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  • Author or Editor: James W. Olmstead x
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Most sweet cherry (Prunus avium L.) cultivars grown commercially in the United States are susceptible to powdery mildew, caused by the fungus Podosphaera clandestina (Wall.:Fr.) Lev. Recently, hybrid populations segregating for resistance to powdery mildew were developed by crossing a mildew-resistant sweet cherry selection, PMR-1, with the susceptible cultivars Bing, Rainier, and Van. Although segregation within these populations indicated a single gene was responsible for the powdery mildew resistance conferred by PMR-1, the gene action could not be determined. Therefore, a reciprocal cross between `Bing' and `Van' was made to determine the allelic state of the susceptible parents used previously. All progeny (n = 286) from this cross were susceptible to powdery mildew. This information, combined with results from previous segregation data, indicate the powdery mildew resistance gene is inherited in a dominant manner and is present in PMR-1 in the heterozygous allelic state. We have named this gene Pmr1. Furthermore, in combination with known pedigree information, we have been able to predict the susceptibility of more than 60 additional commercial and recently released sweet cherry cultivars.

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Most sweet cherry (Prunus avium L.) cultivars grown commercially in the Pacific Northwestern states of the United States are susceptible to powdery mildew, caused by the fungus Podosphaera clandestina (Wall.:Fr.) Lev. The disease is prevalent in the irrigated arid region east of the Cascade Mountains in Washington State. Little is known about genetic resistance to powdery mildew in sweet cherry, although a selection (PMR-1) was identified at Washington State Univ.'s Irrigated Agriculture Research and Extension Center that exhibits apparent foliar immunity to the disease. The objective of this research was to determine the inheritance of powdery mildew resistance from PMR-1. Reciprocal crosses were made between PMR-1 and three high-quality, widely-grown susceptible cultivars (`Bing', `Rainier', and `Van'). Resultant progenies were screened for reaction to powdery mildew colonization using a laboratory leaf disk assay. Assay results were verified by natural spread of powdery mildew among the progeny in a greenhouse and later by placing them among infected trees in a cherry orchard. Segregation within the progenies for powdery mildew reaction fit a 1 resistant: 1 susceptible segregation ratio (P ≤ 0.05), indicating that resistance to powdery mildew derived from PMR-1 was conferred by a single gene.

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A personal computer-based method was compared with standard visual assessment for quantifying colonization of sweet cherry (Prunus avium L.) leaves by powdery mildew (PM) caused by Podosphaera clandestina (Wallr.:Fr.) Lev. Leaf disks from 14 cultivars were rated for PM severity (percentage of leaf area colonized) by three methods: 1) visual assessment; 2) digital image analysis; and 3) digital image analysis after painting PM colonies on the leaf disk. The third technique, in which PM colonies on each leaf disk were observed using a dissecting microscope and subsequently covered with white enamel paint, provided a standard for comparison of the first two methods. A digital image file for each leaf disk was created using a digital flatbed scanner. Image analysis was performed with a commercially available software package, which did not adequately detect slight differences in color between PM and sweet cherry leaf tissue. Consequently, two replicated experiments revealed a low correlation between PM image analysis and painted PM image analysis (r2 = 0.66 and 0.46, P ≤ 0.0001), whereas visual assessment was highly correlated with painted PM image analysis (r2 = 0.88 and 0.95, P ≤ 0.0001). Rank orders of the 14 cultivars differed significantly (P ≤ 0.05) when PM image analysis and painted PM image analysis were compared; however, rankings by visual assessment were not significantly different (P > 0.05) from those by painted PM image analysis. Thus, standard visual assessment is an accurate method for estimating disease severity in a leaf disk resistance assay for sweet cherry PM.

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Although maximizing fruit size is critical for profitable sweet cherry (Prunusavium L.) production, little is known about the cellular differences among and between cultivars that contribute to fruit size differences. A wide range of fruit size exists among sweet cherries, and, due to cultural and environmental differences, significant variation exists among genetically identical fruit from the same cultivar. To determine the relative contributions of flesh cell number and cell size to final fruit size in sweet cherry, equatorial sections of three cultivars with a wide range in final average fruit size [`New York 54' (NY54; 1.4 g fresh weight, 11.8 mm diameter), `Emperor Francis' (EF; 6.1 g, 21.0 mm), and `Selah' (12.8 g, 25.5 mm)] were created from mature fruit. Cells intersecting a transverse line were counted and average cell length was calculated. The average cell numbers were significantly different (P ≤ 0.05) between `NY54', `EF', and `Selah' (26.7, 47.4, and 83.2, respectively), indicating that flesh cell number is the major contributor to differences in fruit size between cultivars. Flesh cell numbers of `NY54', `EF', and `Selah' were similar at bloom and increased rapidly for a short duration after fertilization, suggesting a key developmental period for fruit size differences. To determine the contribution of cell number differences to variation in fruit size within a cultivar, fruit from `Bing' and `Regina' trees exhibiting a range of size due to cultural and environmental differences were measured. In both cases, average cell number was not significantly different (P = 0.9, P = 0.3, respectively), while average cell size was (P ≤ 0.05), further indicating fruit flesh cell number is a genetically controlled trait.

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Most sweet cherry (Prunus avium L.) cultivars grown commercially in the Pacific Northwest U.S. are susceptible to powdery mildew caused by the fungus Podosphaera clandestina (Wall.:Fr.) Lev. The disease is prevalent in the irrigated arid region east of the Cascade Mountains in Washington State. Little is known about genetic resistance to powdery mildew in sweet cherry, although a selection (`PMR-1') was identified at the Washington State Unive. Irrigated Agriculture Research and Extension Center that exhibits apparent foliar immunity to the disease. The objective of this research was to characterize the inheritance of powdery mildew resistance from `PMR-1'. Reciprocal crosses between `PMR-1' and three high-quality, widely-grown susceptible cultivars (`Bing', `Rainier', and ëVaní) were made to generate segregating progenies for determining the mode of inheritance of `PMR-1' resistance. Progenies were screened for susceptibility to powdery mildew colonization using a laboratory leaf disk assay. Assay results were verified by natural spread of powdery mildew among the progeny seedlings in a greenhouse and later by placement among infected trees in a cherry orchard. Progenies from these crosses were not significantly different (P > 0.05) when tested for a 1:1 resistant to susceptible segregation ratio, indicating that `PMR-1' resistance is conferred by a single gene, which we propose to designate as PMR-1.

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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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Carotenoids serve as protective antioxidants, and function in normal vision, bone growth, cell division and differentiation, and reproduction. Winter squash (Cucurbita spp.) is an excellent dietary source of carotenoids. The range of colors from yellow to red in Cucurbita species indicates that increasing carotenoid levels through plant breeding is possible. The objective of this research was to determine the heritability of flesh color in winter squash in both Cucurbita moschata Duchesne and Cucurbita pepo L. Segregating families representing F2, BC1P1 and BC1P2 populations were created in two families of C. pepo (‘Table Gold Acorn’ × PI 314806 and ‘Table King Bush’ × PI 314806) and one family of C. moschata (‘Butterbush’ × ‘Sucrine DuBerry’). Broad-sense heritabilities were calculated for the F2, BC1P1, and BC1P2 populations within each of the three families. Heritabilities ranged from 0.19 to 0.82 for L*, 0.28 to 0.97 for chroma, and 0.12 to 0.87 for hue across all families. Transgressive segregation for color space values L* was identified in the ‘Table King Bush’ × PI 314806 C. pepo population. Our results indicate that it is possible to breed for improved flesh color in Cucurbita, but the population size and number of test locations for evaluation need to be increased to provide better heritability estimates. Cucurbita species are grown throughout the world and their availability and low price makes them an important potential source of carotenoids for human nutrition and health for all ages.

Open Access

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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