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Title:   Author  item
                                            ALCORTA, MARISA - University Of Californiaitem
                                            FIDELIBUS, MATTHEW - University Of Californiaitem
                                            Steenwerth, Kerriitem
                                            SHRESTHA, ANIL - California State University
  Submitted to: Weed Science
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 9/15/2010
                                    Publication  Date: 1/1/2011
                                    Citation: Alcorta, M., Fidelibus, M.W., Steenwerth, K.L., Shrestha, A. 2011. Effect of vineyard row orientation on growth and phenology of glyphosate-resistant and glyphosate-susceptible horseweed (Conyza canadensis L. Cronq.). Weed Science. 59:55-60.Interpretive Summary: Horseweeds (Conyza canadensis L. Cronq.) have become increasingly common and difficult
to control in San Joaquin Valley (SJV) vineyards, due in part, to the evolution of a glyphosate resistant (GR) biotype. The development of weed suppressive vineyard designs in which the trellis design, spacing, and row orientation, combine to cast dense shade on the weed canopy zone (WCZ) may reduce weed growth, but the relevance of such a system to horseweeds, which can grow to be as tall, or taller, than a typical grapevine trellis, is uncertain. Also unknown is whether the GR and glyphosate susceptible (GS) biotypes would perform similarly under such conditions. Therefore, we compared the growth and development of potted GR and GS horseweeds in vinerows oriented East-West (EW) and North-South (NS). Rows oriented EW allowed less light penetration to the WCZ than NS rows throughout the study, and horseweeds of both biotypes responded to low light levels by producing leaves with larger specific leaf area and leaf area ratios than those in the NS rows. Also, the leaf, stem, and root dry weight of the horseweed plants in the EW rows was reduced by 30% compared to the horseweeds in NS rows. Leaf number was also reduced in the horseweed plants in the EW rows, but only for the GS biotype. Row orientation did not affect phenological development or the number of seeds produced by the GR or GS horseweeds, but GR horseweeds budded, flowered, and set seed
approximately 1 wk earlier than the GS horseweeds. Thus, shade associated with the EW vinerows reduced horseweed growth, but not fecundity, and GR horseweed biotypes reached reproductive maturity earlier than the GS biotype.
                                Technical Abstract: Horseweeds (Conyza canadensis L. Cronq.) have become increasingly common and difficult
to control in San Joaquin Valley (SJV) vineyards, due in part, to the evolution of a glyphosate resistant (GR) biotype. The development of weed suppressive vineyard designs in which the trellis design, spacing, and row orientation, combine to cast dense shade on the weed canopy zone (WCZ) may reduce weed growth, but the relevance of such a system to horseweeds, which can grow to be as tall, or taller, than a typical grapevine trellis, is uncertain. Also unknown is whether the GR and glyphosate susceptible (GS) biotypes would perform similarly under such conditions. Therefore, we compared the growth and development of potted GR and GS horseweeds in vinerows oriented East-West (EW) and North-South (NS). Rows oriented EW allowed less light penetration to the WCZ than NS rows throughout the study, and horseweeds of both biotypes responded to low light levels by producing leaves with larger specific leaf area and leaf area ratios than those in the NS rows. Also, the leaf, stem, and root dry weight of the horseweed plants in the EW rows was reduced by 30% compared to the horseweeds in NS rows. Leaf number was also reduced in the horseweed plants in the EW rows, but only for the GS biotype. Row orientation did not affect phenological development or the number of seeds produced by the GR or GS horseweeds, but GR horseweeds budded, flowered, and set seed
approximately 1 wk earlier than the GS horseweeds. Thus, shade associated with the EW vinerows reduced horseweed growth, but not fecundity, and GR horseweed biotypes reached reproductive maturity earlier than the GS biotype.


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