• Residual Soil Nitrogen Evaluations In Irrigated Cotton, 2006

      Silvertooth, J. C.; Soto-Ortiz, R.; Norton, E. R.; Tronstad, Russell (College of Agriculture, University of Arizona (Tucson, AZ), 2007-08)
      Field experiments have been conducted for the past 19 seasons at three Arizona locations on University of Arizona Agricultural Centers (Maricopa, MAC; Marana, MAR; and Safford, SAC. aimed at investigating nitrogen (N) fertilizer management in irrigated cotton (Gossypium spp.) production. The MAC and SAC experiments have been conducted each season since 1989 and the Marana site was initiated in 1994. The original purposes of the experiments were to test N fertilization strategies and to validate and refine N fertilization recommendations for Upland (G. hirsutum L.) and American Pima (G. barbadense L.) cotton. The experiments have each utilized N management tools such as pre-season soil tests for NO₃⁻-N, in-season plant tissue testing (petioles) for N fertility status, and crop monitoring to ascertain crop fruiting patterns and crop N needs. At each location, treatments ranged from a conservative to a more aggressive approach of N management. The integrity of the experimental sites at each location was maintained in each consecutive season. Results at each location revealed a strong relationship between the crop fruit retention levels and N needs for the crop. This pattern was further reflected in final yield analysis as a response to the N fertilization regimes used. The higher, more aggressive N application regimes did not consistently benefit yields at any location. Generally, the more conservative, feedback approach to N management provided optimum yields at all locations. In 2001, a transition project evaluating the residual N effects associated with each treatment regime was initiated and no fertilizer N was applied. From 2001 to 2005 the residual N studies were conducted at two of these locations (MAC and MAR). In 2006, the residual N study was conducted only at MAC (the University of Arizona ceased operations at MAR at the end of the 2005 season). Therefore, all N taken-up by the crop was assumed to be derived from residual soil N. However irrigation water analysis showed that NO₃⁻-N concentration levels added to the crop ranged from about 5 to 15 ppm. In 2001- 2005 there were no significant differences among the original fertilizer N regimes in terms of residual soil NO₃⁻-N concentrations, crop growth, development, lint yield, or fiber properties. In 2006 however, significant differences in lint yield among N fertilization regimes for the Maricopa location were found. This suggests a possible pattern associated with the residual fertilizer N effects in relation to the original treatments at the Maricopa site.
    • Review of the 2006 Arizona Cotton Season

      Tronstad, Russell; Nolte, Kurt; Norton, Eric; Norton, Randy; Taylor, Erin; Tronstad, Russell (College of Agriculture, University of Arizona (Tucson, AZ), 2007-08)
    • Susceptibility of Southwestern Pink Bollworm to Bt toxins Cry1Ac and Cry2Ab2 in 2005

      Dennehy, Timothy J.; Unnithan, Gopalan C.; Harpold, Virginia; Carrière, Yves; Tabashnik, Bruce; Antilla, Larry; Whitlow, Mike; Tronstad, Russell; Department of Entomology, The University of Arizona, Tucson, AZ; Arizona Cotton Research & Protection Council, Phoenix, Arizona (College of Agriculture, University of Arizona (Tucson, AZ), 2007-08)
      Bt cotton is an extremely important tool for integrated pest management in the Southwest. It has been a major factor in the current historic low levels of conventional insecticide use in cotton of this region. This is due to Bt cotton’s unprecedented efficacy against the pink bollworm, Pectinophora gossypiella, and its selectivity in favor of key natural enemies of arthropod pests. Due to the pivotal importance of Bt cotton and widespread concerns about the development of pest resistance to transgenic crops, a multi-agency resistance management program was established to monitor and pro-actively manage resistance development in the pink bollworm. This report constitutes results from the ninth year of this monitoring program. Larvae were obtained from bolls collected in cotton fields located throughout the Southwest, cultured in the laboratory, and offspring tested using diet-incorporation bioassays that discriminate between susceptible and resistant pink bollworm. A total of 11 Arizona and four California collections were successfully reared and tested for susceptibility to Cry1Ac using a discriminating concentration of 10 μg Cry1Ac/ml of diet. Susceptibility to Cry2Ab2 was estimated similarly for 12 strains from Arizona and four from California using diagnostic concentrations of 1.0 and 10 μg Cry2Ab2/ml of diet. Success of pink bollworm eradication in suppressing pink bollworm populations in New Mexico and Texas precluded successful collection of samples in those states. No survivors of 10 μg Cry1Ac/ml were detected in any bioassays of 2005 strains (n=5358). The grand mean frequency of PBW survival of 10 μg Cry1Ac/ml in 2005 was 0.000%. A susceptible culture, APHIS-S, used each year as an internal control, yielded 99.3% corrected mortality in tests of 10μg/ml Cry1Ac (n=490). All twelve pink bollworm strains collected in 2005 were highly susceptible to Cry2Ab2, based on contrasts with baseline data collected from 2001-2003. There were no survivors of bioassays of either 1.0 μg Cry2Ab2/ml (n=1,000) or 10 μg Cry2Ab2/ml (n=3425). The susceptible APHIS-S culture had 82.5% corrected mortality in tests of 10 μg/ml Cry2Ab2 (n=200) and 100% mortality in tests of 10 μg/ml Cry2Ab2 (n=120). Field evaluations of efficacy of Bt cotton were conducted by the Arizona Cotton Research and Protection Council in adjacent pairs of Bt and non-Bt fields at 44 Arizona locations. Statewide, large pink bollworm larvae were found in an average of 15% of non-Bt bolls sampled from borders of refuge fields. This was on the low end of the range of infestation levels observed in refuges during the past decade. Bolls from adjacent Bt cotton (Bollgard™) fields yielded an average of 0.28% infested bolls. This value was down slightly from the previous year. Over 70% of the pink bollworm recovered from collections in Bt fields were from bolls that did not express Bt toxin. We conclude that there was no indication of problems with pink bollworm resistance to Cry1Ac or Cry2Ab2 at the locations sampled in 2005. Moreover, Bt cotton continued to exhibit exceptional field performance in Arizona.