Fifty nine morphological characters and isozyme band patterns of glutamate oxaloacetate transminase, peroxidase, glucose phosphate isomerase from fully expanded leaves were used for taxonomic study on 51 taxa consisted of Korean native and principal cultivars of the genus Pyrus. Taxonomic relationships were analyzed by complete cluster analysis method based on Euclidean taxonomic distance of IBM PC SPSS/PC+(ver. 3.0). Among the 39 qualitative morphological characters, a great deal of variations among 51 taxa were observed in immature fruit shape, skin lusterness, hair density on pedicel, anther color, shape of leaf apex and base, hair density on leaf surface, and leaf margin. Considerable variations were found in most tested quantitative characters except in the number of petals and styles. More reliable taxonomic results could be obtained by comparing morphological characters rather than examining isozyme band patterns. Even though there were considerable differences depending upon the methods of investigation, classification of the genus Pyrus by using isozyme band patterns was proved to be a good tool for rapid taxonomic studies.
The nutritional status of ‘Bartlett’ pear growing on P. communis seedling, Old Home × Farmingdale (OH×F), and several Pyrus species seedling rootstocks was compared to those growing on Bartlett seedling rootstock at 3 locations over a 2-3 year period. Few significant differences were found in leaf element content of scions growing on Bartlett seedling rootstock and those on the rootstock clones or other Pyrus seedlings. Nitrogen was higher in the scions on 5 of the OH×F rootstocks but did not seem related to yield efficiency. Generally the leaf element content of Mg and Mn was lower and Fe higher in leaves of trees growing on the OH×F clonal rootstocks when compared to trees on Bartlett seedling. Nutrient uptake and passage through graft unions appeared unrelated to the degree of graft compatability, based on the similarity of nutrient levels of ‘Bartlett’ scions grafted directly on quince rootstock and those with an Old Home interstem. Root system genetics seems to be the controlling factor of nutrient uptake rather than the interstock.
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.
Apical meristems of four pears (Pyrus communis L. cv. Beurre Hardy, P. koehnei Schneider, P. cossonii Coss. and Dur., and a hybrid, P. dimorphophylla Makino × P. fauriei Schneider) were tested for their ability to survive immersion in liquid nitrogen. Plantlets were grown in vitro at 25C or cold-hardened for 1 week at – 1C before cooling at rates of 0.1, 0.3, 0.5, and 0.8 C/rein to –40C, followed by plunging the vials into liquid nitrogen. Vials were thawed for 1 min at 40C. A cryoprotectant mixture of polyethylene glycol, glucose, and dimethylsulfoxide (DMSO) was used. Regrowth of meristems ranged from 0% to 61% for plants grown at 25C and from 5% to 95% for cold-hardened plants. Cold-hardening significantly improved the recovery rates of all species tested. Survival rates increased as cooling rates decreased. Survival rates were not linked to the geographic origin of the species tested.
`Fifty-nine cultivars and wild seedlings of pear (Pyrus spp.) from Eastern Europe were evaluated for resistance to feeding by early instar pear psylla [Cacopsylla pyricola (Foerster)] in a 24-hour assay. `Bartlett' (P. communis L.) and NY 10352 (P. communis × P. ussuriensis Maxim. BC1 hybrid) were used as susceptible and resistant controls, respectively. A. high degree of resistance, measured as increased mortality and reduced frequency of feeding, was found in 11 plant introductions: `Erabasma' (PI 483370), `Krupan Burnusus' (PI 483387), `Topka' (PI 484489), `Zelinka' (PI 483393), `Mednik' (PI 483399), `Karamanlika' (PI 502165), `Katman' (PI 502172), `Smokvarka' (PI 502176), `Obican Vodenac' (PI 502177), a clone thought to be `Smiljerka' (PI 502178), and an unnamed seedling (PI 506382).
Several annual and perennial weed species were effectively controlled with 3-tert-butyl-5-chloro-6-methyluracil (terbacil) in orchards. One-year-old seedling rootstocks of peach, Prunus persica, (L.) Patsch, were most tolerant to terbacil; pear, Pyrus communis, L., and apple, Malus sylvestris, L., seedlings were intermediate; the East Malling (EM) VII clone, Mazzard, Prunus avium, L., and Mahaleb cherry, Prunus mahaleb, L., seedlings were most susceptible. Both surface and soil incorporated applications were toxic, indicating that terbacil was readily leached into the root zone. Applications were made in 2 and 6-year-old experimental blocks and in commercial orchards (age 2–15 years) from 1965 to 1968. No major damage was observed on apple, peach, tart cherry or sweet cherry trees that were established 3 years or longer. Toxicity symptoms manifested as veinal chlorosis were occasionally observed on sandy loam soils at rates 2–3 fold greater than required for satisfactory weed control.
Factors influencing the foliar penetration of naphthaleneacetamide (NAAm) were established by following penetration from a glass vial into pear leaf discs (Pyrus communis L. cv. Bartlett). Penetration through the upper surface was linear for 96 hr, whereas, through the lower surface there was rapid penetration for 48 hr followed by a reduced rate. Uptake of NAAm was proportional to the concentration applied. Penetration was not influenced by pH of treatment solutions ranging from 3.0 to 7.0. Increasing temperature from 5–35°C caused a marked increase in penetration with Q10 values ranging between 1.59 to 5.46. Increasing light intensity resulted in increased penetration through the lower surface up to about 300 ft-c, but had no effect on NAAm penetration through the upper surface. Penetration was greater through the upper than lower surface in expanding leaves, but the reverse was true when leaves were fully expanded. Tween 20 and Triton B-1956 (0.1%) increased NAAm penetration through the lower surface, but to a lesser degree than X-77 (0.1%). No surfactant studied enhanced penetration through the upper surface. Penetration from microdroplets was similar to that from solutions in glass cylinders until the droplets dried. Droplet drying resulted in an immediate increase in penetration.
Cold hardiness and cryogenic survival of micropropagated pear (Pyrus cordata Desv.) shoots were evaluated after pretreatments with ABA and sucrose. Shoot cold hardiness increased by 3 °C, and cryopreserved shoot tip growth increased by 17% after a 4-week 150 μm ABA pretreatment. Low temperature (LT) pretreatments improved the recovery of cryopreserved P. cordata shoot tips. Six to 10 weeks of LT were required for reaching high cryopreservation recovery. ABA and LT treatments produced significant synergistic effects on both cold hardiness and cryopreservation recovery. ABA shortened the LT requirement for high cryopreservation growth from 10 to 2 weeks. The optimal treatment for recovery of cryopreserved shoot tips was a 3 week culture on 50 μm ABA followed by 2 weeks of LT, while the maximum cold hardiness (-22.5 °C) was obtained with 150 μm ABA and 2-week LT. A 4 week culture on 150 μm ABA at 25 °C induced dormancy in 74% of shoot tips, but had little effect on cryopreservation growth unless combined with LT. Control and ABA-treated shoot tips, lateral buds, and leaves had similar cold hardiness (-10 to -12 °C), but LT and LT+ABA-treated shoot tips survived the lowest temperatures (-17 to -23 °C), lateral buds next (-15 to -20 °C), and finally leaves (-14 to -18 °C). An increase in the preculture-medium sucrose concentration from 2% to 7% combined with 2-week LT significantly increased cryopreserved shoot tip growth (0% to 75%) and decreased the LT50 from -7.8 to -12.4 °C. The optimal shoot pretreatment for successful recovery of cryopreserved P. cordata shoot tips was a 3 week culture on either 50 μm ABA or 5% to 7% sucrose medium followed by 2 weeks of LT, and increased shoot tip growth from zero to >70%. Chemical name used: abscisic acid (ABA).
Pear psyllids [ Cacopsylla pyri (L.), C. pyricola (Förster), and C. pyrisuga (Förster)] are major arthropod pests of pear ( Pyrus communis L.) throughout North America and Europe. Both adults and nymphs feed primarily in the vascular tissue of
Powdery mildew (PM) occurs worldwide and is prevalent on susceptible cultivars wherever pears are grown, causing economic losses due to russeted fruit and an increased need for fungicides. A core subset of the Pyrus germplasm collection at the USDA National Clonal Germplasm Repository in Corvallis, Ore., was evaluated for resistance to Podosphaera leucotricha, the causal agent of PM, using greenhouse and field inoculations of potted trees. The core collection consists of about 200 cultivars and species selections, representing most of the genetic diversity of pears and includes 31 Asian cultivars (ASN), 122 European cultivars (EUR), 9 EUR × ASN hybrids and 46 pear species selections. Three trees of each core accession were grafted on seedling rootstocks. In 2001–02, trees were artificially inoculated in a greenhouse, grown under conditions conducive for PM, and evaluated for symptoms. The same trees were subsequently evaluated for PM symptoms from natural field infections during 2003 and 2004. In the greenhouse, 95% of EUR and 38% of ASN were infected with PM. Average PM incidence (percent of leaves infected) in the greenhouse (8% for ASN and 30% for EUR) was much higher than incidence in the field (2% for ASN and 5% for EUR) during 2003. Symptoms were also more severe in the greenhouse, with 46% of ASN and 83% of EUR with PM symptoms having a mean PM incidence of >10%. In the field, 42% and 22% of EUR and 23% and 13% of ASN were infected with P. leucotricha in 2003 and 2004, respectively. Field infection was very low during both years, with percentage leaves infected in ASN and species selections significantly different from EUR. In the field, 6% of ASN with PM symptoms had a mean PM incidence >10% during both years, while 15% and 2% of EUR accessions with PM symptoms had a mean PM incidence >10% in 2003 and 2004 respectively. These results should be very useful to pear breeding programs to develop improved PM resistant cultivars in the future, by using accessions with consistent low PM ratings.