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Abstract

During the early 1900s, several plant exploration trips to the Orient were made to collect Pyrus species from Japan, China, Korea, and Manchuria to search for resistance to fire blight [Erwinia amylovora (Burr.) Winsl. et al.]. Following numerous inoculations in various tree tissues, Reimer (15) reported the four most important species, ranked in descending order of blight resistance, as follows: P. ussuriensis Maxim, P. calleryana Deche, P. betulaefolia Bunge, and P. pyrifolia (Burn.) f. Nak. Pyrus ussuriensis and P. pyrifolia were used in earlier breeding programs and are the parent species of many oriental cultivars. However, their textures and flavors have not been accepted by most Americans. P. calleryana also has been used in the USD A pear breeding program, and selections of the 3rd backcross generation appear useful only for resistance.

Conditions were established for inducing pear (Pyrus communis L.) blossom blast caused by Pseudomonas syringae (Ps) on detached shoots. Highest incidence of infection followed occurrence of a major exotherm in the presence of Ps suspended in water drops on blossom tissue. Eight pear cultivars were evaluated for susceptibility to blossom blast, with the red-fruited `Beurré d'Anjou' sports `Gebhart' and `Columbia' least susceptible and `Doyenné du Cornice', `Beurré d'Anjou', and `Beurré Bosc' most susceptible.

Explants of three rootstock selections Pyrus calleryana Dcne `Oregon Pear Rootstock (OPR) 157', P. betulifolia Bunge `OPR 260', and P. communis L. `Old Home' × `Farmingdale 230' (`OH × F 230') were initiated from forced branches of field-grown trees. `OPR 260' and `OH × F 230' shoots cultured on Cheng medium with IBA proliferated better than those on NAA. NAA and IBA at concentrations >0.5 μm inhibited shoot multiplication. Overall, the best micropropagation medium for `OPR 260' and `OH × F 230' was Cheng medium with 8 μm BA and 0.5 μm IBA. Shoot multiplication of `OPR 157' was best on 8 μm BA and better on low NAA (0.5 μm) or no auxin than on IBA. `OH × F 230' rooted easily (>80%) with all IBA and NAA treatments. The best rooting treatment (42.9%) for `OPR 260' was 10 μm IBA in darkness for 1 week; for `OPR 157' (23.9%), it was a 15-second dip in 10 mm NAA. Only rooted plantlets survived 4 weeks of greenhouse acclimatization. Chemical names used: N⁶-benzyladenine (BA); indole-3-butyric acid (IBA); napthaleneacetic acid (NAA).

Abstract

The genus Pyrus has been classified into at least 22 primary species. These can be grouped by geographical distribution and/or taxonomic relationships. The European group includes P. communis L., P. nivalis Jacq., and P. cordata, (Desv.) Schneid. The North African group contains P. longipes Coss. and Dur., P. gharbiana Trab., and P. mamorensis Trab. The west Asian group consists of P. syriaca Boiss., P. elaeagrifolia Pall., and P. amygdaliformis Vill., P. salicifolia Pall., P. glabra Boiss., P. regellii Rehd., (syn. P. bucharica and P. heterophylla Reg. & Schmalh). The medium to large fruited east Asian species are P. pyrifolia (Burm.) Nak., P. kansuensis, P. ussuriensis Max., P. hondoensis Kik. and Nak., while the Asian “pea” pear species are P. calleryana Dcne., P. betulaefolia Bung., P.fauriei Schneid., P. dimorphophylla Makino, and P. koehnei Schneid. (3, 27). A number of nonprimary species also appear in the literature, which may be either botanical varieties, subspecies, or interspecific hybrids. Among the east Asian group, P. bretschneideri Rehd. is a probable hybrid of P. betulaefolia and the cultivated forms of P. pyrifolia; P. phaeocarpa Rehd. may be a P. betulaefolia × P. ussurensis hybrid, whereas P. serrulata Rehd. is a probable interspecific hybrid involving P. pyrifolia and P. calleryana.

Abstract

Respiration of flower-buds of Pyrus communis L., a late blooming species, and P. calleryana, an early blooming species, was investigated throughout the winter. Respiration of P. calleryana Decne at 5°C was twice as high as that of P. communis, whereas the respiration rates were similar at 25°. A large portion (60–70%) of the respiration at 5° was cyanide resistant in P. calleryana and much less in P. communis. The combination of inhibitors, cyanide (KCN) and salicylhydroxamic acid (SHAM), still only partially inhibited respiration. The residual respiration was much higher for P. calleryana than for P. communis. The nature of the residual respiration is not known.