Fire blight, caused by the gram-negative bacterium Erwinia amylovora (Burrill) Winslow et al., is a dangerous disease of pome fruits, including pear. A pear breeding program for fire blight resistance was initiated in 2003 at the Department of Pomology, Warsaw University of Life Sciences, Poland. Since several Asian species are considered to be potential sources of resistance to fire blight, the susceptible Pyrus communis 'Doyenne du Comice' was crossed with the resistant P. ussuriensis. The F1 full-sib progeny composed of 155 seedlings was tested for susceptibility to fire blight by artificial shoot inoculation. A framework linkage map of both parents was constructed based on 48 AFLP and 32 SSR markers and covered a length of 595 cM and 680 cM in 'Doyenne du Comice' and P. ussuriensis, respectively. For the first time a putative QTL for fire blight resistance in P. ussuriensis linkage group 11 was identified. Another putative QTL in linkage group 4 of 'Doyenne du Comice' seems to indicate that sources of fire blight resistance can be identified also in the susceptible cultivars.
L'Aptien-Albien du Nord de l'Espagne est caractérisé par de grandes masses calcaires lenticulaires décamétriques à hectométriques constituées par des empilements de mud-mounds. Ces mud-mounds possèdent un noyau construit lithifié très précocément avec une charpente réticulée micritique et bacinellide (type thrombolite) attribuée à l'action d'algues bleu-vert et de bactéries. Les cavités interréticu-laires sont généralement complètement oblitérées par de la micrite claire d'origine également cyanobactérienne : les cavités stromatactoïdes sont rares. Les cristallisations et recristallisations sparitiques à l'intérieur du noyau peuvent être aussi d'origine microbienne.
Les grands mud-mounds se développaient selon la séquence suivante : 1-niveau bioclastique basai (faciès A) ; 2-noyau construit, en forme de dôme avec des fortes pentes (30 à 50°) ou de lentille sigmoïde, avec charpente réticulée micritique (faciès B) relayée latéralement par une charpente réticulée bacinellide (faciès C) puis par des faciès à la fois bioconstruits et accumulés (faciès D) ; 3-placage protecteur et couches de recouvrement. Ils se formaient sur de fortes pentes, en bordure des plates-formes à rudistes et orbitolines et face à des milieux de bassin plus profonds et ouverts. Leur implantation et leur développement se faisaient en domaine infralittoral sous la limite d'action des vagues mais l'édification se poursuivait jusque dans la zone intertidale (placage organique protecteur et couches de recouvrement) et pouvait subir des émersions temporaires (éogenèse d'eaux douces).
Several old cultivars, and breeding clones of European pear Pyrus communis L. originating from Belgium, England, Sweden, and Switzerland were evaluated for their resistance/susceptibility to fire blight. Studies were carried out during three consecutive years 2007–2009 in the greenhouse of Warsaw University of Life Sciences, Poland. Strain 691 of Erwinia amylovora was used for artificial infection of plants. Genotypes included in this study considerably varied in their resistance to fire blight. The most resistant was the old English cultivar ‘Hessle’. The other two genotypes, i.e., ‘Gränna Rödpäron’ originating from Sweden, and Pyrus communis FG 1606 from Switzerland were included in a group of low susceptible ones. The most susceptible were Cra Py H 18, Cra Py V 22 and Cra Py W 14 from Belgium.
Since several Asian pear species are considered to be potential source of fire blight resistance, we crossed 'Doyenné du Comice', the susceptible European cultivar, with four Asian pear species. The aim of the study was to establish the level of resistance of each genotype and the mode of transmission of fire blight resistance to each F 1 full-sib progeny. The best sources of resistance were P. ussuriensis 18 and P. ussuriensis var. ovoidea 8 ranked to resistant and highly resistant, respectively. Although pear resistance to fire blight is suggested to be polygenic, distribution of phenotypes in 'Doyenné du Comice' × P. ussuriensis var. ovoidea 8 hybrid family suggests the possibility of monogenic inheritance with the dominance of resistance derived from P. ussuriensis var. ovoidea 8. Polygenic inheritance of pear resistance to fire blight was identified in cross combinations of 'Doyenné du Comice' with P. pyrifolia 6, and contributed by the major gene, with P. ussuriensis 18 and P. calleryana 12. Transgressive segregation was observed within the progenies of susceptible, moderately susceptible and resistant parents.
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