Turf Notes:

Creeping Bentgrass Suffers Severe Heat Stress

Daily temperatures in excess of 90 degrees F during June and July have taken their toll on creeping bentgrass (Agrostis stolonifera) greens throughout Tennessee. Although several newer varieties have improved high-temperature tolerance, creeping bentgrass grows best at air temperatures from 60 to 75 F (Schlossberg et al., 2001). The growth of shoots (leaves, tillers and stems) and roots of this cool-season turfgrass slows as air temperatures rise above this optimum range. Unfortunately, when exposed to prolonged periods of high temperatures, bentgrass thins, and plant leaves become weak and may turn dark green, as one or more physiological processes is disrupted. Eventually chlorophyll is destroyed, and aerial shoots turn yellow, then brown.

Depending on the species and variety, direct high-temperature kill of cool-season turfgrasses usually occurs when the temperature of plant tissue reaches 100 to 130 F. The high-temperature hardiness of creeping bentgrass is most often superior to that of creeping red fescue and perennial ryegrass (Lolium perenne), but less than that of tall fescue. The actual lethal temperature depends on the amount of time plants are exposed and the ‘hardiness’ level and type of plant tissue. Generally, at a given extreme high temperature, the degree of injury increases with increasing relative humidity.

Since the optimum soil temperature range (e.g., 50 F to 65 F) for root growth of creeping bentgrass is lower than that for shoots, the leaves and stems of plants suffering high-temperature stress may continue growing while roots do not (Beard, 1973). When carbohydrates become limited, root cells have a lower priority than the cells of leaves and stems, where photosynthesis is taking place. Roots of plants experiencing severe heat stress for an extended period of time usually become spindly and brown before the walls of plant cells finally collapse and plants die. Roots mature more rapidly as the temperature of the soil increases above the optimum range. In New Jersey, when roots of ‘Penncross’ creeping bentgrass were exposed to a soil temperature of 95 F and shoots were kept at 68 F for 10 days, root production and the chlorophyll content of leaves decreased while root mortality increased (Huang and Liu, 2003). After 15 days at a soil temperature of 95 F, the growth, photochemical efficiency and relative water content of shoots also decreased (Huang and Liu, 2003).

It is difficult to consider turfgrass heat stress independent of soil moisture status. During hot summer months, creeping bentgrass commonly experiences heat stress in combination with a lack of transpiration. When turfgrasses are actively growing and soil is moist, water moves from soil pores into root hairs and is transported through the plant to leaves by way of vascular tissue, or xylem. Although some water is required in support of photosynthesis (6 CO2 + 12 H2O + light + chlorophyll ÷ C6H12O6 + 6 O2 + 6 H2O, or more simply 6 CO2 + 6 H2O + light + chlorophyll ÷ C6H12O6 + 6 O2), most of the water taken up by turfgrasses eventually moves into the atmosphere through small openings (stomates) in the leaves. Since energy is consumed as water vaporizes from the surface of leaves, the transpiration of water by turfgrasses results in a cooling effect. Evapotranspiration or ET (the return of moisture to the air through evaporation from soil and plant surfaces, and transpiration by plants) helps maintain favorable internal plant temperatures by cooling through the latent heat of vaporization. Drought-stressed turfs are very prone to high-temperature injury. When plants exposed to high temperatures become overheated, leaves often curl, stomates close and evapotranspiration is restricted. Plants that are not transpiring water may not take up or transport mineral nutrients and pesticides.

Too much water is also very problematic. A bentgrass rootzone containing excessive organic matter that holds water may eventually become anaerobic. Several species of bacteria thrive in soils low in oxygen. The activity of some (cyanobacteria) results in the formation of biofilms that impede water drainage, creating an anaerobic environment and contributing to the eventual development of a metal sulfide-containing and toxic “black layer” (Brent and Vargas, 2010, Hodges, 1992 and Postgate, 1984).  

Creeping bentgrass golf greens are ‘syringed’ in an effort to prevent both direct and indirect high-temperature injury. Indirect high-temperature injury of creeping bentgrass usually occurs when plants are exposed to high temperatures (air temperature greater than 85 F and soil temperature above 75 F) below the lethal temperature (about 120 F) for a prolonged period of time (DiPaola and Beard, 1992). Routine and very light applications of water during the day may temporarily reduce the temperature of leaves and the air surrounding leaves several degrees. For example, research conducted at Michigan State University on ‘Toronto’ creeping bentgrass revealed that when 1/4 inch of water was applied at noon, the canopy temperature was reduced from 2 to 4 degrees F for two hours following the application (Duff and Beard, 1966). The positive results of syringing can be very short-lived. In North Carolina, in the absence of wilt, syringing a Penncross creeping bentgrass green at 11:00 a.m. or 1:00 p.m. had little effect on the canopy temperature one hour after water was applied (DiPaola, 1984). In New York, applications of 0.12 inch of water between 11:30 a.m. and 3 p.m. resulted in a canopy temperature reduction of 8 degrees F two minutes after syringing and only 1 degree F after 10 minutes (Hawes, 1965). More recently, research conducted in Alabama suggests that the use of fans to direct air (e.g., at the rate of ~ 5.6 ft./sec. or ~ 3.8 mph) across greens surfaces along with syringing may significantly lower the temperature of the bentgrass rootzone compared to syringing alone (Han, et al., 2006).

Indirect high-temperature stress is often the primary cause of an initial decline in quality of creeping bentgrass greens. However, as the supply of carbohydrates is exhausted due to an imbalance between the rate of photosynthesis and that of respiration, secondary stresses may also occur. For example, indirect high-temperature stress may lead to a decline in viability of bentgrass roots, followed by root dieback and an accumulation of fresh organic matter at the soil-thatch interface (Carrow, 1996). This may result in less soil water infiltration, greater moisture retention near turfgrass crowns and a reduction in the amount of oxygen in the soil (Carrow, 1996). Populations of fungal pathogens may increase in response to an abundance of undecomposed organic matter and weakened plants (Carrow, 1996). Eventually, algae and summer annual weedy grasses may invade as the shoot density of bentgrass declines.

Several maintenance practices in addition to syringing and the installation and use of fans may help creeping bentgrass survive heat stress in summer. Considerations when managing creeping bentgrass suffering from heat stress include:

Elevating the cutting height. Mowing too short removes leaf area that could be available for photosynthesis. Closely mowed turfs are often prone to heat stress, as energy reserves become depleted in response to an increased demand for carbohydrates resulting from greater respiration and less production of carbohydrates by photosynthesis (Hull, 1992). Additional foliage may help insulate and buffer the soil against extreme high temperature.

Distributing use patterns to reduce soil compaction and wear injury. Turfs that are weak and thin as a result of heat stress generally recover slowly from wear injury. Frequent changing of cup locations can help prevent excessive wear and compaction due to concentrated foot traffic. Similarly, changing the direction of mowing, and turning the mower on greens surrounds rather than on the surface of the green may reduce wear injury and soil compaction.

Judicious nitrogen fertilization. Limiting nitrogen fertilization of creeping bentgrass greens during hot summer months by no means implies a need to avoid supplemental nitrogen applications. Rather, applying nitrogen at a rate beyond that which is normally recommended (e.g., 0.1 pound nitrogen per 1,000 sq. ft. per week) may not result in improvements in overall turfgrass quality. Research in Iowa has shown that an application of iron rather than nitrogen to cool-season turfgrasses growing in sandy soil may be more beneficial to improve the color of chlorotic, heat-stressed cool-season turfs (DeVetter and Christians, 2007). However, applications of iron before plants became chlorotic did not prevent tissue from yellowing (DeVetter and Christians, 2007).

Hand watering. In summer, creeping bentgrass on greens often suffers midday wilt. During periods of high ET, an internal drought stress develops as the rate of water loss from plants exceeds that of water uptake from soil. Rather than activating the irrigation system to apply water to the entire green surface, the possibility of over-watering areas of a green with adequate soil moisture can be reduced, and water can be conserved, by hand watering. Hand watering supplies bentgrasses with more water than syringing, and raises the leaf water potential (a measure of plant-water status – the lower the leaf water potential, the greater the internal drought stress) for a longer period of time compared to syringing (Peacock, 1999).

Alternating mowing and lightweight rolling. Research here at the University of Tennessee has demonstrated that alternating mowing with lightweight rolling (Speed Roller, Diversified Manufacturing Inc.) three days each week has the potential to reduce total maintenance cost, particularly for courses using walk-behind mowers, without compromising overall turf quality or bentgrass greens speed during summer (Strunk, 2006 and Strunk, et al., 2006). The use of grooved rollers should be avoided when mowing creeping bentgrass greens suffering from high-temperature stress.

Topdressing. Many sands contain particles with sharp edges that can damage leaves as they contact them. To limit injury to heat-stressed plants, topdress in the early morning or in the evening when it is cooler and light is less intense. Irrigate to incorporate sand into foliage rather than matting or brushing the newly topdressed turf.

Avoiding abrasive cultivation practices. The combination of high temperature and poor soil aeration may quickly lead to a decline in the quality of bentgrass greens. Turfgrasses have a greater demand for oxygen as temperatures increase (Kurtz and Kneebone, 1980). Conventional vertical mowing and aeration with large-diameter coring tines are too aggressive when creeping bentgrass is suffering severe heat stress. Routine hydraulic aeration or spiking (e.g., with 1/4-inch diameter hollow tines or solid quadratines) to vent greens improves the balance of air and moisture in the rootzone (Vavrek, 1999).   

Removing dew and using preventative fungicide programs on turf areas with a history of disease problems. Turfs growing on shaded sites with limited air circulation are often disease-prone. The surface of turfs shaded in the morning may remain moist for several hours after sunrise. This moist environment may be conducive to the development of algae and several diseases. Increased relative humidity resulting from poor air movement restricts transpirational cooling and also favors disease development. Early-morning dew removal (Cropper and Williams, 2009; and Danneberger and Vargas, 1983) and preventative fungicide applications (Golembiewski and McDonald, 2010; and Smith and Walker, 2009) may reduce the severity of certain diseases including dollar spot (Sclerotinia homeocarpa) and anthracnose (Colletotrichum graminicola).
      
Plant growth regulator (PGR) applications. Research conducted for two years on a Penncross creeping bentgrass green in New Jersey suggests that sequential applications of the PGR Primo® (trinexapac-ethyl) at 0.04 pound active ingredient per acre (e.g., 0.125 fl. oz. Primo® Maxx per 1,000 square feet) per application on two-week intervals from late June through August may improve turfgrass performance in summer (Xu and Huang, 2010). On several sampling dates, the foliar application of the PGR resulted in significant improvement in overall turf quality, stand density, tissue chlorophyll content and the rate of photosynthesis (Xu and Huang, 2010). Avoid combining treatments with PGRs such as Primo®, flurprimidol (Cutless® 50W) or paclobutrazol (Trimmit®) along with treatments of a dimethylation-inhibiting (DMI) fungicide. The growth-regulating properties of a DMI fungicide in addition to those of a PGR may limit bentgrass growth during and recovery after high-temperature stress. Fenarimol (Rubigan®), myclobutanil (Eagle®), triadmefon (e.g., Accost®, Bayleton® …) and triticonazole (Trinity® and Triton®) are DMI fungicides.

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Literature Cited:
      Beard, J.B. 1973. Turfgrass: Science and culture. Prentice-Hall, Englewood Cliffs, N.J.
      Berndt, W. L., and J. M. Jr. Vargas. 2010. The nature and control of black layer. Golf Course Manage. 78(4):p. 104-108.
      Carrow, R.N. 1996. Summer decline of bentgrass greens: Understanding the cause of this problem not only helps you cure SBD, but also lets you initiate preventive maintenance. Golf Course Manage. 64(6):p. 51-56.
     Cropper, K., and D. Williams. 2009. Towards reducing fungicide use in the control of dollar spot (Sclerotinia homeocarpa F.T Bennett) disease on creeping bentgrass (Agrostis stolonifera L.). Int. Ann. Meet. p. [53752].
     Danneberger, T. K., and J. M. Jr. Vargas. 1983. Forecasting anthracnose development on annual bluegrass from weather data. p. 47-48. In Proceedings of the 53rd Annual Michigan Turfgrass Conference. East Lansing, MI: January 18-19, 1983. East Lansing: Michigan State University.
      DeVetter, D., and N. Christians. 2007. Summer-induced chlorosis. Iowa Turfgrass Res. Rep. p. [1-2].
      DiPaola, J. M. 1984. Syringing effects on the canopy temperatures of bentgrass greens. Agron. J. 76(6):p. 951-953.
      DiPaola, J.M. and J.B. Beard. 1992. Physiological effects of temperature stress. P. 232-268. In Waddington, D.V., R.N. Carrow and R.C. Shearman (eds.) Turfgrass Monogr. 32. Amer. Soc. of Agron. Madison, WI.
      Duff, D.T., and J.B. Beard. 1966. Effects of air movement and syringing on the microclimate of bentgrass turf. Agron. J. 58:495-497.
     Golembiewski, R.C., and B.W. McDonald. 2010. Evaluation of fungicides and kelp for the preventative control of anthracnose in Oregon, 2009. PDMR: Plant Dis. Manage. Rep. 4:p. T010.
      Han, D., E.A. Guertal, and S. Phillips. 2006. Fans and syringing for cooling bentgrass greens: Further exploration. Golf Course Manage. 74(5):p. 106-110.
     Hawes, D.T. 1965. Studies of the effects of Poa annua as affected by soil temperature, and observations of soil temperature under putting green turf. M.S. thesis, Cornell Univ., Ithaca, N.Y.
      Hodges, C. F. 1992. Interaction of cyanobacteria and sulfate-reducing bacteria in subsurface
black-layer formation in high-sand content golf greens. Soil Biol. Biochem. 24:15-20.
      Huang, B., and X. Liu. 2003. Physiological responses of creeping bentgrass to high soil temperature. Proc. Annu. Rutgers Turfgrass Symp. 12:p. 45.  
     Hull, R. 1992. Energy relations and carbohydrate partitioning in turfgrass. pp. 175-205.
In D.V. Waddington, R.N. Carrow, and R.C. Shearman (eds.) Turfgrass. Amer. Soc. of Agron.,
Madison, Wis.
     Kurtz, K. W., and W. R. Kneebone. 1980. Influence of aeration and genotype upon root growth of creeping bentgrass at supra-optimal temperatures. Int. Turfgrass Soc. Res. J. p. 145-148.
      Peacock, C. H. 1999. Syringing and hand-watering quench greens' thirst. Grounds Maint. 34(7):p. G10-G12.
      Postgate J. R. 1984. The sulfate-reducing bacteria. Cambridge University Press, Cambridge.
      Schlossberg, M.J., K.J. Karnok, and G. Jr. Landry. 2002. Estimation of viable root-length density of heat-tolerant 'Crenshaw' and 'L93' creeping bentgrass by an accumulative degree-day model. J. Am. Soc. Hortic. Sci. 127(2):p. 224-229.
     Smith, D. L., and N. R. Walker. 2009. Evaluation of fungicide programs for the management of dollar spot and brown patch of creeping bentgrass, 2008. PDMR: Plant Dis. Manage. Rep. 3:p. T025 [1]-T025 [2].
      Strunk, W.D. 2006. Mowing and Light-Weight Rolling of Creeping Bentgrass (Agrostis stolonifera L.) Putting Greens During Summer Heat Stress Periods in the Transition Zone. M.S. Thesis: University of Tennessee, Knoxville.
      Strunk, W.D., J.C. Sorochan, C. Hall, J.S. McElroy, and T. Samples. 2006. The economics of managing creeping bentgrass putting greens in the transition zone. Agron. Abstr. p. [10].
     Vavrek, B. 1999. Ten techniques to try when greens nearly die. USGA Green Sec. Rec. 37(6):p. 8-10.
     Xu, Y., and B. Huang. 2010. Responses of creeping bentgrass to trinexapac-ethyl and biostimulants under summer stress. HortScience. 45(1):p. 125-131.
       Zhang, X., K. Wang, and E. H. Ervin. 2010. Optimizing dosages of seaweed extract-based cytokinins and zeatin riboside for improving creeping bentgrass heat tolerance. Crop Sci. 50(1):p. 316-320.

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The University of Tennessee Institute of Agriculture Turfgrass Team:
Drs. James Brosnan, Brandon Horvath, Tom Samples and John Sorochan, Plant Sciences Department; and Drs. Frank Hale and Alan Windham, Entomology and Plant Pathology Department.