Twenty-one pear cultivars and breeders’ selections with interspecific pedigrees involving Pyrus ussuriensis Max. or P. pyrifolia (Burm.) Nakai crossed with P. communis were assessed for resistance to pear psylla (Cacopsylla pyricola Förster) using a nymphal feeding antixenosis assay. The proportion of nymphs live and present on the plants varied from 0.52 for Purdue 77-73, a P. ussuriensis × P. communis hybrid, to 0.08 for ‘Zelinka’, a P. communis cultivar from eastern Europe. Two P. ussuriensis × P. communis hybrid selections (NY 10355 and NY 10359) and the P. communis landrace cultivars, Batjarka and Zelinka, were the most resistant to nymphal feeding. NJ B9 T1 T117 may also be a useful source of resistance. The most resistant germplasm should be valuable genetic resources for the breeding of new pear cultivars with resistance to pear psylla.
Richard L. Bell
Pear psylla are serious pests of pear throughout North America and Europe. Inhibition of nymphal feeding has been identified as a key component of resistance, being correlated with inhibition of oviposition, mortality, and delayed development. Breeding for resistance would be aided by knowledge of the inheritance and relative breeding values of resistant parents. Nine seedling populations derived from crosses among five resistant European P. communis cultivars (‘Batjarka’, ‘Erabasma’, ‘Ilinjaca’, ‘Spina Carpi’, and ‘Zelinka’), a clone labeled ‘Obican Vodenac’, and two susceptible P. communis parents (‘Bartlett’ and ‘Sunrise’) were used in this study. The seedlings, ‘Bartlett’ as a susceptible control and NY10353 (a P. communis × P. ussuriensis backcross hybrid) as a moderately resistant control, were assayed for short-term survival and nymphal feeding after an infestation period of 2 days. There were no significant differences among the progenies and controls in mean proportion of live nymphs after 2 days. The distributions of live and feeding nymphs indicated some dominance for susceptibility in some crosses. There were few significant differences among crosses in the proportion of actively feeding nymphs; however, the two crosses involving ‘Erabasma’ exhibited significantly less feeding, indicating that ‘Erabasma’ transmitted resistance to nymphal feeding to its progeny to a greater degree than the other four resistant parents.
Richard L. Bell
Pear psyllids (Cacopsylla spp., Hemiptera: Psyllidae) are major pests of European pear (Pyrus communis L.) in North America and Europe. In breeding resistant cultivars, a thorough understanding of the nature of host resistance and susceptibility is essential for designing efficient and accurate methods to identify resistant host genotypes. A developmental assay was performed on six genotypes previously identified as highly resistant, moderately resistant, or susceptible to nymphal feeding. Plants were infested with first instars of Cacopsylla pyricola (Förster), and feeding, mortality, and larval stage were observed for 34 days. Host genotypes significantly differed in feeding antixenosis at day 1 after infestation. Differences in antibiosis were also evident, as mortality of nymphs was greater on the feeding-resistant genotypes. There were also differences in the total numbers of surviving psylla and those developing to adults. Survival was directly related to the feeding antixenosis. There were no significant differences among the feeding-susceptible and moderately resistant genotypes in population age structure, except at day 19, reflecting the advanced stage of the few nymphs that did survive. Early feeding antixenosis was associated with mortality, either through a lack of feeding stimulation, feeding inhibition, or because of a direct toxic effect of the resistant plant genotypes. The short-term feeding antixenosis assay is predictive of long-term mortality and, to a lesser extent, with delayed development. ‘Bartlett’ and the wild seedling Y-167 were confirmed as susceptible, ‘Karamanlika’ and NY10352 were confirmed as moderately resistant, and Bacui-1 and Bacui-2 were confirmed as highly resistant.
Richard L. Bell
A collection of 60 genotypes-of pear (Pyrus sp.) from East Europe were assayed for ovipositional antixenosis in no-choice cage tests. Five single plant replicates of each genotype, 'Bartlett' (P. communis L.), a susceptible and NY10352 (P. ussuriensis × P. communis BC1 hybrid)' as a resistant control, were stripped to the 6 youngest, but fully expanded leaves, and enclosed in single-plant cages. Four mating pairs of summer-morph adult pear psylla were introduced into each cage. The number of eggs laid on each plant was counted after 5 days. Egg hatch was determined 7 days later, and leaf area was measured. Twelve genotypes, 8 of which are P. communis, were found to be highly resistant to oviposition. These same genotypes had previously been found to exhibit high levels of nymphal feeding antixenosis and antibiosis. The results will be discussed in light of ovipositional cues and host acceptance-related behavior of the pear psylla.
Richard L. Bell
Sixteen interspecific backcross hybrid selections from various breeding programs have been selected as prospective parents for breeding for resistance of European-type pears to the pear psyllids (Cacopsylla spp.). The Pyrus communis × P. pyrifolia (n = 6) backcross selections are derived mostly from NJ 1, an open-pollinated P. pyrifolia seedling, and the Pyrus communis × P. ussuriensis (n = 9) backcross selections are derived from Illinois 76, an open-pollinated P. ussuriensis seedling, and one Pyrus communis × P. ussuriensis cultivar. Ratings of psylla resistance have been based primarily on multiyear orchard observations under no-pesticide and minimal pesticide conditions. To select the best prospective parents, data on fruit quality and tree traits were analyzed. Fruit characteristics included harvest date, fruit size and shape, skin color, percentage blush, russet, overall appearance, texture (flesh fineness), texture type, juiciness, overall grittiness and grit size, flavor acceptability and type, aroma, and a quality index, which was an unweighted total of the scores for appearance, texture, grit, flavor, and aroma. For this report, comparisons were made to ‘Bartlett’, the most widely grown U.S. pear cultivar. Both the P. communis × P. pyrifolia and Pyrus communis × P. ussuriensis backcross hybrid groups had significantly lower quality indices than ‘Bartlett’, and most individual traits were similar in this respect. There were significant differences among selections for all traits as were differences between years within genotype for most traits with some exceptions. Harvest date, percentage blush, appearance, juiciness, flavor, and the quality index were relatively stable from year to year. Flesh texture type varied within each group. The P. communis × P. pyrifolia selection NJ Rock R23 T252 had the highest quality index of the selections. For eight traits, various selections ranked higher than ‘Bartlett’, although the differences were not significantly higher with the exception of the russet score. Five selections appear to have sufficient quality and are being used as parents.
Richard L. Bell
Preconditioning effects of cytokinin in the shoot proliferation medium on explant quality and subsequent adventitious regeneration of `Bartlett' and `Beurre Bosc' pear were investigated. The basal medium for regeneration consisted of half-strength MS macro- and micronutrients, MS organics, 30 g·liter–1 sucrose, 6 g·liter–1 agar, and 10 μM thidiazuron (TDZ), and 1 μM NAA. Leaves from BAP medium were more effective than those from media with 2-iP or kinetin in spite of the increased leaf size of shoots cultured with 2-iP (28% vs. 10%). Use of leaves from in vitro-rooted shoots did not increase regeneration frequency (19.5% vs. 31%) of `Bartlett'. Actively expanding leaves are more suitable explants than larger, fully expanded leaves. Liquid medium overlays and incubation in liquid medium decreased regeneration frequency when compared with agar-solidified medium. Among auxins in regeneration induction phase media, IBA (0.5 or 1.0 μM) resulted in greater regeneration than NAA.
Richard L. Bell
Discs of cambial tissue were excised from actively growing shoots of `Bartlett' pear, and explanted directly on regeneration induction media. The basal medium was 1/2 strength MS macro-nutrients, MS micro-nutrients and organics, 8 g/l agar, and 30 g/l sucrose. Phytohormone treatments consisted of a factorial design of NAA (0 and 5μM) and TDZ (1, 2, 3, 4, and 5μM). After 4 weeks incubation in the dark, the explants were transferred to auxin-free media with identical concentrations of TDZ. There was an absolute requirement for auxin in the induction medium, as all discs on auxin-free initial media died without callusing. Maximum shoot regeneration 4 weeks after transfer to expression media was obtained with an initial medium containing 5μM NAA and 3μM TDZ, from which 30% of the explants produced one or more adventitious shoots. This rate of regeneration is similar to that obtained in some experiments with in vitro leaf explants, and provides an alternative system for regeneration of pear.
Richard L. Bell and Richard H. Zimmerman
Combining ability for transmission of juvenile period duration was studied in a large pear breeding population. The 92 parents, consisting of cultivars and selections of Pyrus communis L. and its interspecific hybrids with P. pyrifolia (Burm.) Nakai and P. ussuriensis Maxim., as well as genotypes of P. calleryana Decne., were crossed in 298 combinations. General combining ability was highly significant and of much larger magnitude than specific combining ability, indicating that juvenile period length was under predominantly additive genetic control. Selection of parents based on their juvenile period or their combining ability constants is likely to result in significant reduction in mean juvenile period.
Richard L. Bell and L. Claire Stuart
Four genotypes of pear (Pyrus spp.) of East European origin, a susceptible control, `Bartlett' (P. communis L.), and a moderately resistant control, NY 10352 (P. ussuriensis Maxim. × P. communis B C1 hybrid), were artificially infested with pear psylla (Cacopsyll a pyricol a Foerster) nymphs in the laboratory. Ten neonate first instars were placed on each of the two youngest leaves of four small trees per genotype. On PI 506381 and PI 506382, wild seedlings of P. nivalis Jacq., all nymphs died within 5 days. Mortality and development of nymphs on PI 502173, a wild P. communis seedling, was similar to that observed on `Bartlett', with 43% and 45% of the nymphs surviving to adulthood, respectively. On `Karamanlika' (PI 502165) and NY 10352, 15% of the nymphs developed into adults. Increased mortality and delayed development of nymphs was associated with feeding inhibition. The mode of host plant resistance to pear psylla nymphs in these accessions of East European pear is, therefore, similar to that previously characterized for NY 10352, in which the resistance is derived from germplasm of East Asian origin.
Richard L. Bell and Tom van der Zwet
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.