Kentucky bluegrass comes back to fairways
Kentucky bluegrass is the primary turfgrass species for home lawns, grounds, parks and sports fields in the cool-arid and cool-humid parts of this country. At one time, most golf-course fairways in these regions consisted of Kentucky bluegrass. However, in the late '60s and early '70s, the popularity of Kentucky-bluegrass fairways waned, for several reasons: * Intolerance to low mowing heights; * Susceptibility to diseases, such as leaf spot, necrotic ring spot and summer patch; * Moderate (at best) traffic tolerance; * Inability to compete with Poa annua; * Undesirable playing surface.
Golfers complained of the poor playing conditions on Kentucky-bluegrass fairways, and superintendents were finding that their fairways had converted to annual bluegrass. As a result, courses planted fewer fairways to Kentucky bluegrass and breeders placed less emphasis on developing new cultivars. However, recent developments in Kentucky bluegrass have renewed interest on fairways.
Kentucky bluegrass qualities Kentucky bluegrass typically forms medium- to high-quality turf, particularly in the cool, semi-arid portions of the United States. Its subsurface rhizomes help it form dense sod and recover well from injury. Cultivars that tolerate mowing heights of 0.5 to 0.75 inch provide the best fairway turf conditions.
Kentucky bluegrass prefers fertile, well-drained soils and sunny sites. Some cultivars display improved shade tolerance, but dense shade still is a limiting factor to Kentucky-bluegrass adaptation. Kentucky-bluegrass cultivars that do tolerate shade are able to do so primarily by their resistance to powdery-mildew disease.
Kentucky bluegrass tolerates a range of soil pH, but iron chlorosis is a problem when pH exceeds 7.5. Chlorosis is more pronounced on cultivars with light-green genetic color. Kentucky bluegrass has moderate-to-high wear tolerance and tolerates medium to high soil compaction reasonably well. Cultivars classified as aggressive are better adapted to intense traffic.
Kentucky bluegrass has excellent winter hardiness. It recovers quickly from desiccation injury and is more tolerant of direct low-temperature injury than annual bluegrass and creeping bentgrass. Growing points on rhizomes allow Kentucky bluegrass to initiate new growth and recover from injury that may occur from freezing, desiccation and winter traffic.
New Kentucky bluegrass developments Several new cultivars perform well at low mowing heights (see table, below). The National Turfgrass Evaluation Program (NTEP) trials that began in 1995 include a number of locations maintained as fairway turfs. These trials, which will continue through 2000, are in their fourth growing season and are providing useful information to those individuals who are interested in cultivar adaptation for fairway use.
Superintendents will find this information helpful when selecting cultivars that are well adapted to their location and needs. For fairway conditions, select Kentucky-bluegrass cultivars that display aggressive characteristics, such as: * Rapid establishment * Tolerance to low mowing * Fine texture * High shoot density * Strong lateral spread * High sod tensile-strength values * Rapid recuperative potential * Outstanding turfgrass quality * Excellent disease resistance
Cultivars with these characteristics tolerate fairway conditions, mix effectively with perennial ryegrass, recover rapidly from divot injury and compete better with Poa annua.
Interest is growing among private turfgrass breeders to develop new cultivars with improved disease resistance and tolerance of low mowing heights. However, Kentucky bluegrass is not an easy species to improve. It has a wide range of chromosome numbers and tends to be highly apomictic (it develops seed without fertilization).
These characteristics make it difficult to incorporate new traits in improved cultivars, so progress will be relatively slow.
Disease problems Disease problems limit Kentucky-bluegrass use in intensively managed fairway conditions. Necrotic ring spot, summer patch, powdery-mildew and leaf-spot diseases reduce Kentucky-bluegrass performance in many regions. To a lesser extent, diseases like dollar spot, brown patch and snow mold can also be a problem.
Susceptibility to leaf-spot diseases is one of the reasons early cultivars did not perform well under fairway conditions, particularly in the cool-humid portions of the central and eastern United States. As the disease progresses, the melting-out stage weakens and thins the turf, allowing weeds such as Poa annua to invade the stand. It also stresses the turf and limits its ability to recover from environmental stress. Leaf-spot resistance is an essential characteristic for cultivars used in fairway turfs.
In the 1960s and 1970s, breeders thought that cultivars with leaf-spot resistance would improve Kentucky-bluegrass performance under fairway conditions. However, these cultivars soon succumbed to pathogens such as summer patch and necrotic ring spot. These diseases seemed to be more severe on closely mowed, heavily fertilized and intensively trafficked turfs-typical conditions for most fairways. Data from the 1995 NTEP Kentucky-bluegrass trial indicates that cultivars with resistance to necrotic ring spot and summer patch do exist. Superintendents should use this information when selecting cultivars to use in fairway turfs. Turfgrass breeders are continuing to improve Kentucky-bluegrass disease resistance and tolerance.
New Kentucky-bluegrass cultivars with improved disease resistance and fairway adaptation also may be the answer for golf-course superintendents with perennial ryegrass fairways and who are dealing with gray leaf spot (Pyricularia grisea). This disease, called "blast" by many in the industry, has devastated perennial-ryegrass fairways in many parts of the East and Midwest. Under the right conditions, blast can wipe out infected perennial-ryegrass turf in as little as 48 hours. We know of no resistance among commercial perennial-ryegrass cultivars, and fungicidal control is both limited and costly.
Fortunately, Kentucky bluegrass is not susceptible to gray leaf spot. Thus, in areas where gray leaf spot is a potential problem, superintendents should consider mixing one of the new, improved cultivars of Kentucky bluegrass with perennial ryegrass, either in new seedings or into existing stands. The Kentucky bluegrass will ensure that gray leaf spot will not devastate the entire fairway, and it also will improve the recuperative potential of the turf.
Earthworms Earthworm castings are a common complaint of golfers playing on ryegrass fairways. Superintendents have found that incorporating Kentucky bluegrass into perennial-ryegrass fairways reduces this problem. Kentucky bluegrass adds density that seems to cover up the earthworm castings, making them less evident.
Traffic tolerance Kentucky bluegrass has medium-to-high tolerance of traffic stress. However, its traffic tolerance is best at mowing heights in excess of those typical for fairways. Older cultivars were not selected or adapted for use under low mowing heights. When placed under fairway conditions, these cultivars were weak, disease-prone, and susceptible to traffic injury. They did not recover well from wear injury and compaction stress. As turfgrass breeders have renewed their interests in developing cultivars suited for fairway conditions, traffic tolerance under low-mowing conditions has improved.
For example, we maintain the 1995 NTEP Kentucky bluegrass trial at the University of Nebraska under fairway conditions, including trafficking on a daily basis. Many of the cultivars and experimental lines in this study are performing well under intense traffic and low mowing. The more aggressive cultivars with high shoot density and strong rhizome systems seem to perform best under fairway conditions and intense traffic. They also recovery rapidly from divot injury and environmental stress.
Poa annua competition One of the biggest concerns golf-course superintendents have about Kentucky-bluegrass use in fairways is the potential for Poa annua competition. In the past, annual-bluegrass competition has been severe in many Kentucky-bluegrass fairways. Over time, these Kentucky-bluegrass fairways slowly converted to annual bluegrass. Control of Poa annua in Kentucky-bluegrass turfs is difficult. Kentucky bluegrass establishes slower than most other turfgrasses. Thus, Poa annua competition is enhanced by its ability to establish more rapidly than Kentucky bluegrass, particularly under traffic stress and in compacted soils.
New Kentucky-bluegrass cultivars show promise for stronger competition with Poa annua. They form high-quality, dense turfs that limit the ability of weeds to encroach. They also have strong rhizome systems that enhance their ability to recover quickly from divot injury. Further, data from NTEP trials indicates that several new cultivars have more rapid establishment and stronger rhizome development than older cultivars.
Certain irrigation practices can help limit Poa annua competition. New Kentucky-bluegrass cultivars with excellent drought-avoidance characteristics respond well to deep, infrequent irrigation, making it easier to manipulate soil moisture and reduce annual-bluegrass encroachment.
Turfgrass breeders have placed considerable emphasis on breeding extremely dark-green cultivars. Even though these improved cultivars form dense, high-quality turfs, some Poa annua is bound to invade their stand. These cultivars, though appealing to the eye, do not blend well with annual bluegrass, whose light-green appearance is quite evident. Thus, it may be best to select Kentucky-bluegrass cultivars with lighter green color rather than darker ones.
Playing conditions Kentucky-bluegrass fairway turfs were notorious for allowing the ball to settle into the turf. This made it difficult to hit the golf ball with the desired backspin and control. Golfers often complained about hitting "flyers" from Kentucky-bluegrass fairways. Older, less-improved cultivars formed an open and upright turf, particularly at mowing heights that were suited for their persistence. The fairway playing quality of these cultivars was not desirable. Their playing quality would improve as mowing height was lowered, but these cultivars could not persist long under these conditions.
In the last 5 years, industry plant breeders have increased their efforts to improve Kentucky-bluegrass performance under fairway conditions. As a result, several new cultivars and experimental lines that tolerate close mowing and intense traffic, and provide excellent playing quality are available today. These cultivars form dense turfs of high shoot density and fine texture, even at 0.5-inch mowing heights. The golf ball does not settle into these turfs, making it easier to strike the ball with control.
Trends and future developments The industry is excited about the potential use of Kentucky bluegrass as a fairway turf. It is evident that improved cultivars suitable for fairways are available. These cultivars will likely play a significant role in the cool-arid and cool-humid portions of the United States. They also will have an impact in areas where gray leaf spot is affecting perennial-ryegrass fairways.
The 1995 NTEP Kentucky-bluegrass evaluation trial will continue to provide valuable cultivar performance information through 2000. In the fall of 2000, NTEP will initiate a new trial. In the meantime, industry turfgrass breeders are continuing to develop new cultivars with improved fairway performance. Thus, this trend and interest will continue into the future, creating even more choices for superintendents selecting improved cultivars.
Dr. Robert C. Shearman is a professor of horticulture at the University of Nebraska (Lincoln, Neb.) and special projects coordinator for the National Turfgrass Evaluation Program (Beltsville, Md.).
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