Search Results

Growers in California’s San Joaquin Valley produced >25% of the world’s raisins in 2012, with a farm-gate value of >$590 million, making the United States the leading global producer of raisins. California’s traditional raisin-making method is a laborious process in which clusters of grapes (Vitis vinifera) are harvested by hand onto paper trays, which are left in the vineyard to dry. The drying fruit may need to be turned or rolled, tasks requiring manual labor, and the trays of dried raisins are also picked up by hand. Most California raisins continue to be made in this way, but in recent years, the declining availability and increasing cost of labor has prompted many growers to implement one of two mechanized production systems, “continuous tray” (CT) or “dry-on-vine” (DOV). In CT systems, machines are used to pick the berries, lay them onto a tray, and pick up the dried raisins. The CT system could be considered a short-term strategy: it is compatible with existing conventional ‘Thompson Seedless’ raisin vineyards and has been widely adopted. The DOV system could be considered a medium-term strategy: it is best suited for vineyards specifically designed for DOV, with early ripening grapevine cultivars on expansive trellis systems, which ensures timely drying, and capitalizes on the fact that sunlit row middles are not needed for fruit drying. Grapevine breeding programs are currently working toward the development of raisin grape cultivars with fruitful basal nodes, with fruit that dry naturally upon ripening. This is a long-term strategy to further reduce labor needs by enabling mechanical pruning in winter and eliminating the need for cane severance in the summer.

Four AM fungal isolates (Glomus sp.) from disparate edaphic conditions were screened for effects on leaf gas exchange of `Volkamer' lemon (Citrus volkameriana Ten. and Pasq.) plants of similar size under conditions of increased soil water deficit stress and recovery from stress. Mycorrhizal and non-mycorrhizal plants were grown in 8-L containers for 10 weeks under well-watered conditions in a glasshouse and then subjected to three consecutive soil-drying episodes of increased severity (mean soil water tension reached –0.02, –0.06, and –0.08 MPa, respectively). Gas exchange measurements were made on the last day of each soil-drying episode. Plants were irrigated after each soil-drying episode, and measurements were repeated on the following 2 recovery days, when soil remained moist. All measurements were made at mid-day with a LI-COR 6200 portable photosynthesis system. The effect of AM fungi on leaf gas exchange fluxes varied depending on the isolate and the intensity of soil water stress. Leaf gas exchange fluxes always were highest for plants colonized by Glomus mosseae (Nicol. & Gerde.) isolate 114C, except during the third soil-drying episode, when all mycorrhizal plants had similar, and lower, gas exchange fluxes compared with non-mycorrhizal plants. During recovery from the third soil-drying episode, Glomus mosseae isolate 51C had lower leaf gas exchange fluxes compared with all other plants. Our results show that AM fungi can alter leaf gas exchange fluxes of citrus, under conditions of optimal P nutrition, in an isolate-specific manner.

In Florida, gibberellic acid (GA₃₎ is applied to citrus in the late summer or early fall to reduce senescence-related peel disorders of fresh fruit and to increase juice yield of processing oranges. Heavy rainfall may occur daily during this time that could reduce the efficacy of GA₃ sprays. Experiments were conducted in 1998-99 and 1999-2000 to test the effect of timed “wash off” treatments on the peel color and peel puncture resistance (PPR) of `Hamlin' orange (Citrus sinensis [L.] Osb.) fruit that were previously treated with GA₃. In Oct. 1998 and 1999, the canopy of 14- or 15-year-old trees were sprayed to runoff (≈10 L) with GA₃ (45 g a.i./ha) and a non-ionic surfactant (Silwet, 0.05%). For the next 4 (1998-99) or 5 (1999-2000) h, three different GA₃-treated trees each hour were then sprayed with ≈20 L of tap water to simulate rainfall that might remove or dilute the GA₃. An additional three trees did not receive a GA₃ or a washoff treatment. Fruit were harvested in Nov. 1998 and Jan. 1999 and Dec. 1999 and Jan. 2000 and evaluated for PPR and color. Data were subjected to regression analysis to determine the relationship between peel variables and time until washoff. In 1998-99, PPR and peel hue (level of green color) increased linearly with time until washoff, indicating that some GA₃ uptake was still occurring after 4 h. In 1999-2000, PPR and hue increased linearly until about 3 h before washoff. Therefore, heavy rainfall within 3 to 4 h of application may reduce GA₃ effectiveness, even when a surfactant is used.

Forchlorfenuron (CPPU), a synthetic cytokinin, applied after fruit set increases the size and firmness of table grapes (Vitis vinifera L.) beyond what is possible without CPPU treatment. However, treatment with CPPU may inhibit coloring of ‘Flame Seedless’ grapes, limiting its use in growing areas where color has been consistently poor. In contrast, application of abscisic acid (ABA) to ‘Flame Seedless’ grapes may cause fruit softening, which is undesirable, but its primary effect is to increase anthocyanin content and fruit color. Thus, we hypothesized that application of CPPU followed by ABA might increase the size and firmness of ‘Flame Seedless’ grapes without excessively inhibiting coloring. Grapes were treated with 0 or 20 g·ha⁻¹ CPPU (applied at fruit set) and 0, 300, or 600 mg·L⁻¹ ABA (applied at veraison) in 2005 and with 0, 5, 10, 15, or 20 g·ha⁻¹ CPPU and 0, 200, 400, or 600 mg·L⁻¹ ABA in 2006. Both plant growth regulators (PGRs) increased berry mass, but grapes treated with CPPU were as firm, or firmer, than nontreated grapes, whereas those treated with ABA were of similar or lesser firmness. Treatment with CPPU generally reduced soluble solids and red berry color, whereas treatment with ABA reduced titratable acidity and increased red color. The PGRs did not interact to affect any of the fruit quality variables measured, so beneficial effects of CPPU or ABA were apparent whether the grapes were treated with either or both PGRs. Thus, the combined use of CPPU and ABA may be a desirable cultural practice for ‘Flame Seedless’.

Four AM fungal isolates (Glomus sp.) were screened for effects on growth of `Volkamer' lemon (Citrus volkameriana Ten. and Pasq.) under well-watered conditions. Plants were inoculated with an isolate of AM fungi, or non-inoculated. Non-mycorrhizal plants received more phosphorus (P) fertilizer than mycorrhizal plants because mycorrhizae enhance P uptake. Mycorrhizal and non-mycorrhizal plants were grown in 8-liter containers for 3 months in a glasshouse. Plants were then harvested, and root length colonized by mycorrhizal fungi, leaf P concentration, and plant growth were determined. Root length colonized by AM fungi differed among isolates; control plants were non-mycorrhizal. Leaf P concentration was in the optimal range for all plants; however, plants colonized by Glomus mosseae Isolate 51C had higher leaf P concentration than non-mycorrhizal plants. Plants colonized by Glomus AZ112 had higher leaf P concentration than all other plants. All plants had similar canopy leaf area, shoot length, and shoot dry mass. Plants colonized with AM fungi, except Glomus mosseae Isolate 51C, had longer root length and greater root dry mass than non-mycorrhizal plants. All mycorrhizal plants had lower shoot:root dry mass and leaf area:root length ratios than non-mycorrhizal plants. Our results showed that under optimal P nutrition and well-watered conditions, AM fungal isolates differentially altered the morphology of citrus plants by stimulating root growth.

It is desirable to mix gibberellic acid (GA₃) with other commonly applied materials to reduce application cost. However, applying GA₃ with some compounds can reduce its efficacy or cause phytotoxicity. We conducted experiments in 1997-98 and 1998-99 to determine if GA₃ (ProGibb) can be tank-mixed with fosetyl-Al (Aliette), or avermectin (Agri-Mek) and oil, without reducing GA₃ efficacy. In addition, we compared Silwet and Kinetic adjuvants for enhancement of GA₃ efficacy. Five tank mixes were tested along with a nonsprayed control. These included 1) GA₃; 2) GA₃ and Silwet; 3) GA₃ and Kinetic; 4) GA₃ Silwet, and fosetyl-Al; and 5) GA₃, Silwet, avermectin, and oil. All compounds were applied at recommended concentrations. In September 1997 or October 1998, about 2.5 gal (9.5 L) of each tank mix was applied with a hand sprayer to 14- or 15-year-old `Hamlin' orange (Citrus sinensis) trees on sour orange (Citrus aurantium) rootstock. Peel puncture resistance (PPR), color, and juice yield (% juice weight) were evaluated monthly between December 1997 and March 1998, and December 1998 and January 1999. In both years, fruit of treated trees usually had higher PPR and were less yellow in color than fruit from control trees. There were tank mix effects on juice yield in January of both seasons and February 1998. Gibberellic acid was most effective at enhancing juice yield when applied singly or with avermectin and oil. In both seasons there were dates when GA₃ applied singly was superior at enhancing juice yield than a tank mix of GA₃, Silwet and fosetyl-Al, indicating that GA₃ was incompatible with fosetyl-Al. Neither Kinetic nor Silwet adjuvants consistently enhanced GA₃ effects on peel quality or juice yield over GA₃ alone.

California table grape (Vitis vinifera) growers cover the canopies of late-season varieties with plastic (polyethylene) covers to shield the fruit from rain. Green- or white-colored covers are commonly used, but there is lack of information whether either cover might be preferable based on canopy microclimate or fruit quality. In late September, ‘Redglobe’ (in 2011) and ‘Autumn King’ (in 2012) table grapevines were covered with green or white plastic, or left uncovered, and canopy microclimate, fungal and bacterial rot incidence, and fruit yield and quality at harvest, and after postharvest storage, were evaluated. Green covers were more transparent and less reflective than white covers, and daily maximum temperature difference in the top center of the canopies of grapevine with green covers was consistently >5 °C than that of grapevine subjected to other treatments, but covers had little effect on temperatures in the fruit zones, which were not enveloped by covers. Effects on relative humidity (RH) depended on location within the canopy and time of day; RH peaked in early morning and was at a minimum in late afternoon. All cover treatments had relatively similar peak RH in south-facing fruit zones and the top center of the canopy. However, in the north-facing fruit zone, vines with green covers had higher RH at night than vines subjected to other treatments. Both covers consistently reduced evaporative potential in the top center of the canopy, but not in fruit zones. Treatment effects on condensation beneath the covers were inconsistent, possibly due to differences in canopy size, variety, or season, but south-facing cover surfaces generally had less condensation than the top or north-facing surfaces. About 0.5 inch of rain fell on 5 Oct. 2011, but no rain occurred during the 2012 experiment. In 2011, green covers delayed fruit maturation slightly, but not in 2012. Covers did not affect vineyard rot incidence, the number of boxes of fruit harvested, or postharvest fruit quality in 2011, but fruit from covered grapevine had less postharvest rot in 2012 than fruit from noncovered grapevines, even though a measurable rain event occurred in 2011 but not in 2012. In conclusion, our results suggest that white covers may be preferable to green since green covers were associated with higher temperatures in both seasons and higher RH in the ‘Autumn King’ trial of 2012, but otherwise performed similarly.

In two experiments, various combinations of ethephon, with or without 1-aminocyclopropane carboxylic acid (ACC), were applied to the fruiting zone of ‘Selma Pete’ raisin grapes (Vitis vinifera) to determine whether any could serve as a defoliant, and if so, whether defoliation improved subsequent vine drying of the grapes. In the first experiment, the fruiting zone was treated on 8 Aug. 2013 with a control (water) and one of four plant growth regulator (PGR) treatments: 1000 ppm ethephon, 1000 ppm ethephon plus 1000 ppm ACC, 2000 ppm ethephon, and 2000 ppm ethephon plus 1000 ppm ACC. In the first experiment, treatment with any of the PGRs hastened leaf senescence, but leaf greenness, measured with a SPAD meter, declined most rapidly in leaves from vines treated with 2000 ppm ethephon or 2000 ppm ethephon plus 1000 ppm ACC, and defoliation was best in vines treated with 2000 ppm ethephon plus 1000 ppm ACC. None of the treatments in the first study affected berry composition, hastened berry drying, or ultimately affected raisin moisture or quality. In a second experiment, initiated 18 days later, a factorial design was employed to determine whether three chemical treatments, a control (water spray), 2000 ppm ethephon, and 2000 ppm ethephon plus 1000 ppm of ACC, might interact with fruiting zone orientation (east or west facing) to affect leaf senescence or berry drying. The second study confirmed that 2000 ppm ethephon and 2000 ppm ethephon plus 1000 ppm ACC induced rapid leaf senescence. Defoliation proceeded more rapidly in the second study and by 13 days after treatment, vines treated with 2000 ppm ethephon plus 1000 ppm ACC had less than one leaf layer remaining in the fruiting zone compared with more than 2.5 leaf layers in untreated vines. Treatments again had no effect on berry fresh weight or composition, but grapes on west-facing vines treated with 2000 ppm ethephon plus 1000 ppm ACC dried significantly better than grapes on vines subjected to other treatments, possibly because the higher temperatures of west-facing vines coupled with better defoliation of the 2000 ppm ethephon plus 1000 ppm ACC treatment was sufficient to improve grape drying compared with vines subjected to other trellis orientation and chemical treatment combinations. Therefore, we conclude that treatment with ethylene-promoting PGRs can defoliate the fruiting zone of ‘Selma Pete’ grapes with divided canopies, and such defoliation treatments may enhance berry drying when drying is initiated later than normal.

Gibberellic acid (GA₃) increases juice yield of processing oranges, but results are inconsistent. Preliminary research suggested that this variability might be related to application timing. Therefore, we conducted an experiment to determine the optimal time to apply GA₃ for increasing juice yield of `Hamlin', `Pineapple', and `Valencia' sweet oranges [Citrus sinensis (L.) Osb.]. Mature trees of each cultivar were sprayed with ≈10 L of a solution of GA<sub>3</sub> (45 g·ha<sup>-1</sup> a.i.) and organo-silicone surfactant (Silwet, 0.05%) between 2 Sept. and 9 Dec. 1998, and 25 Sept. and 9 Dec. 1999, or remained non-sprayed (control). Generally, the earliest application dates were most effective at maintaining peel puncture resistance above that of control fruit, while the latest application dates resulted in the most green peel color at harvest. Juice yield of `Hamlin' and `Valencia', but not `Pineapple', was increased by GA₃ at some application timings and harvest dates in both years. The increase in juice yield was related to time between application and harvest; juice yield of `Hamlin' was greatest ≈2 months, and `Valencia' ≈5 months after GA₃ application. Treated fruit often had lower juice Brix than non-sprayed fruit, a phenomenon that often paralleled treatment effects on peel color. When treatments did not increase juice yield but reduced juice Brix, then yield of solids was sometimes lower than for non-treated fruit. Treatments generally delayed flowering of `Pineapple' and `Valencia' but not `Hamlin'.

Poor coloration of red grapes grown in warm regions is a frequent problem that decreases production efficiency. Most table grape growers use ethephon to improve color, but its influence on color development is erratic, and it may reduce berry firmness. Application of S-abscisic acid (ABA) to grapes can increase the anthocyanins in their skins, but no protocols have been established regarding its potential commercial use. Therefore, we evaluated the effects of ABA and ethephon treatments on fruit quality characteristics, including those related to firmness and color, on `Flame Seedless' grapes (Vitis vinifera L.) in several experiments over three consecutive seasons. Abscisic acid had few effects on berry weight or juice composition, but it increased berry softening and skin anthocyanin concentrations. The effect of ABA on berry firmness was similar to ethephon. With respect to skin anthocyanin concentration and fruit color characteristics, 300 mg·L^–1 ABA applied at veraison was superior to the other ABA concentrations and to ethephon applied at any of the times tested. Moreover, any concentration of ABA between 75 and 300 mg·L^–1 applied after veraison improved color better than ethephon applied at the same time. There was a highly significant inverse curvilinear relationship between skin anthocyanin concentration and the lightness and hue of the berries. Anthocyanin concentrations between 0.01 and 0.04 mg·cm^–2 had little effect on berry lightness and hue, so researchers should consider measuring color, not just anthocyanins, when evaluating the quality of red table grapes.