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Article Excerpt
THE RUSSIAN WHEAT APHID is a relatively important pest of winter wheat and other small grains in the USA. Feeding by D. noxia on susceptible wheat leads ultimately to the death of the plant if infestation is uncontrolled (Walters et al., 1980; Gilchrist et al., 1984; Deol et al., 2001). In general, infested plants develop white, yellow, or purple longitudinal streaks on leaves and stems, exhibit rolled leaves, and often display a prostrate growth habit. In older plants, rolling of the flag leaf may trap the emerging awns, causing the heads to bend. Diuraphis noxia was first reported in the USA in 1986, and by 1993 had caused direct and indirect losses of more than $800 million in the western USA (Morrison and Peairs, 1998). Additional losses have been incurred since then, primarily in Colorado and parts of neighboring states including northeastern New Mexico, southwestern Nebraska, and western Kansas (Berzonsky et al., 2002). Various management approaches have proven effective in alleviating damage from D. noxia (Quisenberry and Peairs, 1998) and natural biological controls have continued to evolve (Noma et al., 2005). Considerable loss reduction has been attributed to the widespread adoption of D. noxia-resistant cultivars by producers of winter wheat in areas with frequent D. noxia infestations (Berzonsky et al., 2002). Approximately 25% of wheat acreage in Colorado has been planted with cultivars resistant to D. noxia over the past 5 yr (Colorado Agricultural Statistics Service, 2004).
Some minor biotypic variation was previously reported in D. noxia collections from the Great Plains, but it was not considered of practical significance as no differential host-plant reactions were observed (Bush et al., 1989; Shufran et al., 1997). However, various infestations of D. noxia reached economically damaging levels on resistant cultivars in eastern Colorado in spring 2003 and required pesticide applications (Haley et al., 2004). This apparent failure of resistant cultivars led to characterization of a novel strain of D. noxia since designated as biotype 2 that is distinguished from biotype 1 on the basis of its virulence to Dn4-based resistance in wheat. This is the first report of biotypic variation in D. noxia in the new world, although a number of biotypes are recognized in the Old World that vary in virulence to different resistance sources (Basky, 2003). Puterka et al. (1992) examined eight D. noxia isolates, including one from the USA, and seven unique virulence patterns were identified. One isolate from the former Soviet Union was virulent to P1372129, the donor parent of the Dn4 resistance gene deployed in virtually all the resistant cultivars grown in Colorado. Basky (2003) demonstrated virulence of a Hungarian D. noxia isolate to wheat lines expressing Dn1, Dn2, and Dn4. More recently, Smith et al. (2004) reported an isolate of D. noxia in Chile that proved highly virulent to Dn4-expressing wheat lines, but avirulent to those expressing Dn2, Dn5, Dn6, Dnx, and Dny.
A previous study (Jyoti and Michaud, 2005) compared the performance of both D. noxia biotypes on three wheat cultivars, Trego, Halt, and Stanton, under standardized conditions, and characterized substantial differences in both aphid biology and plant responses. Subsequent observations (Jyoti, 2004, unpublished data) suggested additional disparity between biotypes with respect to their responses to temperature. A previous study (Qureshi et al., 2005) evaluated colonization of commercial wheat cultivars by the two biotypes and reported some differential responses. The present experiments were designed to evaluate the performance of both biotypes of D. noxia on a wider range of commercially available cultivars, particularly those that are most extensively grown in Colorado and Kansas. The objectives were to compare colony growth rates of the two D. noxia biotypes on eight cultivars at two constant temperatures and assess the progression of plant damage symptoms.
MATERIALS AND METHODS
Aphid Colonies
A colony of D. noxia was established from a single apterous virginopara (wingless parthenogenetic female) collected from wheat in Ellis County, Hays, KS, in autumn 2002 and has been in continuous culture at Agricultural Research Center-Hays ever since. This colony was identified as biotype 1 D. noxia on the basis of its reduced survival and reproduction on the resistant cultivars Halt and Stanton (Jyoti and Michaud, 2005). A colony of biotype 2 D. noxia was initiated from material provided by USDA-ARS in Stillwater, OK, in fall of 2003. This colony had been cultured continuously in isolation at Stillwater since its collection from an infested field of 'Prairie Red' in eastern Colorado in the spring of 2003.
Colonies of biotype 1 D. noxia were maintained on Trego, and those of biotype 2 on Halt. The two biotypes were isolated in separate growth chambers (Percival Model 1-37VL, Percival Scientific, Inc., Perry, IA) held in separate buildings at the Agricultural Research Center-Hays. Colonies of both biotypes were maintained at a temperature of 22 [+ or -] 1[degrees]C under cool-white fluorescent lights with a photoperiod of 16:8 (L/D) h. Fresh trays of 10-d-old wheat seedlings were introduced for each colony at intervals of approximately 2 wk. Then, each tray of plants was manually infested with 25 fourth instars of D. noxia using a camel's hair brush, and the old tray of plants removed, autoclaved, and discarded before the production of any alate (winged) aphids that could pose a hazard for cross-contamination....
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