Before the 1980s, research and breeding activities in tall fescue took place without knowledge of Neotyphodium endophytes, although problems in livestock performance and health associated with tall fescue, such as summer slump and fescue foot, were well known (see Chapter 16). Initially it was thought that summer slump in tall fescue could be addressed by selecting for low perloline alkaloid production in tall fescue (Bush and Buckner, 1973; Watson et al., 1978). Stand survival and animal performance for a tall fescue population selected for low ergot alkaloid concentration were found to be intermediate between those of corresponding, nonselected E+ and E- populations (Hill et al., 2002). While selection for low ergot alkaloid concentration may still have merit, to date this approach has not resulted in the development of improved tall fescue cultivars. Infection with a wild-type endophyte benefits the plant through increased resistance or tolerance to insects, diseases, drought, and nematodes (see Chapter 9, Chapter 8, Chapter 4, and Chapter 10, respectively), and other stresses, as reviewed elsewhere (Popay and Bonos, 2005; Malinowski et al., 2005a). Tall fescue research conducted before the early 1980s generally should be interpreted with caution or in some cases dismissed because plant genotype effects were quite possibly confounded with endophyte status (see Chapter 11).
After the endophyte was determined to be involved in tall fescue toxicosis, researchers focused their attention on developing E- cultivars. Viable endophyte can be killed in tall fescue seed using a combination of heat and humidity (Welty et al., 1987), such as occurs when seed is stored on-farm at ambient conditions for a year or more (Williams et al., 1984). Because the endophyte is transmitted maternally via seed, E- cultivars can be developed and maintained by allowing only E- plants to produce seed for subsequent generations. This fact has been exploited in developing both E+ and E- versions of some tall fescue cultivars, such as KY-31 and Jesup. In some areas of the world, such as Argentina (De Battista, 2005) and parts of Europe (Brilman, 2005), endophyte infection in tall fescue seed that is sold is strictly limited by law.
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Fig. 19-1. Effect of endophyte infection on stress tolerance in tall fescue. Plots were grown in the southern United States and subjected to periodic drought stress. (Left) E- KY-31; (Right:) E+ KY-31. Contrast between the two images is more vivid in the color pictures on the accompanying CD. (Photo courtesy of J.H. Bouton, The Noble Foundation.)
Fig. 19-2. Endophyte mycelia being inserted into a tall fescue seedling to introduce a novel endophyte. (Photo by B.N. Stearns, The Noble Foundation.
Extensive plantings of E- tall fescue were made in the 1980s to alleviate tall fescue toxicosis. However, it became obvious that the endophyte conferred a major advantage to the plant, particularly in terms of improved persistence (Fig. 19-1). Many producers have continued using "toxic" endophyte tall fescue because of the expected high risk of stand losses of E- tall fescue (Ball, 1997). Endophyte free tall fescue is a viable option where plant stress is minimized, including heavily irrigated conditions (Asay et al., 2001), sites with deep soils that have very good water-holding capacity (Hopkins and Alison, 2006), or areas with favorable climates such as the U.S. Upper Midwest (Brummer and Moore, 2000). Breeders have continued to select E- tall fescue for persistence and vigor, but in at least one instance (Bouton et al., 2001) selection has been unsuccessful.
Novel endophyte tall fescue is defined here as a host-endophyte association that did not previously occur in nature. Bacon and Siegel (1988) proposed the use of nontoxic, novel endophytes in targeted grass cultivars as a means of mitigating tall fescue toxicosis while retaining agronomic benefits to the grass. Neotyphodium endophytes do not naturally infect other plants. Therefore, techniques were developed to infect endophytic fungi into E- tall fescue seedlings in the laboratory (Latch and Christensen, 1985). A novel endophyte can be introduced into a tall fescue population by making a small longitudinal slit in an E- seedling just below the apical meristem and inserting fungal mycelia (Fig. 19-2). Some of the seedlings form stable symbioses (see Chapter 14) with the novel endophyte; of these, a percentage will go on to produce seed containing viable novel endophyte. Breeders typically will examine a number of elite tall fescue populations infected with a range of novel endophytes to identify a superior combination (e.g., Bouton et al., 2002).
This process has been employed to infect tall fescue with naturally occurring novel endophytes, such as the cultivar Jesup with
MaxQ (a trademark of Grasslanz Technology Ltd., Palmerston North, New Zealand, commercialized in the US by Pennington Seed Co., Madison, GA in ‘Jesup' tall fescue) (Bouton et al., 2002), and the germplasm HiMag with Strains 4 and 9 (Nihsen et al., 2004). As of 2007, Jesup MaxQ was the only tall fescue cultivar infected with a novel endophyte that was commercially available in the United States. This same combination is marketed as Jesup MaxP (registered trademark of Grasslanz Technology Ltd., Palmerston North, New Zealand) in Australia. Novel endophyte infected populations can serve subsequently as germplasm for further cycles of selection for traits of interest without the need to re-inoculate, assuming infection is not lost due to improper seed storage, poor transmission, or other causes.
Tall fescue cultivars infected with a novel, nontoxic endophyte result in animal weight gains equivalent to those from E- versions and also can persist as well as tall fescue infected with wild-type endophyte (Bouton et al., 2002; Nihsen et al., 2004; Hopkins and Alison, 2006). For extensive reviews of the use of endophytes in tall fescue cultivar development, the reader is referred to Bouton and Easton (2005), Brilman (2005), and Bouton and Hopkins (2003).
Viability of endophytes, including that of novel endophytes, can be lost in seed before planting. Tall fescue seed with a dead novel endophyte will perform the same as the E- version of that cultivar. Proper seed production, processing, packaging, and storage techniques can improve greatly the likelihood of delivering to the end user seed that contains a high level of infection with viable endophyte (see Chapter 20). Seed containing a novel endophyte should be planted as soon as practical and should not be exposed to heat or high humidity. A high level of transmissibility (generally 90% or greater) to progeny is essential for tall fescue-novel endophyte combinations that are to be released as cultivars.
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