Golf Course Management

FEB 2018

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78 GOLF COURSE MANAGEMENT 02.18 have provided larger and more consistent re- ductions (4). Double-cutting and lightweight rolling are other practices that are useful to maintain acceptable playability (green speed) under a light-rate, frequent nitrogen fertiliza - tion program without intensifying anthrac- nose (12,15). Timing of granular nitrogen applications has been shown to affect suppression of an - thracnose, with spring applications being much more effective than fall applications in reducing disease severity (4,27). Thus, super - intendents who rely on late-season granular nitrogen fertilization and struggle to con - trol anthracnose may want to redistribute a greater portion of their granular fertilization to the spring. Recent studies evaluating nitrogen sources (urea, ammonium sulfate, ammonium nitrate, potassium nitrate) for light, frequent fertiliza - tion led to the discovery that deficiencies in potassium nutrition and moderately acidic soil pH will increase the susceptibility of annual bluegrass to anthracnose (28). Potassium When soil potassium is very low (<50 ppm), anthracnose will be more severe (27). interpretation of soil test data varies depend - ing on the extractant used by the laboratory conducting the test, as well as on the mineral - ogy of the soil (3). The tissue concentration of potassium in leaf clippings was also useful for assessing the likelihood that anthracnose would be respon - sive to potassium fertilization in our studies. Plots with tissue concentrations below 1.9% were considered deficient and suffered exten - sive damage from anthracnose (27). Annual potassium fertilization rates ranging from 1.1 to 4.4 pounds/1,000 square feet (5.37-21.48 grams/square meter) increased tissue potas - sium into the range of 2.4% to 3.0% and greatly reduced anthracnose. Root-zone pH Although root-zone pH did not have as striking an effect on anthracnose as nitrogen or potassium fertilization, plots with moder - ately acidic soil (pH < 5.5) exhibit greater an- thracnose severity compared with plots with a slightly acidic to neutral soil pH (27). The sampling depth for testing pH was the same as described for our potassium research; sam - pling depth was matched to the root zone (surface 2 inches), and was composed of top - dressing sand mixed with thatch. The pH of root zones composed of silica sand and thatch can be adjusted relatively eas - ily compared with the root zones of finer-tex- tured or calcareous soils. Putting greens that received liming (calcium carbonate) treat - ments that adjusted the root zone pH between 6.0 and 6.5 had less anthracnose, but they also had higher levels of extractable soil calcium. To determine the role of calcium, if any, in suppressing anthracnose, we have been con - ducting experiments to determine whether a greater level of soil calcium across a range of soil pH conditions will influence anthracnose severity. Matching the soil calcium of plots across multiple soil pH levels has been chal - lenging, but the most recent data indicates that increased soil calcium under acidic (pH = 5.6) conditions does not appear to affect an - thracnose. However, this research is ongoing. Mowing and rolling Mowing, rolling, topdressing and irriga- tion are often practiced to produce a high- quality playing surface rather than to improve the agronomic health of the grass. Producing excellent playability and healthy turfgrass at Nitrogen fertilization treatments were applied to research plots for the nitrogen rate study. Photo by James Murphy An accurate sampling of the root-zone pro- file is necessary for a correct assessment of the soil potassium status. The sampling depth for testing should be restricted to the depth of the root zone (surface 2 inches; 5 cm), which is largely composed of sand (from topdressing) mixed with thatch (organic matter). This area is often referred to as the mat layer. This is especially important when putting greens are grown on layered profiles such as those that are established on push-up construction. Any inclusion of a nutrient-rich soil layer from below the turf 's root zone will bias the test and result in an inaccurate interpretation that there is more available potassium than the turf root system can access. Our research, which used the Mehlich-3 extractant method to estimate the availabil - ity of potassium in root-zone samples com- posed of silica sand and thatch, identified a soil potassium level of 40 to 50 ppm as being critically low and likely to result in greater an - thracnose severity if potassium fertilization was not applied. It is important to note that only a modest amount of potassium fertiliza - tion (~1 pound/1,000 square feet) was needed to offset a potassium deficiency in the mat layer (27). Superintendents are reminded that

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