Turfgrasses make up a majority of commercial and residential landscapes in the United States. Sixteen million hectares of turfgrass is irrigated each year, which makes it the most irrigated crop when all species are taken into consideration (The Lawn Institute, 2010). Additionally, an estimated $2.8 billion U.S. of gasoline, $700 million U.S. of pesticides, and $5.2 billion U.S. of fossil fuel-derived fertilizers are used to manage lawns (The Lawn Institute, 2010). Turfgrasses provide erosion control, quicken restoration of disturbed soils, sequester CO2, dissipate urban heat, and reduce noise and visual pollution (Beard and Green, 1994). Individual homeowners manage many small areas of turfgrass. Therefore, adaptations of proper cultural practices are important to optimize the environmental, economic, use, and aesthetic impact of turfgrasses in home lawns. The most important cultural practice might be turfgrass selection, particularly when the home lawn is managed in the transition zone of the United States, which stretches from central New Jersey to the panhandle of Texas. Two cool-season turfgrasses that have desirable agronomic qualities and can be grown in the transition zone are tall fescue and hybrid bluegrass.
Tall fescue [Festuca arundinacea Shreb synonym Schedonorus phoenix (Scop.) Holub] is the dominant cool-season perennial grass in the United States (Ju et al., 2006). Tall fescue provides great use as a lower maintenance turfgrass in the transition zone compared with other cool-season grasses. The plant’s well-developed root system, modest nitrogen requirements, and aesthetically pleasing dark green color make it a popular choice for home lawns, parks, golf course roughs, and other low-traffic situations. However, the major limiting factor to tall fescue’s success as a turf in the transition zone is its susceptibility to the fungal pathogen R. solani Kuhn (Piper and Coe, 1919), which causes the disease brown patch in the summer months. Brown patch is particularly problematic when nighttime temperatures average 21 °C, relative humidity exceeds 85%, and the leaf wetness duration period is prolonged (Couch, 1995; Fidanza et al., 1996; Gross et al., 1998). No tall fescue cultivars are immune to R. solani; thus, applications of fungicides are necessary to effectively reduce brown patch spread (Yuen et al., 1994). Brown patch infestations not only decrease the aesthetic appearance of tall fescue, but can thin the turf stand, which may lead to infestation of undesirable weed species (Ferrell et al., 2003). Tall fescue’s bunched-type growth habit limits recovery from brown patch infection (Turgeon, 1999).
Hybrid bluegrass may be an alternative cool-season grass to grow in the transition zone. Hybrid bluegrass (Poa pratensis L. × Poa arachnifera Torr.) was created by crossing kentucky bluegrass (Poa pratensis L.) and texas bluegrass (Poa arachnifera Torr.) (Read et al., 1999). This cross results in a grass that has the agronomic qualities of kentucky bluegrass but the heat and drought characteristics of texas bluegrass. Hybrid bluegrass has a rhizomatous growth habit; thus, it exhibits better recuperative ability than a bunch-type grass like tall fescue and can fill in bare spots in a turfgrass stand. Hybrid bluegrass exhibits superior range in adaption compared with traditional cool-season grass cultivars; it can be grown in lawns as far south as Atlanta and Dallas. The rhizome system allows for turf recovery with less need for reseeding compared with tall fescue (Turgeon, 1999). Hybrid bluegrass’ shade tolerance compares with tall fescue and it is resistant to brown patch. In one study, ‘Thermal Blue Blaze’ hybrid bluegrass exhibited greater visual quality than traditional kentucky bluegrass and tall fescue in conditions of high temperature, drought, and salinity stress (Suplick-Ploense et al., 2002). Gross photosynthesis was reduced 21% in ‘Thermal Blue’ hybrid bluegrass, 30% in kentucky bluegrass, and 27% in tall fescue when plants were subjected to high temperature and drought conditions (Su et al., 2007). Electrolyte leakage was significantly less in hybrid bluegrass than kentucky bluegrass or tall fescue. Hybrid bluegrass use is limited by its slow establishment period, higher nitrogen requirements, and susceptibility to diseases such as dollar spot (Sclerotinia homeocarpa) F.T. Bennett and summer patch (Magnaporthe poae) Landschoot and Jackson (Serensits et al., 2011). Mixing hybrid bluegrass with tall fescue may increase the quality of the turf stand when compared with individual monocultures of these species.
Seeding mixtures of different species increase the genetic variability of a turfgrass stand (Donald, 1963). Environmental stresses such as drought, nutrient availability, salinity, cold and heat tolerance, and competition from weeds have varying impacts on individual species. Proper selection of species for seeding combinations should focus on alleviating potential damage from pathogens, insects, weeds, drought, or shade. A grass species that is resistant to a specific disease should be mixed with a grass species that is resistant to a different disease. From an epidemiological perspective, polystands of different plant cultivars or species reduce the rate that a disease spreads. Mitchell et al. (2002) demonstrated that increasing species diversity in grasslands decreased pathogen load. The best defense against a ubiquitous pathogen such as R. solani might be to keep a polystand as close to a 1:1 sensitive grass to resistant grass species ratio. A study by Dunn (2001) documented the reduction in brown patch and improved quality seen in mixtures of kentucky bluegrass and tall fescue when compared with the monoculture of tall fescue. However, another study by Hunt and Dunn (1993) showed that mixing tall fescue with kentucky bluegrass and perennial ryegrass (Lolium perenne L.) did not impact disease incidence when compared with a monoculture of tall fescue. Therefore, more information is needed on different kinds of seeding combinations to fully elucidate the epidemiological impact of multiturfgrass systems.
Establishment period may be an important consideration when selecting mixtures of turfgrass species. Grasses that are slow to establish might be more susceptible to weed infestations and should then be mixed with a turfgrass species that establishes quickly. The growth habit of grasses should be considered as well. For example, tall fescue has good wear tolerance but exhibits a bunch-type growth habit, which limits its recovery from excessive traffic or thinning resulting from disease. Mixing in a rhizomatous grass such as hybrid bluegrass or kentucky bluegrass with tall fescue would ideally result in a polystand that has good wear tolerance and recuperative ability. Agronomic traits of the two grasses are important. Turfgrass species with contrasting colors would not be aesthetically pleasing in a polystand. The grasses should be able to tolerate similar mowing regimes and fertilizer application (Davis, 1958). Mixing turf-type tall fescue with hybrid bluegrass has potential to become a successful polystand. Although tall fescue is a bunched-type species, turf-type tall fescues establish quickly and could serve as a nurse crop for the slower establishing hybrid bluegrass. Therefore, it is important to evaluate how combinations of tall fescue and hybrid bluegrass compare with monocultures when evaluating disease pressure, weed infestations, and turfgrass density over multiple years. Research was conducted to evaluate the impact of seeding rate combinations of ‘Greenkeeper’ tall fescue and ‘Thermal Blue Blaze’ hybrid bluegrass on turf species dynamics, weed species encroachment, disease severity, and sod strength.
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