Apple Progeny Research Articles

Texture analysis in an apple progeny through instrumental, sensory and histological phenotyping

Phenotypic analysis of texture traits was performed in an apple progeny by three complementary approaches: two classical instrumental measurements (compression and penetrometry), sensory assessment and histological screening. The progeny was composed of 141 individuals harvested over 2 years. Sensory and instrumental texture were assessed at harvest and after 2 and 4 months of cold storage. Histological screening was performed by combining macro-vision of outer parenchyma sections and image analysis on fruits after 2 months storage. Harvest year was observed to have a major impact on texture phenotypes followed by storage and genetic factors. Principal component analysis of data from the instrumental texture evaluations showed that the two methods complemented each other in characterizing the texture of the apple progeny. Compression parameters correlated better than penetrometry variables with sensory descriptors related to crispness, firmness, and graininess. Cell size distribution differentiated individuals in the apple progeny. It correlated with instrumental texture analyses and with juiciness perception. All measured texture related traits showed that they were all under genetic control with high heritability values. Higher values were obtained for fruits after 2 months storage. These results provide ground for future search of new apple texture QTLs.

Deciphering the genetic determinism of bud phenology in apple progenies: a new insight into chilling and heat requirement effects on flowering dates and positional candidate genes

The present study investigates the genetic determinism of bud phenological traits using two segregating F(1) apple (Malus × domestica) progenies. Phenological trait variability was dissected into genetic and climatic components using mixed linear modeling, and estimated best linear unbiased predictors were used for quantitative trait locus (QTL) detection. For flowering dates, year effects were decomposed into chilling and heat requirements based on a previously developed model. QTL analysis permitted the identification of two major and population-specific genomic regions on LG08 and LG09. Both 'chilling requirement' and 'heat requirement' periods influenced flowering dates, although their relative impact was dependent on the genetic background. Using the apple genome sequence data, putative candidate genes underlying one major QTL were investigated. Numerous key genes involved in cell cycle control were identified in clusters within the confidence interval of the major QTL on LG09. Our results contribute towards a better understanding of the interaction between QTLs and climatic conditions, and provide a basis for the identification of genes involved in bud growth resumption.

Genetic determinism of anatomical and hydraulic traits within an apple progeny

The apple tree is known to have an isohydric behaviour, maintaining rather constant leaf water potential in soil with low water status and/or under high evaporative demand. However, little is known on the xylem water transport from roots to leaves from the two perspectives of efficiency and safety, and on its genetic variability. We analysed 16 traits related to hydraulic efficiency and safety, and anatomical traits in apple stems, and the relationships between them. Most variables were found heritable, and we investigated the determinism underlying their genetic control through a quantitative trait loci (QTL) analysis on 90 genotypes from the same progeny. Principal component analysis (PCA) revealed that all traits related to efficiency, whether hydraulic conductivity, vessel number and area or wood area, were included in the first PC, whereas the second PC included the safety variables, thus confirming the absence of trade-off between these two sets of traits. Our results demonstrated that clustered variables were characterized by common genomic regions. Together with previous results on the same progeny, our study substantiated that hydraulic efficiency traits co-localized with traits identified for tree growth and fruit production.

Repeatability and number of growing seasons for the selection of custard apple progenies

This study aimed to estimate the repeatability coefficient and determine the minimum number of samples required for effective selection for yield of custard apple. Twenty progenies were evaluated in randomized blocks, five replications and four plants per plot. The fruits were collected, counted and weighed every two days of the year. Estimates of the repeatability coefficients were obtained by the methods of analysis of variance - ANOVA and principal components - PC. The estimates from the repeatability analysis of biennial data are higher than those based on individual years. The estimates of the PC method were accurate even in the first harvest, unlike ANOVA. Four biennia were sufficient to ensure effective progeny selection of custard apple.

The threat of wild habitat to scab resistant apple cultivars

Evaluations of plant resistance to pathogens are rarely made using isolates from wild habitats, although the heterogeneity of such habitats may generate pathogen diversity which could be a source of new virulence in cultivated habitats. The aim of this study was to investigate whether scab resistance factors, identified and characterized in apples using isolates of Venturia inaequalis from a cultivated habitat, remained effective against isolates from a wild habitat. Three V. inaequalis core collections originating from the cultivated apple Malus × domestica and from two wild species, M. sieversii and M. sylvestris, were established to maximize pathogen diversity. For each core collection, 10 isolates were inoculated in mixtures onto 51 genotypes from an apple progeny segregating for two qualitative resistance genes and six quantitative resistance loci (QRL). On each apple genotype, isolates that contributed to the scab symptoms were identified within the mixture using microsatellite markers. The most frequently detected isolates were inoculated singly to compare their aggressiveness according to their host origin. The results showed that isolates from a wild habitat were able to infect the susceptible apple genotypes. However, these isolates were never more aggressive than isolates from the cultivated habitat on the resistance factors tested. It can therefore be concluded that the resistance factors used in this study, identified with V. inaequalis isolates from a cultivated habitat, remained effective against isolates from M. sylvestris and M. sieversii.

JUVENILITY OF APPLE SEEDLINGS: CORRELATION BETWEEN TREE GROWTH HABIT AND FLOWERING INDEX ON DIFFERENT APPLE PROGENIES AT THE MIDDLE WEST OF SANTA CATARINA STATE - BRAZIL

Around the World many investigations have already been done to overcome the challenge of the long juvenile period of apple seedlings. To collaborate with apple breeders, a study has been done at Epagri / Cacador Experimental Station in order to investigate the correlation between some parameters of tree growth habit with juvenility phase on apple, expressed as flowering index - FI. After three and a half years from seed germination, a total of 1.287 pre-selections from eight apple progenies grafted on Marubakaido/M-9 inter-stem have been investigated. The following variables were evaluated: a) plant height - PH; b) length of the last annual growth of the leader - AGL; c) length of the internodes - IL; d) type of leader - TL. Only for TL a reasonable good correlation with FI was observed. There was significant negative correlation between length/thinness of the leader and intensity of blooming on all the eight apple progenies studied. The correlation values varied from R=0,27 for the progeny 'Imperatriz' × 'Fred Hough' to R=0,49 for 'Imperatriz' × M-46/94, 'Imperatriz' x 'Catarina' and 'Imperatriz' × 'Baronesa', with an average value of R=0,44. For all the other variables, the correlation was very poor, even not significant in most cases. For plant height, mentioned in the literature as a good parameter for selecting apple seedlings to shorten juvenile period, not only a very poor correlation with FI was observed, but even some negative correlation between both. The average for all progenies for this parameter was R=0,07 only, being not significant. For internodes length, even showing a negative correlation for all progenies as expected, the correlation values were highly variable. The highest negative correlation was observed for the progeny 'Imperatriz' × M-46/94 - R=-0,42 - and the poorest for the progeny M-72/90 × 'Gala' - R=-0,017. These results indicate that for apple progenies grafted on Marubakaido/M-9 inter-stem, the only criterion that offers a certain guarantee for pre-selections leading to a short juvenile period on apple seedlings seems to be the type of leader.

A study of scab resistance in 16 apple progenies using parents with partial scab resistance

A study was conducted to examine the inheritance of partial resistance to apple scab [Venturia inaequalis (Cke.) Wint.] using as parents a set of apple (Malus × domestica Borkh.) cultivars that show a range of field resistance to the disease, but are not known to carry any major-gene resistance. Akane, Alkmene, Antonovka, Carola, Chehalis, Goldgelbe, Katja and Summerred were used as pollen parents, and Spartan and an unnamed selection of Splendour × Gala parentage were used as seed parents in a factorial 2 × 8 mating design. One hundred seedlings of each family were planted in a randomized complete block design at a site with heavy annual natural scab infections. Each seedling was rated for infection severity for 3 consecutive years using a six-point categorical scale. A χ2 procedure was used to estimate the contribution of each parent to field resistance (analogous to general combining ability, GCA) and the contribution of the particular male-female combination (analogous to specific combining ability, SCA). Infection severity tended to increase over the 3 yr of the study. Antonovka and Chehalis had the highest analogous GCA for resistance (defined here as absence of sporulating lesions). Specific combining ability was present in 2 of 3 yr, but was small in magnitude relative to GCA. The field resistance of a given parent tended to be a poor predictor of its breeding value. However, 13 of the 16 families had at least 30% of seedlings with a potentially useful degree of partial resistance (low severity of infection). The results are discussed in the context of breeding apples with durable scab resistance.Key words: Fruit breeding, Malus, partial resistance, Venturia inaequalis

GENETIC CHARACTERIZATION AND MAPPING OF THE MAJOR POWDERY MILDEW RESISTANCE GENE PLBJ FROM MALUS BACCATA JACKII

Powdery mildew, caused by the obligate biotrophic ascomycete fungus Podosphaera leucotricha, is one of the main fungal diseases in commercial apple production. Only a few major resistance sources are currently used in apple breeding programmes to include mildew resistance. Some of them such as Pl1 from Malus robusta and Pl2 from Malus zumi have already been reported to be not fully active against the whole race spectrum of the apple powdery mildew fungus. New mildew resistance sources are needed that could extend the genetic basis for mildew resistance breeding. The mildew resistant Malus bacctata accession no. 419 (M. baccata var. jackii) was selected as crossing parent to produce an apple progeny segregating for resistance. Based on clearly resistant/susceptible individuals, an AFLP-based bulked segregant analysis (BSA) was performed. Several polymorphic AFLP markers were identified. Among them one marker co-segregated with the resistance phenotype. This AFLP fragment was cloned and a specific SCAR primer pair (called PLBJ-SCAR) was developed. By using an SSR-based genome-scan, the Plbj gene was located close to the SSRs CH02a10 and CH02c11 on the apple linkage group LG 10. For a verification of the monogenic inheritance of Plbj, as well as for testing the practical usefulness of the developed SCAR marker, a second population has been raised in 2004 from a cross between ‘Cox Orange’ and M. baccata 419. The correspondence between the mildew resistance phenotype and the PLBJ-SCAR is demonstrated also for this apple family.

EXPLORING THE GENETIC DETERMINISMS OF ARCHITECTURAL AND FUNCTIONAL TRAITS IN AN APPLE PROGENY

Traits related to tree architecture and tolerance to abiotic stress represent innovative targets for the selection of new cultivars in the contexts of integrated fruit production (IFP) and global climatic changes. Controlling tree form and leaf functioning is thus a desirable breeding objective, but efforts have been hampered by a poor knowledge of their determinisms in apple. In the last four years, the genetic basis of architectural and ecophysiological traits was investigated in a F1 apple progeny (‘Starkrimson’ × ‘Granny Smith’ cross) of 123 genotypes. A fine phenotyping was performed over the first 3 years of growth in the open field, collecting both topological and geometrical traits along the scaffold axes and at the whole tree level. After replication of the progeny, the leaf gas exchange responses to different air droughts were studied on 1-year-old plants in a controlled environment chamber, through five increasing leaf-to-air vapour pressure deficits (VPD, from 0.5 to 3 kPa). Stomatal conductance (gsw), net CO2 assimilation (An) and transpiration (E) rates were measured, and instantaneous water use efficiency (WUE, An / E) computed. All traits were analyzed through linear models and broad sense heritability values (h²b) were estimated. A quantitative trait loci (QTL) analysis was carried out using a consensus genetic map. For each trait, QTL detection was performed on the best linear unbiased predictor (BLUP) of genotypic values, computed from the linear model. For architectural traits, high h²b values were estimated and QTLs detected for many topological and geometric traits: (i) at tree scale, surface and volume; (ii) on the trunk, mean internode length and number of sylleptic and proleptic axillary shoots; (iii) on lateral shoots, orientation, mean internode length, and number of proleptic axillary shoots. Regarding ecophysiological traits h²b values were also high, indicating a good repeatability of ecophysiological measurements in controlled conditions. QTLs were detected for gsw and E suggesting a strong genetic control of these two traits. The putative agronomic interests and potential values for selection of the architectural and ecophysiological traits are discussed.

QTL mapping of aroma compounds analysed by headspace solid-phase microextraction gas chromatography in the apple progeny ‘Discovery’ × ‘Prima’

Improving fruit quality of apple varieties is an important but complex breeding goal. Flavour is among the key factors of apple fruit quality but in spite of the analytical and biochemical knowledge about volatiles little is known about the genetic and molecular bases of apple aroma. The aim of this study was to use a saturated molecular linkage map of apple to identify QTLs for aroma compounds such as alcohols, esters and terpenes, but also for a number of unidentified volatile compounds (non-targeted analysis approach). Two parental genetic maps were constructed for the apple cultivars ‘Discovery’ and ‘Prima’ by using mainly AFLP and SSR markers. ‘Discovery’ and ‘Prima’ showed very different volatile patterns, and ‘Discovery’ mostly had the higher volatile concentrations in comparison with the Vf-scab resistant ‘Prima’ which has its origin in the small-fruited apple species Malus floribunda. About 50 putative QTLs for a total of 27 different apple fruit volatiles were detected through interval mapping by using genotypic data of 150 F1 individuals of the mapping population ‘C3’ together with phenotypic data obtained by head-space solid phase microextraction gas chromatography. QTLs for volatile compounds putatively involved in apple aroma were found on 12 out of the 17 apple chromosomes, but they were not evenly dispersed. QTLs were mainly clustered on linkage groups LG 2, 3 and 9. In a first attempt, a LOX (lipoxygenase) candidate gene, putatively involved in volatile metabolism, was mapped on LG 9, genetically associated with a cluster of QTLs for ester-type volatiles. Implications for aroma breeding in apple are discussed.

Production costs and fruit yield profitability in the initial harvest of custard apple trees

The aim of this study was to estimate the production cost and economic indicators associated with the production and sales of fruits from 20 custard apple progenies during the initial five harvests, in order to identify the harvest season from which custard apple exploitation becomes profitable, as well as the most promising progenies from an economic point of view. The fruit yield data upon which the present work was based were obtained during the period from 2001 to 2005, in an experiment that evaluated 20 custard apple half-sibling progenies, under sprinkler irrigation. The progenies were evaluated in a random block design with five replicates and plots consisting of four plants each. The exploitation of custard apple progenies only showed to be a profitable agribusiness after the fourth year. Before that, only A3 and A4 progenies in the second year, and P3 and P11 in the third year provided profitable incomes. Considering the methodological assumptions imposed concerning the time period analysis and the prices as of July 2007, the most important profitability indicators (operating profit, return index and equilibrium price) evidenced that the A4 progeny is the most recommended, although other progenies are also highlighted, such as FJ1 and FJ2. As already discussed, the progenies showing the highest average yields of five harvests are not always the most economically recommendable ones.

Shaping the shoot: the relative contribution of cell number and cell shape to variations in internode length between parent and hybrid apple trees

Genetic control of plant size and shape is a promising perspective, particularly in fruit trees, in order to select desirable genotypes. A recent study on architectural traits in an apple progeny showed that internode length was a highly heritable character. However, few studies have been devoted to internode cellular patterning in dicotyledonous stems, and the interplay between the two elementary cell processes that contribute to their length, i.e. cell division and elongation, is not fully understood. The present study aimed at unravelling their contributions in the genetic variation of internode length in a selection of F(1) and parent genotypes of apple tree, by exploring the number of cells and cell shape within mature internodes belonging to the main axes. The results highlighted that both the variables were homogeneous in samples collected either along a sagital line or along the pith width, and suggest that cell lengthening was homogeneous during internode development. They allowed the total number of cells to be estimated on the internode scale and opened up new perspectives for simplifying tissue sampling procedures for further investigations. Differences in internode length were observed between the genotypes, in particular between the parents, and partly resulted from a compensation between cell number and cell length. However, genetic variations in internode length primarily involved the number of cells, while cell length was more secondary. These results argue for an interplay between cellular and organismal control of internode shape that may involve the rib meristem.

Development of an STS map of an interspecific progeny of Malus

Simple sequence repeat (SSR) markers developed from Malus, as well as Prunus, Pyrus and Sorbus, and some other sequence-tagged site (STS) loci were analysed in an interspecific F1 apple progeny from the cross ‘Fiesta’ × ‘Totem’ that segregated for several agronomic characters. A linkage map was constructed using 259 STS loci (247 SSRs, four SCARs and eight known-function genes) and five genes for agronomic traits—scab resistance (Vf), mildew resistance (Pl-2), columnar growth habit (Co), red tissues (Rt) and green flesh background colour (Gfc). Ninety SSR loci and three genes (ETR1, Rt and Gfc) were mapped for the first time in apple. The transferability of markers from other Maloideae to Malus was found to be around 44%. The loci are spread across 17 linkage groups, corresponding to the basic chromosome number of Malus and cover 1,208 cM, approximately 85% of the estimated length of the apple genome. Interestingly, we have extended the top of LG15 with eight markers covering 25 cM. The average map density is 4.7 cM per marker; however, marker density varies greatly between linkage groups, from 2.5 in LG14 to 8.9 in LG7, with some areas of the genome still in need of further STS markers for saturation.

Mapping of the apple powdery mildew resistance gene Pl1 and its genetic association with an NBS‐LRR candidate resistance gene

AbstractMolecular markers for the major apple powdery mildew resistance gene Pl1 were identified and are presently used in marker‐assisted selection in apple breeding. However, the precise map position of the Pl1 gene in the apple genome was not known. The objectives of this investigation were the identification of the Malus linkage group (LG) carrying the Pl1 locus, mapping of the resistance gene by simple sequence repeat (SSR) markers, and the analysis of genetic associations between the Pl1 gene and the numerous NBS‐LRR resistance gene candidates already mapped in the apple genome. A two‐step linkage mapping was used, based on two different apple families. The identification of LG 12 carrying Pl1 was performed indirectly by mapping the SCAR marker AT20 in an apple progeny for which there was a core genetic map but no mildew data available. Then, the position of Pl1 on LG 12 was determined by SSR markers in a second population which has been scored for mildew over 6 years in a greenhouse and in the field. The SSR Hi07f01, previously mapped on LG 12 [Tree Genet. Genomes, 2 (2006), 202] cosegregated with AT20 and was closely linked (∼1 cM) to the Pl1 gene. The TIR‐NBS‐LRR resistance gene analogue 15G11 mapped by the SSCP technique was also closely linked to the Pl1 resistance locus and might be a candidate for Pl1 itself, a second powdery mildew major resistance gene (Pld, [Theor. Appl. Genet., 110 (2004), 175]), or two scab resistance genes (Vg, [IOBC/WPRS Bull., 23 (2000), 245]; Vb, [Genome, 49 (2006), 1238]) which all seem to be located in a common R gene cluster at the distal end of apple LG 12.

Wide range QTL analysis for complex architectural traits in a 1-year-old apple progeny

The present study aimed at investigating the genetic determinisms of architectural traits in a 1-year-old apple (Malus x domestica Borkh.). F1 progeny. A precise phenotyping including both tree topology and geometry was performed on 123 offspring. For a wide range of developmental traits, broad-sense heritability was estimated and quantitative trait loci (QTLs) were investigated. Several loci controlling geometry were identified (i) for integrated traits, such as tree surface and volume; (ii) for traits related to the form of long sylleptic axillary shoots (LSAS), such as bending and basis angle; and (iii) for traits of finer components, such as internode length of the trunk and LSAS. Considering topology, 4 QTLs were mapped for the total number of sylleptic branching in the tree, suggesting a strong and complex genetic control that was analysed through colocalisations between QTLs mapped for the different shoot types (long, medium, short). Two QTLs were also mapped for a phenological trait (date of bud break). When several QTLs were detected for a trait, a linear model was built to test epistatic effects and estimate the whole percentage of variability explained. The discussion focuses on particular colocalisations and on the relevance of traits to further tree development.

Development of a multiallelic SCAR marker for the scab resistance gene Vr1/Vh4/Vx from R12740-7A apple and its utility for molecular breeding

A major scab resistance gene initially called Vr1 was identified in the apple cultivar “Regia” derived from the Malus scab resistance source R12740-7A (Russian seedling, RS). A codominant, multiallelic sequence characterized amplified region (SCAR) marker was developed from a random amplified polymorphic DNA marker identified by bulked-segregant analysis. Additional alleles of the AD13 marker locus proved to be informative for the analysis of genetic relationships within Malus including putative relatives of RS. Separate linkage maps were created for the two families derived from crosses with “Regia”. Using phenotypic data from the greenhouse scab tests, the recombination frequency between Vr1 and AD13-SCAR was between 6 and 17%. The Vr1 locus appeared to be closely linked to the Vx [Hemmat et al. J Am Soc Hortic Sci, 127:365–370, 2002], Vr2 [Patocchi et al. Theor Appl Genet, 109:1087–1092, 2004], and the Vh4 gene [Bus et al. Mol Breed, 15:103–116, 2005a]. Our linkage analysis of the molecular markers identified by Hemmat et al. [J Am Soc Hortic Sci, 127:365–370, 2002] for two scab resistance factors from RS (Vr and Vx) indicate that both genes are separated by a large distance on apple linkage group 2 [Boudichevskaia et al. Acta Hortic, 663:171–175, 2004]. This is in agreement with the results of Bus et al., [Mol Breed, 15:103–116, 2005a] who concluded that (1) the RS-derived gene Vh2 is identical to Vr, (2) the RS-derived gene Vh4 is identical to Vx and Vr1, (3) Vh2/Vr and Vh4/Vr1/Vx map on opposite sides of LG 2. One of our main goals was the verification of the Vr1-SCAR within a practical apple-breeding program. The utility of the AD13-SCAR was evident after 2 years under natural scab infection conditions in both families investigated. This is the first report about the confirmation of a molecular marker for a RS resistance factor in a 2-year field experiment. A multiplex polymerase chain reaction assay based on two codominant SCARs for Vf and Vr1 was tested in an apple progeny segregating for both genes. The result of the two-marker approach is discussed with respect to scab races, which are able to overcome the Vf resistance gene.

Phenotyping progenies for complex architectural traits: a strategy for 1-year-old apple trees (Malus x domestica Borkh.)

The aim of this study was to define a methodology for describing architectural traits in a quantitative way on tree descendants. Our strategy was to collect traits related to both tree structural organization, resulting from growth and branching, and tree form and then to select among these traits relevant descriptors on the basis of their genetic parameters. Because the complexity of tree architecture increases with tree age, we chose to describe the trees in the early stages of development. The study was carried out on a 1-year-old apple progeny derived from two parent cultivars with contrasted architecture. A large number of variables were collected at different positions and scales within the trees. Broad-sense heritability and genetic correlations were estimated and the within tree variability was analyzed for variables measured on long sylleptic axillary shoots (LSAS). These results were combined to select heritable and not correlated variables. Finally, the selection of variables proposed combines topological with geometric traits measured on both trunks and LSAS: (1) on the trunk, mean internode length, and number of sylleptic axillary shoots; (2) on axillary shoots, conicity, bending, and number of sylleptic axillary shoots born at order 3. The trees of the progeny were partitioned on the basis of these variables. The putative agronomic interest of the selected variables with respect to the subsequent tree development is discussed.

The PRI Apple Breeding Program

and initiated an intensive study of crosses between cultivated apples and crab-like forms. A summary of Crandall’s work, published in University of Illinois Bulletin 275 (Crandall, 1926), describes all the material in the program. Crandall, infl uenced by the rediscovery of Mendel, was attempting to determine inheritance patterns in Malus. Ironically, he did not work with characteristics that would enable him to identify single genes or evaluate his material for disease resistance. The voluminous data he collected and maintained at the University of Illinois has never been completely analyzed, although the designations reported in Bulletin 275 describing his species sources have become justly famous. The selection he labeled Malus fl oribunda 821 was the original source of the V f gene. Fortunately, his breeding material was maintained following his retirement in 1927, but remained unevaluated until L.F. Hough joined the department in 1942 as a graduate student and plant-breeding assistant. Spring 1943 was unusually cool and wet, resulting in a severe epidemic of scab that defoliated all susceptible unsprayed apples trees. Hough noted that one progeny, an F 2 of M. fl oribunda 821 × Rome Beauty, segregated at a ratio of 1 resistant : 1 susceptible, suggesting the involvement of a single gene, later termed V f . Hough published his results with the Proceedings of the American Society of Horticultural Science in his famous paper “A Survey of the Scab Resistance of the Foliage of Seedlings in Selected Apple Progenies” (Hough, 1944). The paper was brought to the attention of J.R. Shay, a young professor who joined the Purdue’s Department of Botany and Plant Pathology from Arkansas and who had received his PhD from the University of Wisconsin where a program on the pathogenicity of Venturia inaequalis had been under investigation. (In the l940s, considerable interest in the genetics of the ascomycetes had been engendered by the work of Beadle and Tatum in Neurospora.) Shay and Hough decided to collaborate on a scab-resistance breeding program with the objective to advance genetic resistance identifi ed by Hough into improved horticultural backgrounds and identifi cation of additional resistant Malus germplasm through testing of material in arboreta and other collections. The fi rst screened hybrid progenies were planted in the fi eld in 1947 and began fruiting at Illinois in l951. A formal cooperative program was established in 1945 between the agricultural experiment stations associated with Purdue University and the University of Illinois.

The inheritance and durability of scab resistance in apple progenies

In order to select the appropriate parent cultivars and maintain the durability of resistance, it is important to clarify the mechanisms of inheritance of scab resistance depending on the parents. It has been known that the progeny segregation ratios based on scab-resistance do not depend only on the genotype of the resistance locus but also on the genetical makeup of the donor and recipient parents as well as on the susceptible parent.
 The aim of this study has been to demonstrate what factors in the Vf, Vr and VA scab-resistant cultivars — combined with susceptible and resistant parents — affect the inheritance and durability of resistance in seedlings in their first 4 year's growing four years' growth. After inoculating apple seedlings sown in 2001 with the suspension of Venturia inaequalis (Cke.) Wint. in the greenhouse, we studied the segregation ratios of the progenies into reaction classes. Seedlings showing resistance in the greenhouse were also evaluated for scab-resistance after they had been moved to the field and had naturally been infected with the pathogen in 2002 and 2004.
 The majority of our results obtained in the greenhouse test, similarly to earlier experiences, have not justified monogenic inheritance at the phenotypic level. The effect of susceptible parent cultivars on the segregation ratio of progenies have become apparent again. The high infection rate of seedlings in the field trials, which had previously exhibited varying degrees of resistance in the greenhouse test, has raised concern. Our data has raised further doubts, concerning the durability of Vf resistance in Hungary. It is assumed that the composition of natural field populations of Venturia inaequalis in Szigetcsép has changed. The complexity of Vf resistance has been confirmed. The high infection rate in the progenies derived from Vf resistant cultivars draws the attention to the importance of utilizing additional sources of resistance.

Resistance gene analogues identified through the NBS-profiling method map close to major genes and QTL for disease resistance in apple

We used a new method called nucleotide-binding site (NBS) profiling to identify and map resistance gene analogues (RGAs) in apple. This method simultaneously allows the amplification and the mapping of genetic markers anchored in the conserved NBS-encoding domain of plant disease resistance genes. Ninety-four individuals belonging to an F1 progeny derived from a cross between the apple cultivars 'Discovery' and 'TN10-8' were studied. Two degenerate primers designed from the highly conserved P-loop motif within the NBS domain were used together with adapter primers. Forty-three markers generated with NBS profiling could be mapped in this progeny. After sequencing, 23 markers were identified as RGAs, based on their homologies with known resistance genes or NBS/leucine-rich-repeat-like genes. Markers were mapped on 10 of the 17 linkage groups of the apple genetic map used. Most of these markers were organized in clusters. Twenty-five markers mapped close to major genes or quantitative trait loci for resistance to scab and mildew previously identified in different apple progenies. Several markers could become efficient tools for marker-assisted selection once converted into breeder-friendly markers. This study demonstrates the efficiency of the NBS-profiling method for generating RGA markers for resistance loci in apple.