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  • Author or Editor: James W. Olmstead x
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A detached leaf disk assay for screening sweet cherry (Prunus avium L.) genotypes for susceptibility to powdery mildew (PM) [Podosphaera clandestina (Wallr.:Fr.) Lev.] was developed by evaluating the effects of photoperiod (24 hours light, 0 hours light, 14 hours light/10 hours dark), substrate nutrient content (sterile distilled water, 1% sucrose), leaf age (old, young, emergent), and leaf explant size (intact leaf, 30 mm, 20 mm) on PM growth on leaves from the susceptible cultivar Bing. The only parameter described that had a significant (P ≤ 0.001) effect on PM growth was leaf age. Old leaves, designated as the third fully expanded leaf from the basal end of current-year's shoot growth, were never infected with PM under controlled inoculations. In the absence of significant differences between treatments, those parameters with the highest treatment means were selected for subsequent evaluation. To test the leaf disk assay, 14 sweet cherry cultivars were screened in two experiments, and rated according to level of PM susceptibility. Rank sum comparison of results from cultivars used for leaf disk screening agreed with earlier field rankings of the same cultivars. The developed leaf disk assay greatly reduced the space required to screen sweet cherry cultivars, and was a repeatable and objective predictor of field resistance that may be useful for screening germplasm or breeding populations.

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Vaccinium arboreum (VA) is a wild blueberry species that exhibits wider soil pH tolerance and greater ability for iron and nitrate uptake than cultivated Vaccinium species, including southern highbush blueberry (SHB, V. corymbosum interspecific hybrids). The ability of VA and SHB to respond to iron deficiency by rhizosphere acidification was investigated. Rooted cuttings of the VA genotype FL09-502 and SHB ‘Emerald’ were transplanted to a hydroponic system filled with complete nutrient solution. After 14 days of acclimation at 45 µm iron, plants were transferred to unbuffered nutrient solutions containing 90 or 10 µm iron. ‘Emerald’ and FL09-502 plants grown in 10 µm iron exhibited less iron uptake and lower chlorophyll, total iron, and active iron contents than plants grown in 90 µm iron. Generally, there were no species-level differences in iron or nitrate uptake. Neither FL09-502 nor ‘Emerald’ acidified the rhizosphere in either the nutrient solution or in a gel-based assay, regardless of external iron concentration. A screen of 18 additional genotypes of VA and SHB confirmed that this response is absent in these taxa. Thus, rhizosphere acidification is not part of the iron deficiency response of SHB and VA. In addition, the ability to acidify the soil is not likely to be responsible for the wider soil pH tolerance of VA.

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Understanding the genetic control of fruit size in sweet cherry (Prunus avium L.) is critical for maximizing fruit size and profitable fresh market production. In cherry, coordinated cycles of cell division and expansion of the carpel result in a fleshy mesocarp that adheres to a stony endocarp. How these structural changes are influenced by differing genetics and environments to result in differing fruit sizes is not known. Thus, the authors measured mesocarp cell length and cell number as components of fruit size. To determine the relative genotypic contribution, five sweet cherry cultivars ranging from ≈1 to 13 g fresh weight were evaluated. To determine the relative environmental contribution to fruit size, different-size fruit within the same genotype and from the same genotype grown in different environments were evaluated. Mesocarp cell number was the major contributor to the differences in fruit equatorial diameter among the five sweet cherry cultivars. The cultivars fell into three significantly different cell number classes: ≈28 cells, ≈45 cells, and ≈78 cells per radial mesocarp section. Furthermore, mesocarp cell number was remarkably stable and virtually unaffected by the environment as neither growing location nor physiological factors that reduced final fruit size significantly altered the cell numbers. Cell length was also significantly different among the cultivars, but failed to contribute to the overall difference in fruit size. Cell length was significantly influenced by the environment, indicating that cultural practices that maximize mesocarp cell size should be used to achieve a cultivar's fruit size potential.

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Because of financial and labor concerns, growers are interested in using machine harvesting for fruit destined to be fresh marketed. Machine harvest of highbush blueberry (Vaccinium corymbosum) has typically been used to obtain large volumes of fruit destined for processing. Bush architecture, easy detachment of mature berries compared with immature berries, loose fruit clusters, small stem scar, firm fruit, and a concentrated ripening period are breeding goals to develop cultivars amenable to machine harvest. In the University of Florida (UF) southern highbush blueberry [SHB (Vaccinium corymbosum hybrids)] breeding program, sparkleberry (Vaccinium arboreum) has been used in wide crosses in an attempt to introgress traits that may be valuable for machine harvesting, namely upright growth habit with a narrow crown and long flower and fruit pedicels creating loose fruit clusters. Two eras of sparkleberry hybridization experiments have occurred since the early 1980s. The first era used darrow’s evergreen blueberry (Vaccinium darrowii) as a bridge between sparkleberry and tetraploid SHB, with the recently released cultivar FL 01-173 (sold under the trademarked name Meadowlark) as an example of the end product. The second era has used chromosome doubling to develop polyploid sparkleberry selections that were directly crossed with tetraploid SHB. After 1 year of evaluation, a SHB × (SHB × sparkleberry) population developed for linkage and quantitative trait locus mapping showed abundant variation for length:width ratio of the plant, but similarity to the highbush phenotype for peduncle and pedicel length of the fruit. These first evaluations indicate evidence of introgression and provide an initial step toward improved cultivars for mechanical harvesting.

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Northern highbush (NH) blueberry (Vaccinium corymbosum) and southern highbush (SH) blueberry (V. corymbosum hybrids) have fruit that vary in firmness. The SH fruit is mostly hand harvested for the fresh market. Hand harvesting is labor-intensive requiring more than 500 hours/acre. Rabbiteye blueberry (V. virgatum) tends to have firmer fruit skin than that of NH blueberry and has been mostly machine harvested for the processing industry. Sparkleberry (V. arboreum) has very firm fruit. With the challenges of labor availability, efforts are under way to produce more marketable fruit using machine harvesting. This could require changing the design of harvesting machine and plant architecture, and the development of cultivars with fruit that will bruise less after impact with hard surfaces of machines. The objectives of this study were to determine the fruit quality of machine-harvested SH blueberry, analyze the effect of drop height and padding the contact surface on fruit quality, investigate the effect of crown restriction on ground loss, and determine the effect of plant size on machine harvestability. The fruit of ‘Farthing’, ‘Scintilla’, ‘Sweetcrisp’, and several selections were either hand harvested or machine harvested and assessed during postharvest storage for bruise damage and softening. Machine harvesting contributed to bruise damage in the fruit and softening in storage. The fruit of firm-textured SH blueberry (‘Farthing’, ‘Sweetcrisp’, and selection FL 05-528) was firmer than that of ‘Scintilla’ after 1 week in cold storage. Fruit drop tests from a height of 20 and 40 inches on a plastic surface showed that ‘Scintilla’ was more susceptible to bruising than that of firm-textured ‘Farthing’ and ‘Sweetcrisp’. When the contact surface was cushioned with a foam sheet, bruise incidence was significantly reduced in all SH blueberry used in the study. Also, the fruit dropped 40 inches developed more bruise damage than those dropped 20 inches. Ground loss during machine harvesting was reduced from 24% to 17% by modifying the rabbiteye blueberry plant architecture. Further modifications to harvesting machines and plant architecture are necessary to improve the quality of machine-harvested SH and rabbiteye blueberry fruit and the overall efficiency of blueberry (Vaccinium species and hybrids) harvesting machines.

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The Outstanding Fruit Cultivar Award is a medal presented annually by the Fruit Breeding Working Group of the American Society for Horticultural Science (ASHS) for noteworthy new fruits released over the previous 35 years. Since 1987, 38 cultivars have been recognized with medals presented at the Annual Conference of the Society. The awards celebrate the progress achieved by fruit breeders and their contributions to the world’s fruit industry.

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Previously, when selecting for flavor in the University of Florida southern highbush blueberry (SHB, Vaccinium corymbosum L. hybrids) breeding program, sugar/acid ratios and breeder preference were the only factors considered. A more precise method of evaluating flavor would include volatile compounds that may also contribute to the flavor experience. Therefore, volatile profiles of five SHB cultivars (Farthing, FL01-173, Scintilla, Star, and Sweetcrisp) were compared using gas chromatography–mass spectrometry. All cultivars were harvested on four separate dates within the harvest season, and fruit from each cultivar were also harvested at four developmental stages on the first harvest date. Among the cultivars, soluble solids content and volatile production tended to increase with fruit maturity, whereas titratable acidity decreased. All volatile components were more variable than measures of sugars and acids during the harvest season. Many of the volatiles present varied significantly between harvest dates, resulting in significant genotype × environment interactions during the harvest season. A closer examination of linalool, trans-2-hexenol, trans-2-hexenal, hexanal, and 1-penten-3-ol, five volatile compounds commonly associated with blueberry flavor, showed cultivar, developmental stage, and harvest date differences for each volatile. ‘Star’ experienced the least variation through the harvest period.

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Field performance of southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrids) cultivars Emerald, Jewel, and Primadonna derived from softwood cuttings (SW) and tissue culture (TC) was evaluated in Citra and Haines City, FL, in 2010–12. Both fields were planted in Apr. 2010 on sandy soil amended with pine bark. Plant height and width were recorded at both locations, from which plant canopy volume was calculated. Additionally, whole plants were harvested at planting and after the first growing season, after the first fruit harvest, and after the second growing season. Average plant height and width, number of major canes, and total shoot number were determined at each sampling date. Dry weights for roots, crowns, canes, shoots, and leaves were obtained. Although propagation method affected plant canopy volume during the first season, no effects were observed by the end of the second growing season. At planting and after the first and second growing seasons, TC plants of the three cultivars had more major canes. Total shoot number per plant was greater for TC ‘Jewel’ at all dates but ‘Emerald’ TC plants had more shoots only at planting and after the first growing and harvest seasons. Tissue culture resulted in increased plant dry weights of ‘Jewel’ and ‘Emerald’ after the first and second growing seasons. There were no significant differences in total number of shoots or plant dry weight between TC and SW-derived ‘Primadonna’ plants at any point during the study.

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