| Horseweed (Conyza canadensis) is an increasingly common pest in San Joaquin Valley (SJV) vineyards. The eco-physiological factors that contributed to this invasion are yet unclear, but horseweed is tolerant to commonly used herbicides, and a glyphosateresistant (GR) biotype was discovered in the SJV in 2007. The impact of either horseweed biotype on vineyard productivity is uncertain, but recent greenhouse experiments showed that the GR biotype is more vigorous than the glyphosatesusceptible (GS) biotype. Thus the emerging GR biotype may be more competitive than the GS biotype, but the fitness of these weeds in vineyards has not been measured. Therefore, the relative competitiveness of GR and GS horseweed biotypes with young grapevines was tested. Grapevines growing with GR or GS horseweeds produced fewer leaves, and amassed less total dry matter than vines grown without weeds. The GR biotype also reduced grapevine stem mass and length, but the GS biotype did not. The GR weeds accumulated more than twice as much dry matter as GS weeds, whether in competition with grapevine or not.As pressure increases to reduce chemical herbicide use, integrated weed management (IWM) methods are being sought. Row orientation in vineyards has been shown to affect the light quantity and quality reaching the weed canopy zone, and to reduce growth and development of a common vineyard weed. Therefore, the relative fitness of GR and GS horseweeds in contrasting light environments was also tested. Potted GR and GS horseweeds were placed under the grapevine (Vitis vinifera L.) canopy in rows oriented East-West (EW) and North-South (NS). Greater shading in EW rows caused a reduction in the leaf, stem, and root dry matter of the GR and GS horseweeds, as well as lower specific leaf areas and leaf area ratios compared to those plants in the NS rows. Horseweed leaf number was lower in plants grown in EW rows compared to those in NS rows, but only for GS horseweeds. Although row orientation did not affect phenological development or the number of seeds produced by the GR or GS plants, the GR horseweeds budded, flowered, and set seed 1 week earlier than the GS horseweeds regardless of row orientation. |
