and sand particle shape (round versus subangular) indicated that these factors had little effect on disease severity (20).

The cost and interference to play can be significant challenges to implementing a routine topdressing program on putting greens. Frequent topdressing (100 pounds per 1,000 square feet every two weeks) during the playing season will reduce anthracnose severity (19, 20, 26). Use of finer sand (particles ≤ 0.5-mm) for topdressing enables the application of more sand with better incorporation during the summer and a similar or occasionally better suppression of anthracnose than coarser sand (35).

Heavier topdressing during the spring is also effective at suppressing anthracnose and is critically important if a frequent topdressing program during the summer is not practiced (12). The rate of sand topdressing needed during the spring to suppress anthracnose ranged from 400 to 800 pounds per 1,000 square feet (12, 34), which is a quantity sufficient to fill both the surface thatch and turf canopy layers. Higher amounts of sand will likely be needed when filling coring holes.

Heavy fall topdressings are less effective than summer and spring topdressing at suppressing anthracnose (34).

Combining BMPs

Increased nitrogen, higher mowing and greater topdressing are the most effective management regime for suppressing anthracnose (13). And, in two of the three years of the study with moderate epidemics, this best management regime suppressed anthracnose to levels that would be acceptable at many golf courses without the use of fungicides.

The playability of putting surfaces managed with these BMPs may not be acceptable at some golf courses. The green speed often dropped below the 9.5- to 10.5-foot range under 0.125-inch mowing height, especially when combined with higher nitrogen fertilization.

Only one compromise among the three BMPs (lower mowing height to 0.090 inch; 2.3-mm) was needed to achieve faster green speed (> 10 feet) more than 90 percent of the time (13). Fortunately, lowering the mowing height presented a modest risk for increasing disease severity compared with decreasing nitrogen fertilization.

Sand topdressing was effective in lowering disease under both nitrogen fertilization levels. However, the reduction in the disease was more dramatic under lower nitrogen fertilization (2 pounds per 1,000 square feet per year). Similarly, higher topdressing reduced disease under both mowing heights, but the reduction was more substantial under the lower (0.090-inch) mowing height.

Superintendents should recognize that routine topdressing is most beneficial under lower mowing and lower nitrogen fertilization. Lowering the mowing height to increase green speed will not greatly increase the risk for anthracnose when combined with higher nitrogen and routine topdressing practices.

Fungicide efficacy

These BMPs improve fungicide efficacy and make a reduction in fungicide inputs feasible (11). Acceptable disease control was achieved with reduced fungicide rates or fewer threshold-based applications of fungicides (extending the interval between sprays) when implementing BMPs (greater nitrogen and higher mowing).

Fungicide efficacy was always best under greater nitrogen fertility, where the fungicide rate could be lowered to 25 percent of label rates (a 75 percent reduction) and still provide adequate disease control. The number of fungicide applications to control anthracnose was reduced by 80 percent when mowing height was higher (0.125 inch), and nitrogen fertilization was greater (4 pounds per 1,000 square feet per year).

Preventive fungicide applications have typically been considered more effective than curative applications for the control of anthracnose (1, 24), and mixtures of two or more active ingredients applied as a tank mix or prepackaged product often perform better than individual products (33).

Preventive applications are often recommended at least three to four weeks before the typical onset of the disease symptoms on sites with a previous history of anthracnose. The purpose of preventive applications is to reduce the population of Colletotrichum cereale in the turfgrass before the occurrence of environmental conditions conducive to anthracnose (hot, humid weather). However, following the suggested BMPs with early curative sprays can result in significant reductions in fungicide inputs and excellent disease control.

Fungicides should be applied in 2 gallons of water per 1,000 square feet, using nozzles that produce a medium-to-coarse droplet size to optimize control (5, 24). Currently, 13 fungicide groups control anthracnose on cool-season turf (Table 1). Of these groups, the demethylation inhibitors (DMI), the quinone outside inhibitors (QoI or strobilurins), benzimidazoles, polyoxins and nitriles have been the most efficacious against anthracnose.

Chemical classes, including the phosphonate, dicarboximide, phenylpyrrole and SDHI fungicides, have been most effective when applied in a mixture with other anthracnose fungicides (Table 2).

It is good practice to avoid the continuous use of any product to reduce the potential for fungicide resistance. For anthracnose, there are reports of resistance for the benzimidazole and QoI fungicide groups. Also, there are indications of reduced sensitivity (reduced interval and/or rate response) with the DMI fungicides (1, 36). Please refer to The Fungicide Resistance Action Committee (FRAC; www.frac.info/) and follow the strategies to reduce the risk of resistance to fungicides.

Research Takeaways

  • Plant nutrient deficiencies, ultralow mowing and excessively wet or dry irrigation programs are critical stress factors that significantly increase the risk of severe damage from anthracnose.
  • Avoiding deficiencies in nitrogen and potassium is critical to reducing anthracnose severity.
  • The combination of three BMPs (increased nitrogen, higher mowing and greater topdressing) can suppress anthracnose to acceptable levels without the use of fungicides.
  • To improve green speed at lower mowing heights (< 0.125 inch), use higher nitrogen and routine topdressing practices to reduce anthracnose severity.
  • Routine topdressing is most beneficial under conditions of lower mowing and lower nitrogen fertilization.
  • Fungicide efficacy is improved, and a reduction in fungicide inputs (as much as 80 percent) is feasible with the adoption of BMPs.
  • Following these BMPs with early curative sprays can result in significant reductions in fungicide inputs and excellent disease control.

Funding

This work was supported by the USDA National Institute of Food and Agriculture Hatch Multistate Project 0206183 through the New Jersey Agricultural Experiment Station, Hatch Multistate Project NJ12294. The authors thank the Environmental Institute for Golf, GCSA of New Jersey, Tri-State Turf Research Foundation, United States Golf Association, the New Jersey Turfgrass Association and Rutgers Center for Turfgrass Science for the financial support of this work.

Acknowledgments

The authors acknowledge the contributions of Joseph A. Roberts, Charles J. Schmid, James W. Hempfling, Ruying Wang and Kyle Genova.

James Murphy (jamurphy@njaes.rutgers.edu) is an Extension specialist in turfgrass management, and Bruce Clarke is director of the Center for Turfgrass Science and an Extension specialist in turfgrass pathology in the Department of Plant Biology at Rutgers, The State University of New Jersey, New Brunswick, N.J. John Inguagiato is an associate professor in turfgrass pathology in the Department of Plant Science and Landscape Architecture at the University of Connecticut, Storrs.

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