Matthew W. Fidelibus

Application timing and concentration of abscisic acid affect the quality of ‘Redglobe’ grapes

Summary The plant hormone abscisic acid (ABA), the concentration of which increases in grape berry skins at the onset of maturation (veraison), appears to be involved in the regulation of anthocyanin accumulation. Preliminary tests suggested that exogenous applications of ABA could improve grape berry colour, but its high cost precluded the development of commercial applications. Recently, a lower-cost ABA production method was developed, which led to the evaluation of different concentrations of ABA, applied at or around veraison, on the quality of ‘Redglobe’ grapes. In two of three years of tests, several ABA treatments enhanced the anthocyanin content of grape skins. ABA, applied at approx. 300 mg l–1 at veraison, may be required to reliably increase pigmentation, and improve the colour of ‘Redglobe’ grapes. Although the total anthocyanin content was increased by ABA treatment, anthocyanin composition was not affected. Applications of ABA had few effects on fruit size or composition, although they did cause fruit softening, which is undesirable. A secondary objective of this study was to determine how the anthocyanin content of berry skins affected berry colour characteristics. Strong curvilinear relationships between anthocyanin content and lightness and hue showed that these colour characteristics were saturated by anthocyanin contents over 0.02 mg cm–2 of skin. These data suggest that colour measurements may be needed to evaluate the effect of cultural practices on colour in table grapes.

Growth, Phenology, and Intraspecific Competition between Glyphosate-Resistant and Glyphosate-Susceptible Horseweeds (Conyza canadensis) in the San Joaquin Valley of California

Abstract Experiments were conducted in 2006 to 2008 to study growth, phenology, and competitive ability of glyphosate-resistant (GR) and -susceptible (GS) biotypes of horseweeds from San Joaquin Valley (SJV), CA. When grown alone, in pots, the GR horseweeds consistently developed more rapidly than the GS weeds, as evidenced by their earlier bolting, flowering, and seed set; the GR horseweeds set seeds nearly 25 d (approximately 190 fewer growing degree days) sooner than the GS horseweed. At seed set, the relatively slow-developing GS horseweeds had amassed 40% more shoot dry matter than the GR weeds at the same phenological stage, but neither biotype was consistently more fecund than the other. Although the GR biotype had lower shoot dry mass than the GS biotype when grown alone, in mixed populations under increasing levels of competition (in a replacement series design) and limited resources (mainly moisture), the GR weeds were not only taller, but also accumulated more dry matter than the GS weeds. Thus, the GR biotype was more competitive than the GS biotype, particularly when grown at high densities and under moisture-deficit stress. Therefore, under California conditions there is no apparent fitness penalty for this particular GR horseweed biotype, and it is likely to persist in the environment and outcompete the GS biotypes regardless of further glyphosate selection pressure. If so, this biotype of GR horseweed is likely to become increasingly common in the SJV until effective management strategies are developed and adopted. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA

Timing and Concentration of Abscisic Acid Applications Affect the Quality of ‘Crimson Seedless’ Grapes

ABSTRACT ‘Crimson Seedless’ is a table grape (Vitis vinifera) cultivar that often fails to develop adequate color, particularly in warm climate regions such as the San Joaquin Valley, California. Growers generally apply ethephon to ‘Crimson Seedless’ grapes to improve their color, but results are erratic. The plant hormone abscisic acid (ABA) stimulates anthocyanin production in some other varieties of grape, so applications of ABA might help improve the color of ‘Crimson Seedless’. Application of ABA at veraison maximized anthocyanin accumulation, but later applications also increased pigmentation and, at the same time, reduced ABAs negative effect on berry puncture resistance. Concentrations of 150 and 300 mg·L− 1 ABA increased anthocyanin levels more than did 75 mg·L− 1 ABA. The relationships between anthocyanin content of the berry skins and the color characteristics lightness (L*), and hue angle (h°), were well described by the functions L* = 22.56x− 0.0865, and h° = 3.356x− 0.513, where x = skin anthocyanin content. Thus, at skin anthocyanin contents ≥ 0.03 mg·cm− 2, relatively large increases in anthocyanin content have relatively small effects on L* and h° of ‘Crimson Seedless’ grapes. Similar results have been obtained with other cultivars of grape. For this reason, we suggest to include direct color measurements, rather than relying only on anthocyanin content when evaluating color on grapes.

Grapevine row orientation affects light environment, growth, and development of black nightshade (Solanum nigrum)

Abstract Row orientation in vineyards can affect the quantity of light intercepted by the crops canopy. Consequently, the light available to weeds growing under the canopy might also be affected, with potential implications for their physiology, growth, and productivity. This hypothesis was tested in 2003 and 2004 in a central California vineyard having rows oriented east–west (EW) and north–south (NS) in a randomized complete block design. In April of both years, potted black nightshade seedlings were placed under grapevines of both row orientations and grown for about 10 wk. Photosynthetically active radiation (PAR) at the weed canopy zone (WCZ) of NS rows was bimodal, with peaks occurring at about 09:30 a.m. and 4:30 p.m. At those times, PAR approached 500 μmol m−2 s−1 (between 30 and 40% of full sun). In contrast, maximum PAR in the WCZ of EW rows was generally less than 75 μmol m−2 s−1 throughout the day. The ratio of red to far-red light was also greater in NS than EW rows in the morning and afternoon. In both row orientations, PAR was suboptimal for nightshade because maximum net photosynthesis occurred at light levels ≥ 500 μmol m−2 s−1, but nightshade in the NS rows had higher net photosynthetic rates than those in EW rows when subjected to higher ambient PAR. Stem extension and phenology of nightshade was not affected by vine row orientation, but plants in EW rows had greater leaf areas, leaf area ratios, leaf weight ratios, and lower specific leaf weights than plants in NS rows. Berry mass, seeds per berry, and estimated seed production was 40, 7, and 20% lower, respectively, for plants in the EW than in the NS rows. Dry mass and total nonstructural carbohydrates (TNC) of nightshade roots were also 25 and 45% lower, respectively, in EW than in NS plants. Thus, grapevine row orientation may affect nightshade fecundity by reducing light in the WCZ. Nomenclature: Black nightshade, Solanum nigrum L.; grape, Vitis vinifera L.

Effect of Vineyard Row Orientation on Growth and Phenology of Glyphosate-Resistant and Glyphosate-Susceptible Horseweed (Conyza canadensis)

Horseweed has become increasingly common and difficult to control in San Joaquin Valley vineyards, due in part, to the evolution of glyphosate resistance. 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. The relevance of such a system to horseweed, which can grow to be as tall, or taller, than a typical grapevine trellis, is uncertain. Also unknown is whether a glyphosate-resistant (GR) biotype and glyphosate-susceptible (GS) biotype would perform similarly under such conditions. Therefore, we compared the growth and development of two potted horseweed biotypes (GR and GS) 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 horseweed 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 horseweed plants 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 biotypes, but the GR biotype budded, flowered, and set seed approximately 1 wk earlier than the GS biotype. Thus, shade associated with the EW vinerows reduced horseweed growth, but not fecundity, and the GR biotype reached reproductive maturity earlier than the GS biotype. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; grape, Vitis vinifera L.

Competitive Effects of Glyphosate-Resistant and Glyphosate-Susceptible Horseweed (Conyza canadensis) on Young Grapevines (Vitis vinifera)

Abstract Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) of California. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype that has been observed to be more vigorous than a glyphosate-susceptible (GS) biotype in the SJV. However, the impact that either biotype may have on grapevine growth has not been assessed. Therefore, two glasshouse experiments were conducted to characterize the competitiveness of GR and GS horseweed biotypes from the SJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8-L plastic pots, alone, or with a single GR or GS horseweed. Additional GR and GS horseweeds were also planted separately in individual pots, and all plants were grown for 14 and 16 wk in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry mass (DM) than vines grown alone. The GR biotype reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of both horseweed biotypes by almost 50% and aboveground DM of GR and GS biotypes by 50 and 75%, respectively. These preliminary findings indicate that competition from horseweed can substantially reduce the growth of young grapevines and that the GR biotype may be more competitive than the GS biotype. Nomenclature: Horseweed, Conyza canadensis L. Cronquist ERICA.; grape, Vitis vinifera L.

Ethephon As a Potential Abscission Agent for Table Grapes: Effects on Pre-Harvest Abscission, Fruit Quality, and Residue

Some plant growth regulators, including ethephon, can stimulate abscission of mature grape berries. The stimulation of grape berry abscission reduces fruit detachment force (FDF) and promotes the development of a dry stem scar, both of which could facilitate the production of high quality stemless fresh-cut table grapes. The objective of this research was to determine how two potential abscission treatments, 1445 and 2890 mg/L ethephon, affected FDF, pre-harvest abscission, fruit quality, and ethephon residue of Thompson Seedless and Crimson Seedless grapes. Both ethephon treatments strongly induced abscission of Thompson Seedless berries causing >90% pre-harvest abscission. Lower ethephon rates, a shorter post-harvest interval, or berry retention systems such as nets, would be needed to prevent excessive pre-harvest losses. The treatments also slightly affected Thompson Seedless berry skin color, with treated fruit being darker, less uniform in color, and with a more yellow hue than non-treated fruit. Ethephon residues on Thompson Seedless grapes treated with the lower concentration of ethephon were below legal limits at harvest. Ethephon treatments also promoted abscission of Crimson Seedless berries, but pre-harvest abscission was much lower (≅49%) in Crimson Seedless compared to Thompson Seedless. Treated fruits were slightly darker than non-treated fruits, but ethephon did not affect SSC, acidity, or firmness of Crimson Seedless, and ethephon residues were below legal limits.

Methyl Jasmonate and 1-Aminocyclopropane-1-Carboxylic Acid Interact to Promote Grape Berry Abscission

The application of methyl jasmonate (MeJA) or ethephon, an ethylene-releasing agent, to Thompson Seedless grapes can reduce fruit detachment force (FDF) and promote the development of dry stem scars on berries, possibly improving the quality of machine-harvested grapes. However, the amount of MeJA or ethephon needed to stimulate abscission may be prohibitively expensive and result in excessive residues. Thus, experiments were conducted to determine whether MeJA might interact with 1-aminocyclopropane-1-carboxylic acid (ACC), a natural biochemical precursor of ethylene, to promote abscission-related processes and possibly reduce the amount of MeJA needed. In a preliminary trial, MeJA (672 and 1,344 mg/L) interacted with ACC (500 and 1,000 mg/L) to reduce FDF by 25 to 70% compared with untreated grapes. However, MeJA and ACC did not interact to affect preharvest fruit drop, although treatment with 672 or 1,344 mg/L MeJA caused 16 to 23% drop by 3 days after treatment (DAT). In a second trial, grapes treated with MeJA, singly, or with ACC, produced ethylene which peaked at 1 DAT, remained elevated at 2 DAT, and declined rapidly thereafter, whereas grapes treated only with ACC maintained moderately elevated ethylene production throughout the 10-day study. Treatment with ACC or MeJA reduced FDF within 1 or 2 DAT, respectively. By 2 DAT, berries began to abscise from MeJA-treated clusters, regardless of whether they were also treated with ACC, but on 3, 4, and 10 DAT, ACC and MeJA interacted to greatly promote preharvest fruit drop. Moreover, the combination of ACC and MeJA also promoted dry stem scar development. Thus, coapplication of MeJA and ACC is more effective at stimulating grape abscission-related processes than either compound applied singly.

Threshold of Horseweed (Conyza canadensis) in an Established ‘Thompson Seedless’ Vineyard in the San Joaquin Valley of California

In recent years, horseweed (Conyza canadensis L. Cronq.) has become a very common weed in vineyards of the San Joaquin Valley, California. Up to 16 horseweeds m−2 have been observed on berms (raised beds) in some vineyards, but the effect of different horseweed population densities on grapevine productivity is not known. A three-year study was conducted at the Kearney Agricultural Center, Parlier, California from 2006 to 2008 to evaluate the effect of different horseweed densities (0, 4, 7, 12, 16, and 18 plant m−2 within the vine rows) on grape yield components, fruit quality, harvest interference, and pruning weights of 30-year-old flood-irrigated ‘Thompson Seedless’ grapevines. Horseweed seedlings were transplanted to the vineyard berms in early spring each year and allowed to grow throughout each season. Horseweeds, regardless of planting density, had no effect on grapevine growth, yield, yield components, fruit quality, or the time needed to harvest the fruit in any year of the study or as a cumulative effect of three years. However, the above-ground dry mass of individual weeds was reduced by intraspecific competition beyond densities of 7 plants m−2. Therefore, the threshold of horseweed in flood irrigated mature raisin vineyards may exceed 18 horseweed plants m−2.

Grape Maturity, Yield, Quality, Sensory Properties, and Consumer Acceptance of Fiesta and Selma Pete Dry-on-Vine Raisins

Timely cane severance is the critical first step in the dry-on-vine (DOV) raisin-making process. If canes are severed too early, the berries will have insufficient soluble solids, limiting yield and grade, but if the canes are severed too late, then the grapes will not dry sufficiently. Further, grape maturity at drying may affect the sensory attributes of the raisins, and therefore consumer acceptance. Thus, production studies were conducted over three seasons to determine the relationship between grape maturity and the yield and quality of DOV raisins made from two important DOV raisin grape (Vitis vinifera L.) cultivars, Fiesta and Selma Pete. Additionally, sensory attributes, and consumer acceptance, of “B and better” raisins made from low or high maturity raisins of each cultivar were conducted in one year. There was a positive curvilinear relationship between berry soluble solids level and raisin grades, comparable to relationships established for tray-dried raisins. However, DOV raisins appeared to achieve higher quality grades than would be expected of tray-dried raisins, especially at soluble solids <18, or >20 Brix. Raisin yield, moisture content, and quality grades varied according to year and cane severance date but, in general, an acceptable balance of these variables was achieved by severing the canes at 19 Brix for Fiesta and 21 Brix for Selma Pete. However, consumer acceptance data suggested that most people prefer Selma Pete raisins and raisins from the most mature Fiesta grapes, all of which were made from grapes with >20 Brix at drying. It may be difficult to consistently achieve 20 Brix and adequate drying of Fiesta, suggesting Selma Pete is the superior variety for DOV raisin production.