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Project description

Ecology and competitive effect of two horseweed biotypes with young grapevine and established vineyards. (06FE012)
Program UC IPM competitive research grants program
Principal
investigators
A. Shrestha, UC IPM, Kearney Agricultural Center
M. Fidelibus, Kearney Agricultural Center
K.J. Hembree, UCCE, Fresno Co.
Host/habitat Grapes
Pest Horseweed Conyza canadensis
Discipline Weed Science
Review
panel
Applied Field Ecology
Start year (duration)  2006 (Three Years)
Objectives Determine the relative competitiveness of glyphosate-resistant and glyphosate-susceptible horseweed biotypes with young grapevines.

Determine the relative fitness of glyphosate-resistant and glyphosate-susceptible horseweeds in vine rows having contrasting light environments.

Determine the relationship between horseweed density and grapevine growth, and fruit yield and quality.

Project
Summary
Horseweed, Conyza canadensis, is an increasingly common pest in San Joaquin Valley (SJV) vineyards. The ecophysiological factors that contributed to this invasion are uncertain, but horseweed is tolerant to commonly used herbicides, and a glyphosate-resistant (GR) biotype was recently discovered in the SJV. The impact of either horseweed biotype on vineyard productivity is uncertain, but preliminary greenhouse experiments showed that the GR biotype is more vigorous than the glyphosate-susceptible (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, we propose to determine the relative competitiveness of GR and GS horseweed biotypes with young grapevines; the relative fitness of GR and GS horseweeds in emerging vineyard trellis systems with contrasting light environments; and the relationship between horseweed density and grapevine growth, and fruit yield and quality.
Final report This study compared the effect of two biotypes of horseweed [glyphosate-resistant (GR) and glyphosate-susceptible (GS)] on young grapevine growth and assessed the effect of varying densities of horseweed on grape yield in an established vineyard. The study also compared the growth and development of the two horseweed biotypes in vineyards with east-west and north-south row orientations. We conclude that it is more important to control horseweed plants in new vineyards compared to established vineyards and that the two biotypes have differential effects on young grapevines. The GR biotype was more vigorous and amassed twice the amount of dry weight as the GS biotype. Although total dry matter of grapevines grown with a single plant of either horseweed biotype was reduced by about 20% compared to those grown without weeds, the GR biotype reduced mainstem length, stem weight, and leaf area of the young grapevines more than the GS biotype. Therefore, the GR biotype found in the Dinuba-Parlier area may be more aggressive than the GS biotype found in the Fresno area. However, densities as high as 50 weeds (GS biotype) per vines had no effect on yield in an established vineyard. Further, the study showed that horseweed growth could be reduced by shade and that vineyards with east-west oriented rows were shadier, and, therefore, more suppressive to the two horseweed biotypes than north-south oriented rows. In conclusion, the critical period for horseweed control in vineyards is the establishment years, and horseweed growth can be suppressed significantly by canopy management strategies which maximize shading of the vineyard floor.
Second-year
progress
Objective 1: Greenhouse experiments were conducted in 2006 and 2007 at the UC Kearney Agricultural Center (KAC), Parlier. The experiment was a randomized complete block design. The treatments were grapevine rooting alone, grapevine rooting with one GR horseweed seedling, grapevine rooting with one GS horseweed seedling, GR horseweed alone, and GS horseweed alone. Seeds from existing pre-identified GR and GS populations were used. In April of each year, each combination of grapevine rooting and weed seedling were planted in different 8-liter pots, with five single-pot replicates of each treatment, for a total of 25 pots each year. The plants were subjected to similar water and fertilizer regimes that were sufficient to prevent deficiency symptoms in grape rootings without weeds. Plants were grown for about eight weeks. Weeds and grapes were harvested and divided into roots, stems, and leaves. Leaf area of grapevines was determined. Plant parts were oven dried, and dry mass of each organ was determined. Data for both years were analyzed using the statistical software package SAS.

Objective 2: Experiments were conducted in 2006 and 2007 in a vineyard at KAC. The vineyard is planted in four blocks, each containing a group of three vine rows oriented EW and a similar group of rows oriented NS. A factorial experimental design was used where row orientation (EW or NS) was one factor and horseweed biotype (GR or GS) the other. Each treatment combination was replicated four times. In April of each year, seeds from pre-identified GR and GS horseweed populations were transplanted to black polyethylene pots containing about 8 L of commercial growth media. Two potted GR or GS horseweed (sub-samples) were placed in each treatment replicate for a total of 32 potted plants. Each pot was placed between the trunks of two grapevines near the middle of the row. The weeds were watered and fertilized as needed. PAR was measured three times a day (0900, 1200, and 1600 h) each week with a ceptometer positioned parallel to the vine row about 0.5 m above the vineyard floor in the weed canopy zone (WCZ). A single leaf of similar physiological age was identified on each horseweed and subjected to photosynthesis measurements to develop light response curves. Height of each plant was recorded weekly. Phenology of the plants was monitored daily and recorded. All plants were harvested as soon as a flower set seed. Then the plants were oven dried and weighed. At harvest, 16 plants (8 of GR and 8 of GS) were divided into leaves, stems, reproductive parts, and roots. Growth media was washed from the roots. Dry mass of all the plant parts was measured. Data are being analyzed.

Objective 3: Experiments were conducted in 2006 and 2007 in a 40-year-old Thompson Seedless vineyard at KAC. The experimental design was a randomized complete block with four replications of each treatment. Each row was divided into six plots. Each plot contained four vine trunks. Each plot, within a row, was randomly assigned to one of five weed density treatments: 0, 10, 20, 30, 40, or 50 horseweed plants per vine. The horseweed densities were established by transplanting seedlings in late March within the vine rows. Plots were hand watered for a few weeks after transplanting. Transplanted weeds were allowed to grow until late-August, harvested, oven dried, and weighed. Grapes were harvested in mid-September. Immediately before harvest, samples of fresh berries were collected from each plot. The sample consisted of 100 berries that were weighed and macerated in a blender. Soluble solids of the filtered juice samples were measured with a temperature-compensating digital refractometer (Palette 101, Atago, Farmingdale, NY). Fruit from each plot was harvested and weighed, and the number of clusters was counted. Time taken to hand harvest each plot was also recorded. In the winter, pruning weights will also be recorded for each plot.

First-year
progress
A greenhouse experiment was conducted at the UC Kearney Agricultural Center (KAC), Parlier. The experiment was a randomized complete block design. The treatments were grapevine rooting alone, grapevine rooting with one GR horseweed seedling, grapevine rooting with one GS horseweed seedling, GR horseweed alone, and GS horseweed alone. Seeds from existing pre-identified GR and GS populations were used. In April, each combination of grapevine rooting and weed seedling were planted in a different 8-liter pot, with five single-pot replicates of each treatment, for a total of 25 pots. The plants were subjected to similar water and fertilizer regimes that were sufficient to prevent deficiency symptoms in grape rootings without weeds. Plants were grown for about 8 weeks. Weeds and grapes were harvested and divided into roots, stems, and leaves. Leaf area of grapevines was determined. Plant parts were oven dried, and dry mass of each organ was determined.

This experiment was conducted in a vineyard at the KAC. The vineyard is planted in four blocks, each containing a group of three vine rows oriented EW and a similar group of rows oriented NS. A factorial experimental design was used where row orientation (EW or NS) was one factor and horseweed biotype (GR or GS) the other. Each treatment combination was replicated four times. In April, seeds from preidentified GR and GS horseweed populations were transplanted to black polyethylene pots containing about 8 L of commercial growth media. Two potted GR or GS horseweed (sub-samples) were placed in each treatment replicate for a total of 32 potted plants. Each pot was placed between the trunks of two grapevines near the middle of the row. The weeds were watered and fertilized as needed. PAR was measured three times a day (0900, 1200, and 1600 h) each week with a ceptometer positioned parallel to the vine row about 0.5 m above the vineyard floor in the weed canopy zone (WCZ). A single leaf of similar physiological age was identified on each horseweed and subjected to photosynthesis measurements to develop light response curves. Height of each plant was recorded weekly. Phenology of the plants was monitored daily and recorded. All plants were harvested as soon as a flower set seed. Then the plants were oven dried and weighed. At harvest, 16 plants (8 of GR and 8 of GS) were divided into leaves, stems, reproductive parts, and roots. Growth media was washed from the roots. Dry mass of all the plant parts were measured.

Experiments were conducted in a 40 year-old Thompson seedless vineyard at the KAC. The experimental design was a randomized complete block with four replications of each treatment. Each row was divided into six plots. Each plot contained four vine trunks. Each plot, within a row, was randomly assigned to one of five weed density treatments; 0, 10, 20, 30, 40, or 50 horseweed plants per vine. The horseweed densities were established by transplanting seedlings in late March within the vine rows. Plots were hand watered for a few weeks after transplanting. Transplanted weeds were allowed to grow until late August, harvested, oven dried, and weighed. Grapes were hand harvested in mid-September. Immediately before harvest, samples of fresh berries were collected from each plot. The sample consisted of 100 berries that were weighed and then macerated in a blender. Soluble solids of the filtered juice samples were measured with a temperature-compensating digital refractometer (Palette 101, Atago, Farmingdale, NY). All the fruit from each plot was harvested and weighed, and the number of clusters was counted. Time taken to hand harvest each plot was also recorded. In the winter, pruning weights will also be recorded for each plot.

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