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Premature leaf fall disease is caused by the hemibiotrophic ascomycete Diplocarpon mali (Y. Harada & Sawamura 1974) (anamorph Marssonina coronaria). Breeding for disease resistance and development of resistant cultivars could help to defeat the disease in the future. The first step in resistance breeding is to assess Malus genetic resources for resistance to M. coronaria. The aim was to proof a detached leaf assay as phenotyping procedure. Disease assessment was performed after 14 days post inoculation (dpi). Different symptoms obtained from the inoculated genotypes indicate a high level of susceptibility in Malus cultivars. In contrast, M. baccata accession MAL0419 was tolerant to M. coronaria. Future experiments will show whether the tolerance of M. baccata accession MAL0419 is inherited and if the genotype is a possible donor for resistance breeding toward M. coronaria.

Fire blight is a common and very destructive disease affecting apple (Malus ×domestica) and pear (Pyrus communis) productions. Caused by enterobacterium Erwinia amylovora, its devastating economic effects ranges from losses in yield to severe damage or death of trees in the orchard, and in extreme cases loss of a whole orchard. The most effective management measure available to producers is the use of antibiotics. However, dependence on antibiotics treatment is not sustainable given risks such as the rapid selection of antibiotic-resistant Erwinia populations and environmental hazards, which have led to their application being forbidden in many countries. Therefore the planting of fire blight-resistant cultivars seems to be the most promising alternative strategy. However until now, there is no fire blight-resistant apple cultivar produced for the global market. Host resistance to fire blight in Malus is thought to be quantitatively controlled. Thus, several quantitative trait loci for resistance have been identified in some accessions of Malus spp. In this study, we explored a segregating population derived from a cross between the apple wild species Malus fusca and the Malus ×domestica cultivar ‘Idared’. F1 progenies used for mapping were artificially inoculated with E. amylovora strain Ea222 at a concentration of 109 CFU/ml in two different years. The averages of percentage lesion length (PLL) of all replicates of each genotype were used as numerical traits for statistical analysis. A Kruskal-Wallis analysis, used to determine marker-phenotype association, revealed two DArT markers (971000 and 970840) with the highest significance. QTL mapping revealed a strong QTL explaining above 60% of the phenotypic variation. This is the first report of a fire blight resistance QTL in M. fusca.

After fire blight inoculation, shoots of apple and pear formed biphenyls and dibenzofurans as defense compounds. The phytoalexins were only present in the transition zone of stems, whereas leaves were devoid of these compounds. In shoots of the apple cultivar ‘Holsteiner Cox’, four biphenyls and two dibenzofurans accumulated in the downward advancing transition zone six weeks after Erwinia amylovora inoculation. In the pear cultivar ‘Conference’, three biphenyls and one dibenzofuran were detected. The total phytoalexin content in the transition zone of pear was much lower than that in apple. In addition to ‘Conference’, two other pear cultivars, ‘Alexander Lucas’ and ‘Harrow Sweet’ were investigated for their phytoalexin formation in response to fire blight inoculation. Differences in the morphological changes as well as in the biphenyl and dibenzofuran concentration and pattern were observed between these three genotypes. 'Harrow Sweet', classified as most resistant to fire blight, showed the highest phytoalexin concentration when compared to ‘Alexander Lucas’ and ‘Conference’ and its transition zone stopped the downward migration 12 days after inoculation. Five biphenyls and one dibenzofuran were detectable in this pear cultivar.

To date the use of natural resistance to manage fire blight epidemics in apple orchards has been limited by the reduced availability of such traits in commercial varieties as well as the very poor fruit quality of resistant wild apple genotypes. Such natural resistance offers great environmental advantages compared to any other disease control methods i.e. less treatments with chemicals and less tractor rides. We undertook the positional cloning of the fire blight resistance gene located on the linkage group 3 of Malus ×robusta 5 (Mr5), a wild apple genotype immune to European strains of Erwinia amylovora. A single candidate gene (FB_MR5) was identified and validated by a transgenic approach, transforming the fire blight susceptible cultivar ‘Gala’. This represents an unprecedented opportunity to deploy Malus-own resistance by cisgenics, similarly to what we recently reported for the scab resistance gene Rvi6, showing that such GM product may represent an effective and sustainable approach in fire blight management. Current state of the research will be presented.

Malus × robusta 5 (Mr5) is a wild apple accession highly resistant to fire blight caused by the bacterium Erwinia amylovora. Only few strains of E. amylovora are able to overcome the resistance, including the avrRpt2EA deletion mutant ZYRKD3-1. The related wild type strain Ea1189, expressing the avrRpt2EA is not able to break resistance of Mr5. The differential transcriptome comparison of Mr5 after inoculation with these two strains gives the opportunity to analyze the host-pathogen interaction of Erwinia amylovora and Mr5 in a system differing only in one single gene: the avrRpt2EA effector gene.

Fire blight, caused by enterobacterium Erwinia amylovora, is the most important bacterial disease affecting pome fruit. It can cause devastating economic losses and is reliably controlled only by the application of antibiotics, which are banned in many European countries due to environmental and sustainability issues and consumers’ mistrust. One solution could be the utilization of fire blight resistant cultivars in apple production. In 2003, we started an approach at Dresden-Pillnitz to detect different mechanisms conferring resistance to fire blight aimed at their com-bination in new cultivars. Four segregating populations were established to map QTLs for fire blight resistance. The donors used were three wild species accessions of Malus baccata, M. fusca, M. ×robusta 5 and the Pillnitz cultivar ‘Rewena’. The susceptible parent in each case was Idared. Grafted scions of each progeny were inoculated with E. amylovora strain Ea222_JKI. Average percent lesion length (PLL) of all progenies was determined. Genetic linkage maps were established using DArT- SCAR-, SNP-, and SSR-markers. Whereas in Rewena no QTL could be determined, major QTLs were detected in M. baccata on linkage group 12, in M. fusca on linkage group 10, and in M. ×robusta 5 on linkage group 3 explaining up to around 50, 85 and 85% of the phenotypic variance, respectively. The fact that all resistance QTLs are located on different linkage groups enhances the chance that different mechanisms are acting in the donors. Additionally, trees of the Idared by M. ×robusta 5 population were planted in an orchard and flowers were inoculated for phenotyping. The QTL on linkage group 3 could be con-firmed after mapping.

In Germany, all activities of fruit genetic resources management are based on the German National Program for Genetic Resources of Agricultural and Horticultural Plants. Three areas of actions, like ex situ conservation in field collections and building safety back-ups, characterization of the material, and utilization of genetic resources in breeding are described for fruit species collected at Dresden-Pillnitz.

Apple Malus × domestica Borkh. has a long period of juvenility during which plants are unable to produce flowers and fruits. Flowering is only possible on trees which have passed the transition from juvenility to the adult stage. Several genes have been identified which control this switch and promote or delay flowering in different plant species. FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1) are two flowering genes of Arabidopsis which are similar, but act antagonistically. FT and TFL1 homologous genes are present in the apple genome, but little is known about their function. Therefore we produced transgenic apple lines which over-expressed the MdFT2 gene under the control of the constitutive CaMV 35S promoter. Transgenic shoots expressing the MdFT2 gene driven by the CaMV 35S promoter flowered in vitro. To study the function of MdTFL1 we silenced the gene by expressing an MdTFL1 specific hairpin under the control of the CaMV 35S promoter. Transgenic plants of different apple cultivars flowered in vitro and later in the greenhouse. Transgenic flowers were evaluated for phenotypic differences and tested on their usability for crossbred breeding programs.

Fire blight is one of the most damaging diseases in the majority of apple (Malus x domestica) producing countries worldwide. The region of the quantitative trait for resistance to Erwinia amylovora and the same data mapped as single gene in the wild apple Malus x robusta 5 was enriched with additional molecular markers. Generated phenotypic data of individuals that carry a crossing over in the region of interest allowed to position the locus coding for resistance into a short section at the top of linkage group 3 flanked by molecular markers.