Screening for resistance to Nectria galligena Bres. in cut shoots of apple

Summary To find a fast and reliable test to assess resistance to Nectria galligena in apple, different methods of inoculation were compared using macroconidia of N. galligena and one-year-old cut shoots from mature trees of Cox's Orange Pippin, IVT 69078-19, James Grieve and Jonathan.With the best inoculation method 11 genotypes were screened for resistance. Elstar, Golden Delicious, Jonathan and Lombart's Calville were highly resistant and the level of resistance of Ingrid marie, Gloster, Melrose, IVT 69078-19, Cox's Orange Pippin, James Grieve and Idared decreased in this order.The best inoculation method proved to be simple, giving results within four to nine weeks after inoculation.     

Screening and identification of resistance to Leptosphaeria maculans (stem canker) in Brassica napus accessions

The occurrence of race-specific resistance genes to the stem canker fungus, Leptosphaeria maculans, was analysed in 453 accessions of B. napus, mainly originating from the Institut für Pflanzengenetik und Kulturpflanzenforschung (IPK) GeneBank. Major resistance genes Rlm1, Rlm2, Rlm4 and the putative RlmBBA gene were investigated using genetically improved strains of the fungus harbouring as few corresponding avirulence genes as possible. In addition, a screening with fully virulent isolates was used to uncover novel resistance sources. Major resistance genes were rarer in frequency and diversity in spring-type cultivars compared to winter types. In the former, 65.7% of the accessions were fully susceptible to all isolates, whereas only 12.2% of the winter types were devoid of at least one R gene. In spring cultivars, the most common R gene, Rlm4 was found in 26.6% of accessions, whereas the other R genes were rare. In winter cultivars, the most common R genes were Rlm2 (more than 45.9–54.0% of the accessions) and Rlm4 (26.4–27.7% of the genotypes). In winter types however, the improvement of the quality of oils, through the generation of single- and double-low genotypes improved the homogeneity of the cvs, whereas it impoverished R gene diversity, including the loss of complete resistance that was harboured by 18.4% of the less advanced accessions, and a reduction in the ratio of accessions harbouring Rlm1. Correlation between the R gene(s) present in the accessions and their field resistance is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The V f gene for scab resistance in apple is linked to sub-lethal genes

Summary V _f is the most widely used resistance gene in the breeding for scab resistant apple cultivars. Distorted segregation ratios for V _f -resistance have frequently been reported. Here we revealed that sub-lethal genes caused the distorted segregation. The inheritance of V _f was examined in six progenies by testing linked molecular markers. Three progenies showed distorted segregations that could be explained by three sub-lethal genes (sl1, sl2 and sl3), of which sl1, sl2 were closely linked to V _f . The s11 gene was located at about 14 cM from V _f and expressed itself only in the presence of another independently segregating sub-lethal gene sl3. Only the double homozygous recessive genotypes (sl1sl1 sl3sl3) were lethal, which occurred at first as dwarf and poor vigour plants during the first three months after germination. The sl2 gene was also linked to V _f and its lethality was expressed prior to seed germination and also required the homozygous recessive presence of sl3. The map position of sl3 has not yet been identified. The linkage of V _f to sub-lethal genes usually results in a shortage of V _f -resistant progenies. But in some exceptional crosses, it will lead to abundance of resistant seedling.An erratum to this article can be found at     

Early selection for partial resistance to powdery mildew,Podosphaera leucotricha (Ell. et Ev) Salm. in apple progenies

Summary Selection of apple for partial resistance to powdery mildew in the nursery before planting trees in the orchard can only be relied upon if genotypic differences in expression do not depend on plant age. Five progenies between susceptible cv. Melrose and five partially resistant genotypes (four CPRO-DLO selections from the cross ‘Elstar × Prima’ and cv. Prima itself) were assessed for mildew resistance following natural infection in the nursery during two successive seasons. Per progeny 60 randomly taken trees were transplanted into the orchard and their mildew resistance levels were assessed for three more years. The mean mildew resistance of the progenies in the nursery was related to that of their resistant parent, but differences in mean resistance levels in the orchard were not consistent with those in the nursery. The individual correlations of resistance between nursery and orchard also varied per progeny. In general the second year's assessment in the nursery gave the best prediction of the orchard performance over three years. In conclusion, nursery selection for partial mildew resistance can be effective, but the efficiency varies between populations.     

Isoenzyme aided selection in the transfer of mildew (Podosphaera leucotricha) resistance fromMalus hupehensis to the cultivated apple

Summary The isoenzymic geneGOT-1 was used as an indicator of ploidy levels and hybridity in the progeny of a 4x × 2x (A878/5,GOT-1 abdn × ‘Gloster 69’,GOT-lcc) cross, derived from the apomictic species (Malus hupehensis), which segregated for mildew resistance. The distinction between resistant and susceptible plants was sharp. After 3 months in a greenhouse, resistant plants showed no signs of sporulation. Of the hybrid seedlings (diploid and triploid) 78% were mildew resistant. This analysis was facilitated through the use of theGOT-lc allele, as a marker of hybridity. It appears that the prospects of transferring high mildew resistance fromM. hupehensis to the cultivated apple are promising; of the 65 hybrid resistant seedlings 18 were diploid. A similar approach in earlier generations might have facilitated the earlier transfer of this resistance to the cultivated apple.     

Screening fruit tree genetic resources in Belgium for disease resistance and other desirable characters

Summary The wide diversity of old fruit-tree cultivars originating or introduced into Belgium during the 18 ^th and 19 ^th centuries was collected as far as feasible over the last fifteen years at the State Plant Pathology Station in Gembloux. Out of the 2400 accessions now collected, one quarter was recovered from old public collections, and three quarters came from farms or gardens. The initial intention was to screen the material for disease resistance and other characters of agronomic interest with a view to using the best cultivars as breeding parents. However, as the collection developed, genetic resources conservation also became an objectiveper se. The collection presently contains 1150 apple, 850 pear and 300 plum accessions, and smaller numbers of other fruit species. Each accession is evaluated in an experimental orchard for at least ten years. In view of the growing public interest in old fruit-tree cultivars, the Plant Pathology Station has for several years been releasing to the nursery trade the better cultivars emerging from the evaluation, namely nine apple and four plum cultivars, and one peach cultivar. The principal features of the apple cultivars are presented in this paper. Since 1988, old apple and plum cultivars have been being used at the Station as parents in a breeding programme, with both controlled and open pollination. In some instances, old apple cultivars have also been crossed with a modern parent carrying the Vf gene for scab resistance. The preliminary observations on some of these seedlings are presented.     

Observations on different mildew sources used in apple breeding at Ahrensburg

Fifty-six populations of common bean (Phaseolus vulgaris L.) were grown in Pontevedra (Northwestern Spain) in four different environments in order to study their genetic diversity in 18 agronomical traits. All characters showed significant differences among populations, and most of them had significant genotype-environment interactions. Broad-sense heritability for this pool of characters ranged from 0.87 (seed length) to 0.12 (seed yield). Sixteen populations which deserve special attention because of their breeding value for earliness, yield, pod and seed size have been identified.     

Genetics and histology of powdery mildew resistance in apple

Summary Eight scab-resistant cultivars and selections along with eight commercial apple cultivars were evaluated for powdery mildew resistance in greenhouse and nursery tests. Dayton, Liberty, Delicious and Tolman Sweet were rated moderately resistant to infection in both greenhouse and nursery tests. Segregation of seedlings among 14 progenies for mildew reaction indicated that mildew resistance is polygenically controlled in this material with additive gene effects. Recovery of mildew resistant seedlings from crosses involving a scab-resistant parent(s) suggested that this material can be useful in developing scab- and mildew-resistant apple cultivars. Histological investigations were conducted to describe mildew symptoms of infected leaves.     

Early determination of genotypes for apple scab resistance by forced flowering of test cross progenies

Summary Apple selections with different major genes for resistance to apple scab (Venturia inaequalis) derived from Malus floribunda and M. pumila were crossed with each other. The progenies were screened as young seedlings for their reaction to V. inaequalis race 1. A gene for resistance from M. pumila, causing stellate necrotic (SN) lesions, was epistatic to a second gene for resistance from M. floribunda, causing irregular chlorotic (Chl) lesions. Although in most cases SN, Chl and susceptible phenotypes were clearly distinct, occasionally reactions were difficult to characterize or varied from one inoculation to another. Selected seedlings showing resistant or susceptible reactions were forced to flower in 16–20 months in the greenhouse and test crossed with susceptible cultivars. Test cross seedlings were screened for scab reaction. The presence of both genes for resistance in a resistant plant was indicated by presence of both Chl and SN resistant phenotypes in the test cross progeny. Chi-square analysis of four large progenies produced a good fit to the expected ratio. The use of the forced flowering technique to determine scab resistance genotypes in 28 months demonstrated its value in breeding apples with multiple disease resistance.     

Optimal sampling strategies for evaluating fruit softening after harvest in apple breeding

Environmental variance components associated with year, tree, and harvest date were estimated for fruit softening after harvest in apple (Malus × domestica Borkh.) to determine their relative importance and design optimum sampling strategies to discriminate genotypes in apple breeding. Fruit were stored after harvest under 20± 2 ^∘C and 80± 5%RH. Softening was evaluated by adapting the change in firmness during storage to a linear regression and defining the regression coefficient as the softening rate. Environmental variances associated with genotype × year interaction, among trees, year × tree interaction, and among harvest dates were all very small, namely, 2.7, 0.1, 5.2, and 5.7%, respectively, to the total variance obtained from the analysis of variance for the softening rate. The variance associated with genotype, at 57.3%, was very large. On the basis of the number of fruit necessary for firmness measurements, two times harvest is an efficient strategy to determine a genotype mean for the softening.     

Dwarfing and lethal genes in apple progenies

Summary Three dwarf types are described, early dwarf, determined by two recessive genes d ₁ and d ₃, in the double homozygous state, and an additional recessive gene d ₄ and crinkle dwarf and sturdy dwarf each also determined by a recessive gene.The gene l, for pale green lethal was found to be closely linked to the gene V _f , for scab resistance.There was evidence of a regrowth promoting gene, G, in some seedlings homozygous for d ₁ and d ₃.     

Screening techniques and sources of resistance to parasitic angiosperms

Parasitic angiosperms cause great losses in many important crops under different climatic conditions and soil types. The most widespread and important parasitic angiosperms belong to the genera Orobanche, Striga, and Cuscuta. The most important economical hosts belong to the Poaceae, Asteraceae, Solanaceae, Cucurbitaceae, and Fabaceae. Although some resistant cultivars have been identified in several crops, great gaps exist in our knowledge of the parasites and the genetic basis of the resistance, as well as the availability of in vitro screening techniques. Screening techniques are based on reactions of the host root or foliage. In vitro or greenhouse screening methods based on the reaction of root and/or foliar tissues are usually superior to field screenings and can be used with many species. To utilize them in plant breeding, it is necessary to demonstrate a strong correlation between in vitro and field data. The correlation should be calculated for every environment in which selection is practiced. Using biochemical analysis as a screening technique has had limited success. The reason seems to be the complex host-parasite interactions which lead to germination, rhizotropism, infection, and growth of the parasite. Germination results from chemicals produced by the host. Resistance is only available in a small group of crops. Resistance has been found in cultivated, primitive and wild forms, depending on the specific host-parasite system. An additional problem is the existence of pathotypes in the parasites. Inheritance of host resistance is usually polygenic and its transfer is slow and tedious. Molecular techniques have yet to be used to locate resistance to parasitic angiosperms. While intensifying the search for genes that control resistance to specific parasitic angiosperms, the best strategy to screen for resistance is to improve the already existing in vitro or greenhouse screening techniques.     

Evidence of gene introgression in apple using RAPD markers

Summary A genomic remnant of Malus floribunda clone 821 introgressed into the cultivated apple M. x domestica Borkh. was identified using randomly amplified polymorphic DNA (RAPD) markers obtained by the polymerase chain reaction (PCR). Using a set of 59 oligonucleotide decamer primers, polymorphic DNA markers were identified among three pooled DNA samples. Based on the presence or absence of bands among bulked apple scab-resistant selections and cultivars, bulked scab-susceptible cultivars, and a M. floribunda clone 821 sample, one primer, A 15, identified amplified fragments in the scab-resistant bulked sample that was also unique to the M. floribunda clone 821. The unique band from M. floribunda clone 821 was amplified in four out of 17 scab-resistant selections/cultivars. This RAPD, designated OA15₉₀₀, identifies an introgressed fragment that has as yet no known function.     

Screening of melon (Cucumis melo L.) germplasm for multiple disease resistance

Summary Melon germplasm was screened for cucumber green mottle mosaic virus (CGMMV), powdery mildew (Sphaerotheca fuliginea), downy mildew (Pseudoperonospora cubensis) and Fusarium wilt (Fusarium oxysporum f. sp. melonis) resistance under artificial conditions except downy mildew for which screening was done under natural epiphytotic conditions. High level resistance to all the four diseases was not recorded in any of the collections tested. Nevertheless, ertheless, resistance to three diseases was located in three germplasm. Wild Cucumis species C. figarei exhibited absolute resistance to CGMMV and Fusarium wilt and high level resistance to downy mildew. Phoot or snapmelon (Cucumis melo var. momordica) — a non-dessert from of Indian origin—was highly resistant to downy mildew and resistant to CGMMV and medium resistant to Fusarium wilt. Iroquois was resistant to powdery mildew and medium resistant to downy mildew and CGMMV.     

Screening and identification of resistance to bacterial soft rot in Brassica rapa

Mist-chamber, field, and detached leaf inoculation procedures identified plants resistant to bacterial soft rot [Erwinia carotovora subsp. carotovora, (Ecc)] in Brassica rapa and related species. The mist-chamber seedling inoculation provided the best correlation of mean disease severity ratings with the field plant inoculation (r = 0.67^**) and was used to identify resistant materials. The optimum mist-chamber incubation conditions to distinguish the resistance of accessions were 23 ^°C and 100% RH and were used for primary screening. A total of 752 accessions of B. rapa and related Cruciferae were screened. In general, accessions of B. oleracea were more resistant than accessions of B. rapa. Within B. rapa, subspecies pekinensis and chinensis were more susceptible than other subspecies. No completely resistant material was found. In species of B. rapa, only 7% of accessions showed some degree of resistance with plant-to-plant variation within the accessions. G30444, G30449, and AVRDC2837 were identified as the most resistant materials in B. rapa by both mist-chamber and field inoculations. The resistance was correlated between an USA isolate (Geneva-1) and two Chinese isolates (RL4-1 and RL-19) of Ecc. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance and selection of some fruit characters of apple. I. Inheritance of low and high acidity

Summary Fruit acidity as such appears to be gouverned by a single major gene, low acidity being a recessive character. The additional genetic variation is explainable by the additive gene action model. This mode of inheritance causes the pH distribution of seedlings in progenies to shift with a higher mid-parent pH towards more low-acid cum sweet (pH 3.8) and fewer high-acid cum sour (pH 2.9) seedlings or vice versa with a lower midparent pH. As a result, in crosses between heterozygotes (Mama) the total proportion of undesirable seedlings -low plus high-acid ones-, remains similar, irrespective of the mid-parent pH. This proportion constituted about one third of the seedlings. A same proportion, but of mainly sour plants, was found in crosses between relatively acid homozygotes (Mama) or between these and similar heterozygotes. It was less than one quarter if the heterozygous partner was medium-acid and a few percentages only when both the homozygous and heterozygous partner were medium-acid. Choosing parents in the absence of information, chances are that the very large majority of the medium-acid types are heterozygotes, that among the relatively acid ones one in two is homozygous and that the odds to find homozygotes among the sour types are probably best. Attention was drawn to the importance of a high sugar content — which is quantitatively inherited — in parents with a relatively low pH to increase the chances of selecting seedlings with a good sugar/acid ratio.     

Pollen and pollination experiments. VI. Heat resistance of pollen

Summary Pollen of dry apple, pear, lily and rose pollen was heated up to 48 h at a range of temperatures. About half or more than half of the pollen grains survived 48 h at 40 C, 24 h at 50 C, 8 16 h at 60 C. 4 8 h at 70 C, more than one hour at 80 C. and between 10 and 20 min at 90 C. Presumably, pollen able to withstand low humidity is also heat resistant, a property which may be usable to make pollen virus free through heat treatment and perhaps to overcome incompatibility.     

Screening for resistance to take-all in wheat,triticale and wheat-triticale hybrid lines

Summary Fifteen triticale and wheat-triticale hybrid lines were evaluated for resistance to the take-all fungus Gaeumannomyces graminis var. tritici and compared with five wheat and two rye lines in inoculated field and pot trials. The triticale and wheat-triticale hybrid lines varied in rye chromosome number and degree of resistance expressed. One line, Venus with seven pairs of rye chromosomes consistently showed levels of resistance intermediate between wheat and rye. A trend was observed where increasing rye chromosome content led to greater resistance but exceptions showed that variation within triticales could not be ascribed to rye chromosome content alone.     

Screening relatives of wheat for snow mold resistance and freeing tolerance

Four hundred and eighty one accessions of species and subspecies of genera Triticum and Aegilops were evaluated for resistance to speckled snow mold caused by Typhula ishikariensis and for freezing tolerance. All Triticum and seven of Aegilops species were severely affected by snow mold. Among surviving Aegilops spp., only the Ae. cylindrica accessions exhibited resistance similar to that of the most resistant winter wheat cultivar, PI 173438. After repeated screening of accessions of Ae. cylindrica, 12 accessions were identified as having resistance similar to that of PI 173438; eleven of those were considered more freezing tolerant than PI 173438, but less than wheat cv. Valjevskaya, the freezing tolerant check. Accessions of Ae. cylindrica with snow mold resistance exceeding that of PI 173438, and with freezing tolerance, are currently being screened. This revised version was published online in August 2006 with corrections to the Cover Date.