Heat Stress and the Decline of Bentgrass
Leon T. Lucas, Ph. D.
Carolinas Golf Association Agronomist
Turf quality on bentgrass golf greens often declines during the summer in regions that have hot and humid weather. Many factors such as environmental stresses, soil properties, management practices and diseases have been associated with the problem. The death of older roots and poor growth of new roots during hot weather resulting in a few short live roots during heat stress periods contribute to the problem.
Optimum air temperature for shoot growth for cool-season grasses is 60 to 75 degrees F and optimum soil temperature for root growth is 50 to 65 degrees F. Shoot growth ceases when air temperature is above 90 degrees and root growth ceases when soil temperature is above 77 degrees F. These temperatures are often exceeded on bentgrass during the summer in the southeastern and central United States where this grass is grown on golf greens. I use an inexpensive hand-held infrared thermometer to measure canopy temperatures of bentgrass on golf greens. During mid-afternoon on a sunny day in the humid southeastern United States, the canopy temperature of healthy looking bentgrass is usually 2 to 4 degrees higher than the air temperature. The soil temperature at the 2 inch depth is usually similar to the air temperature. Temperatures of bentgrass with early symptoms of wilt, such as foot printing, are usually 8 to 10 degrees higher and are usually 15 to 20 degrees higher when blue wilt symptoms are present. On a summer day when the normal high air temperature is near 90 degrees F, these temperatures exceed the temperatures at which the growth of bentgrass shoots and roots cease. A deficiency of water in the soil and plants is related to the increase in canopy temperature.
The energy capture processes and metabolism in bentgrass is less efficient than in warm-season grasses and the efficiency decreases at higher temperatures. These metabolic processes contribute to the poor growth and recovery of this grass from other stresses such as diseases and traffic damage during the summer.
Wet soil conditions that cause low levels of oxygen in the soil contribute to the problem. Even high sand-content-greens with good drainage usually have saturated layers near the surface in the fine textured layer that contains more thatch. The thatch layer creates a perched water table around the crowns and stolons because this finer textured layer must be saturated before water will drain into the course-textured sand below. The excess water in the top 1/2 inch of soil can cause an oxygen deficiency and creates conditions favorable for disease causing fungi. Aerification practices are needed throughout the summer to help keep this layer open for good oxygen relations in the soil.
High soluble salts from too much fertilizer, or from irrigation water high in salts, contribute to the decline of bentgrass. The high salts often accumulate at the soil surface due to evaporation of water and damage or kill the roots and stolons. The damaged roots are more susceptible to root rot fungi and some Pythium species are encouraged by higher salt levels. Increased root rot and decline of bentgrass in North Carolina has been observed in turf samples with over 500 PPM of salts in the top 1/2 inch of sod.
Several diseases are usually associated with the decline of bentgrass during the summer. Pythium species are usually isolated from the declining plants. Research in North Carolina has identified 33 species of Pythium from bentgrass. Some of the species do not cause disease. Rhizoctonia brown patch, or Rhizoctonia species that can cause brown patch, are usually present at the same time. Research with fungicides has indicated that both Pythium and brown patch may be involved in the summer decline of bentgrass. It was discovered that the combination of Aliette plus Fore or Aliette plus Daconil fungicides gave the best disease control and turf quality on bentgrass greens during the summer. The best results were obtained when the fungicides were applied in 2.5 gallons of water per 1000 sq. ft. and not washed off the plants beginning about June 15, or in early summer, before decline symptoms became evident. The fungicides Fore and Aliette Signature that contain a pigment have been shown to give better turf quality that other similar types of fungicides. Some new fungicides, such as Heritage, that have activity on both Pythium and Rhizoctonia species have given good turf quality results in recent tests.
Management practices that encourage root growth will help to reduce that damage from summer stress. Aerification throughout the summer with small solid tines, spikers or a Hydroject, in addition to the normal spring and fall aerifications will help the grass to survive. Careful management of irrigation water to avoid too much or to little water is important. Application of water with hand-held hoses to areas that are beginning to show wilt symptoms will help to lower the canopy temperature and avoid having other areas with too much water in the soil. Improved air movement by removing trees or using fans around greens has helped in many cases. The wind blowing on the turf breaks up the humid air layer on the surface of the grass and the evapotranspiration cooling process in the grass is enhanced. The application of small amounts of water soluble nutrients with a sprayer during stress periods has helped to manage the decline during the summer.
In summary, golfers should expect some decline of turf quality on bentgrass greens in hot and humid regions of the country during the summer. Many different factors can be involved in the summer decline of bentgrass, which is a cool-season grass that grows best during the fall a spring. Some stress factors may be more important on some greens on a course or from one golf course to another. The superintendent tries to use the best management practices during the spring and summer, such as aerification and proper irrigation and fertilization, to make the turf as healthy as possible in the summer.
You can contact me at 919-779-3241, 919-604-4813 orE-mail for more information.