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  • Author or Editor: Tom van der Zwet x
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Of 133 Pyrus accessions (predominantly P. communis L.), collected in Central Europe and previously rated in the resistant U.S. Dept. of Agriculture (USDA) blight scores 10–6, only 77 (57.0%) remained in these scores after an additional 5 years of exposure to fire blight [Erwinia amylovora (Burr.) Winsl. et al.]. Of these, 24 originated from three states in former Yugoslavia. Following several years of severe blight epiphytotics, only 5 (10.4%) of 52 accessions released from quarantine since 1986 and planted at Appalachian Fruit Research Station scored 6 or above. All accessions were highly susceptible to artificial blossom inoculation, and only 10 accessions were at least moderately resistant to artificial shoot inoculations.

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Pear leaf spot, caused by the fungus Fabraea maculata Atk. (anamorph: Entomosporium mespili (DC.) Sacc.) occurs in most areas of the world where pears are grown. Most major cultivars of the european pear, P. communis L., for which data are available are susceptible. Ratings appearing in the literature are sometimes contradictory. This study evaluated resistance/susceptibility within a diverse collection of Pyrus cultivars and other germplasm in a randomized and replicated nursery plot using quantitative measures of disease incidence and severity. The least susceptible genotypes were the P. communis cultivars `Beurre Fouqueray' and `Bartlett', the P. pyrifolia cultivars `Imamura Aki', and the P. communis × P. ussuriensis hybrid NJ 477643275.

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`Shenandoah' is a new European pear (Pyrus communis L.) cultivar which combines resistance to fire blight with fruit of excellent quality. The original seedling tree was selected in 1985 from a cross of `Max Red Bartlett'× US 56112-146, and was tested under the original seedling number, US 78304-057. The fruit of `Shenandoah' is pyriform in shape, and moderately large in size, averaging 72 mm in diameter and 92 mm in height. Skin color at harvest is light green, turning yellow-green when ripe. The skin finish is glossy, and 10% to 20% of the fruit surface is blushed red. There is light tan russet at the calyx. Lenticles are slightly conspicuous, and are surrounded by small, light brown russet. The stem is medium to long (≈25 mm), of medium thickness, and slightly curved. Harvest maturity occurs about four weeks after `Bartlett', and the fruit will store in refrigerated (-1 °C) air storage for at least four months without core breakdown or superficial scald. The flesh texture is moderately fine, juicy, and buttery. Grit cells are moderately small and occur primarily around the core and in a thin layer under the skin. The flavor is aromatic, similar to `Bartlett', and is moderately acidic during the first two months of storage, becoming subacid after longer storage. The tree is moderate in vigor on `Bartlett' seedling and `OHxF 97' rootstocks, and upright-spreading in habit. Shenandoah' blooms in mid-season, similar to `Bartlett'. Yield has been moderately high and precocious, and with no pronounced biennial pattern. Fire blight resistance is similar to `Seckel', with infections extending no further than 1-year-old branches. Artificial blossom inoculations indicate a moderate degree of blossom resistance to fire blight infection.

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Three plant exploration trips were made to Eastern Europe between 1976 and 1980 in search of domestic Pyrus germplasm in five countries between the Baltic and Adriatic Seas. This collection of pear (Pyrus spp.) germplasm in Romania, Poland, Hungary, and Czechoslovakia was preceded by 279 accessions from Yugoslavia (16). Descriptions of Pyrus spp., method of budwood collection and shipment, and the care and maintenance of the plant material at the U.S. Plant Introduction Station, Glenn Dale, Md. were covered in ref. 16. Trees were exposed to natural infection by fire blight [Erwinia amylovora (Burr.) Winsl. et al.], Fabraea leaf spot (Entomosporium maculatum Lev.), and pear psylla (Cocopsylla pyricola Foerst.). In addition, evaluation of fruit size and a preliminary assessment of quality are being carried out. The most interesting accessions are being indexed for virus disorders before release.

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