Postulation of resistance genes to barley diseases in heterogeneous varieties

Resistance to causal agents of diseases is an important varietal characteristic that influences the management practice of crop plants and thus production costs of commodities. At present, almost all European barley varieties possess at least one major gene for resistance to powdery mildew. After hybridizing selected parental varieties, resistance genes often segregate in subsequent generations and, therefore, some varieties comprise lines that differ in the number or combinations of resistance genes. The objective of this research was to describe the various methods available for postulating resistance genes to pathogens in heterogeneous varieties using resistance to powdery mildew of barley as an example. Four spring barleys (‘Orbit’, ‘Malva’, ‘Tocada’ and CLE 233), and a six-row variety of winter barley, F 12872, were screened. For postulating resistance genes, several testing procedures and many Blumeria graminis f.sp. hordei isolates were used. Minimum amounts of seed were determined and different methods of obtaining homogeneous seed samples from heterogeneous varieties were compared. It was found that ‘Orbit’ and ‘Malva’ are composed of three and ‘Tocada’, CLE 233 and F 12872 of two lines with different resistances to powdery mildew. Problems of postulating resistance genes in heterogeneous varieties and the advantages of testing leaf segments instead of young plants are discussed.     

Parasitism ofLeptinotarsa decemlineata [Coleoptera: Chrysomelidae] byEdovum puttleri [Hymenoptera: Eulophidae] in different cultivars of eggplant

Experiments were conducted to quantify parasitism of Colorado potato beetle,Leptinotarsa decemlineata (Say), by the egg parasitoid,Edovum puttleri Grissell, on 3 different cultivars of eggplant,Solanum melongena L. Levels of parasitism were higher (P<0.05) on ‘Black Pride’ than on other cultivars. The percentage of egg masses that were parasitized was 1.2-fold higher (P<0.05) on ‘Black Pride’ than on ‘Harris Special’ and ‘White’. The number of eggs per mass that were parasitized was 1.3- and 1.4- fold greater (P<0.05) on ‘Black Pride’ than on ‘Harris Special’ and ‘White’, respectively. The percentage of eggs that were parasitized per mass and percentage of emerged adult parasitoids did not differ (P>0.05) among cultivars; between 2.1- to 2.6- fold more females than males emerged from eggs on all cultivars during the growing season.Edovum puttleri suppressed the 2nd generation ofL. decemlineata on ‘Black Pride’ and ‘Harris Special’, but did not suppress populations on ‘White’.      

Localization of the rice stripe disease resistance gene, Stv-bi, by graphical genotyping and linkage analyses with molecular markers

 We used graphical genotyping and linkage analyses with molecular markers to determine the chromosomal location of the rice stripe disease resistance gene, Stv-b ⁱ . The stripe resistance gene from the indica rice (Oryza sativa) cv ‘Modan’ was introgressed into several Japanese rice varieties. We found 4 RFLP markers in ‘Modan’, five susceptible parental rice varieties (‘Norin No. 8’, ‘Sachihikari’, ‘Kanto No. 98’, ‘Hokuriku No.103’ and ‘Koganebare’) and four resistant progeny varieties (‘St. No. 1’, ‘Aichi No. 6’, ‘Aoisora’ and ‘Asanohikari’). Graphical genotyping of the resistant progeny revealed a chromosomal segment ascribable to ‘Modan’ and associated with stripe resistance. The chromosomal segment from ‘Modan’ was located at 35.85 cM on chromosome 11. Linkage analysis using 120 F₂ individuals from a cross between ‘Koshihikari’ (susceptible) and ‘Asanohikari’ (resistant) revealed another 8 RFLP markers in the same chromosome. We performed a bioassay for rice stripe resistance in F₃ lines of the F₂ individuals using infective small brown planthoppers and identified an 1.8-cM segment harboring the rice stripe disease resistance gene, Stv-b ⁱ , between XNpb220 and XNpb257/ XNpb254. Furthermore, Stv-b ⁱ was linked by 0.0 cM to a RFLP marker, ST10, which was developed on the basis of the results of RAPD analysis. These DNA markers near the Stv-b ⁱ locus may be useful in marker-assisted selection and map-based cloning of the Stv-b ⁱ gene. Received: 26 September 1997 / Accepted: 4 November 1997     

b-D-glucosidase activity in pelargonium and poinsettia leaves infected by <Emphasis Type="Italic">Botrytis cinerea</Emphasis> Pers. ex Fr.

The involvement of β-D-glucosidase activity in grey mould was studied in two ornamental plant species attacked by Botrytis cinerea. β-D-glucosidase activity in the susceptible pelargonium cultivar ‘Shiva’ gradually increased with the disease development in the leaf spots and their surroundings. The endogenic level of the studied hydrolase in the resistant pelargonium ‘Cascade’ was several times higher than in the susceptible cultivar ‘Shiva’ and in principle underwent no changes after inoculation. The postinfection increase in the activity of β-D-glucosidase noted in the leaves of the susceptible poinsettia cultivar ‘Malibu Red’ was evidently weaker in the intensity, but its tendencies were similar to those of the susceptible pelargonium cultivar. In the leaves of the medium-resistant poinsettia ‘Coco White’ the constitutional level of β-D-glucosidase was 2-3-fold higher in that cultivar than in the susceptible cv. ‘Malibu Red’. In attacked leaves of ‘Coco White’, the enzyme activity continued to increase temporarily until the 3^rd h after inoculation. The process of healthy leaf senescence in both species had no significant influence on the change of the studied enzyme activity which was generally low. A high activity of β-D-glucosidase was also observed in the homogenate prepared from mycelium and in the fungal spores.     

Egg distribution patterns in the almond seed wasp,Eurytoma amygdali

The egg distribution patterns ofEurytoma amygdali Enderlein (Hymenoptera, Eurytomidae), which oviposits singly in green, developing almonds, were studied in the laboratory and in the field. In the laboratory, individual females were caged with a number of almonds and the eggs deposited in each fruit were counted. In the field, eggs were censused in almonds of different varieties at regular intervals, over four seasons (1988–91). In the susceptible ‘Retsou’ variety, eggs were uniformly distributed among fruits, both in the laboratory and in the field, as long as the mean number of eggs per almond was ≤2.5. When the mean number of eggs per almond was higher the egg distributions were random. This suggests that, up to a certain level of infestation, females were able to assess egg load of fruits and oviposit in the less infested ones. The main factor enabling the females to discriminate and select the less infested almonds for oviposition is probably a host-marking pheromone. In 1990, the same patterns of egg distribution were observed in samples taken from 5 other almond varieties that are not as susceptible as Retsou and have thicker pericarp and harder endocarp. In 1991 however, when fruits developed more rapidly than in 1990, egg distributions in 3 of these 5 varieties (Truoito, Ai, and Marcona) were not uniform. Although the mean number of eggs per fruit was low (1.2–2.0), many fruits of these varieties contained no eggs. This suggests that, in some less susceptible varieties, egg distribution might also be affected by certain fruit parameters, unfavourable for oviposition, related to the pericarp thickness and endocarp hardness.     

Factors influencing induction and maintenance of <Emphasis Type="Italic">Vitis rotundifolia</Emphasis> Michx. embryogenic cultures

Unopened leaves, petioles and fully opened leaves from micropropagation cultures of five Vitis rotundifolia Michx. varieties were cultured on induction medium to study their embryogenic response. Among the various explants tested, the maximum number of varieties produced embryogenic cultures from unopened leaves followed by fully opened leaves and petioles. Based on morphological differences, two types of embryogenic cultures were identified. Friable cultures typically arose as proembryonic masses (PEM) on induction medium, whereas somatic embryo production without an intervening PEM stage was observed in compact cultures. Of the five varieties tested, the highest frequency of embryogenic response was observed from fully opened leaves of ‘Supreme’ and unopened leaves and petioles of ‘Delicious’. Attempts to initiate suspension cultures from varieties resulted in proliferation and maintenance of ‘Alachua’ and ‘Carlos’ cultures in liquid medium for 16 weeks. Embryogenic potential of varieties was studied on cultures growing on embryo development medium. The maximum number of cotyledonary stage somatic embryos from 0.2 g proembryonic masses were observed in ‘Carlos’ (379.3) followed by ‘Alachua’ (350.0) and ‘Delicious’ (305.0). Cotyledonary stage somatic embryos germinated when cultured on Murashige and Skoog medium containing 1 μM Benzyladenine (BA). Although high embryo germination rates (80–100%) were observed in the varieties tested, plant recovery from germinated somatic embryos ranged from 6–47%. Embryogenic cultures could be maintained on X6 medium and used in genetic engineering studies.     

Osmoregulation and osmoprotection in the leaf cells of two olive cultivars subjected to severe water deficit

In this study, we compared the efficacy of defense mechanisms against severe water deficit in the leaves of two olive (Olea europaea L.) cultivars, ‘Chemlali’ and ‘Meski’, reputed drought resistant and drought sensitive, respectively. Two-year old plants growing in sand filled 10-dm³ pots were not watered for 2 months. Changes in chlorophyll fluorescence parameters and malondialdehyde content as leaf relative water content (RWC) decreased showed that ‘Chemlali’ was able to maintain functional and structural cell integrity longer than ‘Meski’. Mannitol started to accumulate later in the leaves of ‘Chemlali’ but reached higher levels than in the leaves of ‘Meski’. The latter accumulated several soluble sugars at lower dehydration. ‘Chemlali’ leaves also accumulated larger quantities of phenolic compounds which can improve its antioxidant response. Furthermore, the activity of three antioxidant enzymes catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) increased as leaf RWC decreased. However, differences were observed between the two cultivars for CAT and POD but not for APX. The activity of the first two enzymes increased earlier in ‘Meski’, but reached higher levels in ‘Chemlali’. At low leaf hydration levels, ‘Chemlali’ leaves accumulated mannitol and phenolic compounds and had increased CAT and POD activities. These observations suggest that ‘Chemlali’ was more capable of maintaining its leaf cell integrity under severe water stress because of more efficient osmoprotection and antioxidation mechanisms.     

Leaf pubescence mediates the abundance of non-prey food and the density of the predatory mite Typhlodromus pyri

Plants with leaves having numerous trichomes or domatia frequently harbor greater numbers of phytoseiid mites than do plant with leaves that lack these structures. We tested the hypothesis that this pattern occurs, in part, with Typhlodromus pyri because trichomes increase the capture of pollen or fungal spores that serve as alternative food. Using a common garden orchard, we found that apple varieties with trichome-rich leaves had 2–3 times more pollen and fungal spores compared to varieties with trichome-sparse leaves. We also studied the effects of leaf trichome density and pollen augmentation on T. pyri abundance to test the hypothesis that leaf trichomes mediate pollen and fungal spore capture and retention and thereby influence phytoseiid numbers. Cattail pollen (Typha sp.) was applied weekly to mature ‘McIntosh’ and ‘Red Delicious’ trees grown in an orchard and, in a separate experiment, to potted trees of the same varieties. ‘McIntosh’ trees have leaves with many trichomes whereas leaves on the ‘Red Delicious’ trees have roughly half as many trichomes. With both field-grown and potted trees, adding cattail pollen to ‘Red Delicious’ trees increased T. pyri numbers compared to ‘Red Delicious’ trees without pollen augmentation. In contrast, cattail pollen augmentation had no effect on T. pyri populations on ‘McIntosh’ trees. Augmentation with cattail pollen most likely supplemented a lower supply of naturally available alternative food on ‘Red Delicous’ leaves and thereby enhanced predator abundance. These studies indicate that larger populations of T. pyri on pubescent plants are due, in part, to the increased capture and retention of pollen and fungal spores that serve as alternative foods. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mapping of quantitative trait loci affecting Drechslera teres resistance in barley with molecular markers

 Resistance loci for seedling-stage resistance to net blotch disease (Drechslera teres) in barley were mapped with molecular markers in an F₂ population derived from a cross between the susceptible barley cultivar ‘Arena’ and the resistant Ethiopian landrace ‘Hor 9088’. Disease reactions were scored with first and second leaves of 2-week-old plants 7 and 9 days after inoculation with a single spore-derived isolate. For linkage analysis, 22 RFLP markers and 284 AFLP markers were used. The seven linkage groups covered 1153.3 cM with an average marker interval of 3.76 cM. The resistance was determined to be inherited in a quantitative manner. Altogether, 12 QTLs were mapped with positions depending on the leaf used for testing and the time period after infection. Heritability in the broad sense ranged between 0.21 and 0.37. Received: 26 May 1998 / Accepted: 9 June 1998     

Expression of an engineered cysteine proteinase inhibitor (Oryzacystatin-IΔD86) for nematode resistance in transgenic rice plants

 We have used a genotype-independent transformation system involving particle gun bombardment of immature embryos to genetically engineer rice as part of a programme to develop resistance to nematodes. Efficient tissue culture, regeneration, DNA delivery and selection methodologies have been established for elite African varieties (‘ITA212’, ‘IDSA6’, ‘LAC23’, ‘WAB56-104’). Twenty-five transformed clones containing genes coding for an engineered cysteine proteinase inhibitor (oryzacystatin-IΔD86, OC-IΔD86), hygromycin resistance (aphIV) and β-glucuronidase (gusA) were recovered from the four varieties. Transformed plants were regenerated from all clones and analysed by PCR, Southern and western blot. Detectable levels of OC-IΔD86 (up to 0.2% total soluble protein) in plant roots were measured in 12 out of 25 transformed rice lines. This level of expression resulted in a significant 55% reduction in egg production by Meloidogyne incognita. Received: 4 August 1997 / Accepted: 22 August 1997     

Effects of nodulation on resistance to alfalfa sickness among ten alfalfa cultivars

Summary Ten alfalfa cultivars were used to study the effects of nodulation and variations in resistance to alfalfa sickness. The alfalfa seedlings were planted in sick soil with three treatment,,i.e.: pasteurized sick soil inoculated with Rhizobium ‘Nitragin’ which served as the control, the inoculated non-pasteurized sick soil and the non-inoculated non-pasteurized sick soil. None of the alfalfa cultivars were immune from the sickness. Cultivar ‘Anik’ fromMedicago falcata was among the most resistant cultivars. Three Phytophthora root rot resistant cultivars including ‘Agate’, ‘Apollo’ and ‘Ramsey’ were not resistant to the disease. Alfalfa inoculated with Rhizobium showed greatly improved seedling growth. Correlation coefficients showed that those alfalfa cultivars more resistant to alfalfa sickness produced more dry weight. Dry weight increase due to nodulation (82%) had more than compensated for the loss of dry weight due to alfalfa sickness (33%). The present study suggested that the poor growth of alfalfa on sick soil was attributed to both the soil borne pathogens and the poor nodulation of alfalfa, probably due to the absence of effective Rhizobium in sick soil. Highly significant differences were also obtained among olfalfa cultivars for plant dry weight in the inoculation treatment. Selection for effective Rhizobium strains and for alfalfa genotypes which are resistant to alfalfa sickness and are high in nitrogen fixation rates could improve alfalfa yield in sick soil.     

Inheritance of resistance to Plum pox virus in apricot (Prunus armeniaca L.)

The inheritance of resistance to Plum pox virus (PPV) has been studied in 1,178 apricot hybrids. Seven hundred and eighteen F1 hybrids, obtained from controlled crosses between the susceptible Greek cultivar “Bebecou” and the resistant PPV cultivars of American origin (“Stark Early Orange,” ‘NJA2,” ‘Veecot,” “Sunglo,” “Harlayne,” and “Orangered”) were evaluated for resistance to the PPV-M (Marcus) strain, 8 years after artificial inoculation. The inheritance of resistance to PPV has been additionally studied for the first time in a BC₁ population of 95 apricot hybrids for four vegetative periods. Reaction of each hybrid to PPV-M was scored through visual symptoms, indexing onto GF-305 and double-antibody sandwich enzyme-linked immunosorbent assay tests. Segregation within the hybrids, determined by Chi-squared analysis, fits a 1:1 ratio (P ≤ 0.05) of the resistant vs susceptible, indicating that resistance to PPV is controlled by a single dominant gene locus and that the above six resistant cultivars are heterozygous for the trait. Plants carrying this gene may initially develop disease symptoms on leaves but eventually recover and no virus can be detected in leaves. Susceptible plants show similar symptoms initially but remain symptomatic. Inheritance of resistance to PPV also has been studied in 365 F1 hybrids by crossing the resistant cultivar “Stella” with the susceptible “Bebecou” and the resistant cultivars “Sunglo” and “NJA2,” for 8 years after inoculation. The segregation ratio was 1:0 (resistant/susceptible) suggesting that “Stella” is homozygous for the resistance trait. The purpose of this work was the enhancement of the knowledge of inheritance of resistance to PPV for the selection of new cultivars.     

Identification of QTL involved in field resistance to light leaf spot (Pyrenopeziza brassicae) and blackleg resistance (Leptosphaeria maculans) in winter rapeseed (Brassica napus L.)

 Quantitative trait loci (QTL), involved in the polygenic field resistance of rapeseed (Brassica napus L.) to light leaf spot disease, were mapped using 288 DNA markers on 152 doubled-haploid (DH) lines derived from the cross ‘Darmor-bzh’בYudal’. Over two years (1995 and 1996), the DH population was evaluated for light leaf spot resistance on leaves (L) and stems (S), and for blackleg disease resistance in same field trials. For the L resistance criterion, a total of five and seven QTL were detected in 1995 and in 1996 respectively, accounting for 53% and 57% of the genotypic variation. For the S criterion, three and five QTL were identified in 1995 and in 1996 respectively, explaining 29% and 43% of the genotypic variation. The locations of the QTL detected were quite consistent over the two years (4- and 2-year common QTL for L and S, respectively). Three genomic regions, located on the DY5, DY10 and DY11 groups, were common to the resistance on leaves and stems. In comparison with the QTL for blackleg resistance described by Pilet et al. (1998), two regions on the DY6 and DY10 groups, were associated with the two disease resistances. These ‘multiple disease resistance’ (‘MDR’) QTL may correspond to genes involved in common resistance mechanisms towards the two pathogens or else to clusters of resistance genes. Received: 21 November 1997 / Accepted: 3 March 1998     

Back-cross reciprocal monosomic analysis of Fusarium head blight resistance in wheat (Triticum aestivum L.)

 Fusarium head blight (FHB or scab) caused by Fusarium spp. is a widespread disease of cereals causing yield and quality losses and contaminating cereal products with mycotoxins. The breeding of resistant varieties is the method of choice for controlling the disease. Unfortunately, the genetic basis of scab resistance is still poorly understood. We present the results of a back-cross reciprocal monosomic analysis of FHB resistance using the highly resistant Hungarian winter wheat line ‘U-136.1’ and the highly susceptible cultivar ‘Hobbit-sib’. Resistance testing was performed in a field trial artificially inoculated with a Fusarium culmorum conidial suspension. Five hemizygous families containing ‘U-136.1’ chromosomes 6B, 5A, 6D, 1B, and 4B had a visually reduced spread of infection compared to lines having the ‘Hobbit-sib’ chromosome. Chromosome 2B from ‘U-136.1’ had an increased spread of infection. The critical chromosomes controlling seed weight were 6D, 3B, 5A, and 6B while those controlling deoxynivalenol (DON) content were homoeologous groups 2 and 6, although the latter effects were not significant due to a high coefficient of variation. Results from this and other studies show that chromosomes 6D, 6B, 5A, 4D, and 7A have frequently been associated with scab resistance in a number of wheat cultivars. Research groups now attempting to map scab resistance in wheat using markers should pay special attention to the above-mentioned chromosomes. Received: 31 March 1998 / Accepted: 14 July 1998     

Production and characterisation of the intergeneric hybrids between Dendranthema morifolium and Artemisia vulgaris exhibiting enhanced resistance to chrysanthemum aphid (Macrosiphoniella sanbourni)

Aphids represent the most destructive of chrysanthemum pests to cultivation. Reliable variety sources of resistance and control methods are limited, so development of highly resistant breeding lines is desirable. An intergeneric hybrid between Dendranthema morifolium (chrysanthemum) variety ‘Zhongshanjingui’ and Artemisia vulgaris (mugwort) ‘Variegata’ was attempted. Most of the hybrid embryos aborted at an early developmental stage. Embryo rescue allowed the generation of hybrid plants, whose hybridity was confirmed by a combination of morphological, cytological and GISH analysis. The hybrids were vigorous, flowered normally, and their flower and leaf shape resembled those of the chrysanthemum more than those of the mugwort parent. The hybrids showed much higher resistance to chrysanthemum aphid (Macrosiphoniella sanbourni) than maternal chrysanthemum by inoculation test. The leaves of the hybrid developed a higher density of trichomes and secretory glands compared to the maternal chrysanthemum. GC–MS analysis revealed that ~51% of the essential oil in the hybrid leaves were monoterpenoids and sesquiterpenoids, while the proportion in the chrysanthemum was ~37%, and in the mugwort was ~90%. It is inferred that higher aphid resistance in the hybrid mainly owed to the leaf micromorphology and bioactive essential oil content.     

QTL mapping of fire blight resistance in apple

Fire blight caused by the bacterium Erwinia amylovora is a severe threat to apple and pear orchards worldwide. Apple varieties exhibit a wide range of relative susceptibility/tolerance to fire blight. Although, no monogenic resistance against fire blight has been identified yet, recent evidence indicates the existence of quantitative resistance. Potential sources of fire blight resistance include several wild Malus species and some apple cultivars. F1 progenies of ‘Fiesta’בDiscovery’ were inoculated with the Swiss strain Ea 610 and studied under controlled conditions to identify quantitative trait loci (QTLs) for fire blight resistance. Disease was evaluated at four time points after inoculation. Shoot lesion length and the area under disease progress curve (AUDPC) values were used for QTL analysis. One significant (LOD score of 7.5–8.1, p<0.001) QTL was identified on the linkage group 7 of ‘Fiesta’ (F7). The F7 QTL explained about 37.5–38.6% of the phenotypic variation.     

ABA and proline accumulation in leaves and roots of wild (<Emphasis Type="Italic">Hordeum spontaneum</Emphasis>) and cultivated (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> ‘Maresi’) barley genotypes under water deficit conditions

The purpose of the study was to examine water stress-induced changes in the ABA and proline contents in roots and leaves of a potentially more resistant wild accession of Hordeum spontaneum and the modern cultivar Maresi (Hordeum vulgare). Leaves of H. spontaneum had higher contents of constitutive ABA and proline in comparison to those of ‘Maresi’. A moderate water deficit resulted only in root dehydration, which was higher in ‘Maresi’. Increases of water deficit in roots coincided with an increase of ABA content in roots, followed by that in leaves. The level of proline increased only in leaves and only in the case of H. spontaneum. Under conditions of severe water stress, the root dehydration levels were similar in the both genotypes, whereas leaf dehydration was higher in ‘Maresi’. H. spontaneum, as compared to ‘Maresi’ showed an earlier increase of ABA content in the roots and accumulated more ABA in the leaves. Free proline levels in the roots increased in both genotypes but H. spontaneum exhibited a 2-fold higher proline accumulation than ‘Maresi’. In H. spontaneum the accumulation of proline in the leaves occurred noticeably earlier and to a higher extent than in ‘Maresi’. A possible connection of these modifications with water stress resistance of the investigated genotypes is discussed in this paper.     

Inheritance of resistance to the two-spotted spider mite from L. pimpinellifolium (Jusl.) Mill. accession ‘TO-937’

The two-spotted spider mite (Tetranychus urticae Koch) is an important pest of tomato (Lycopersicon esculentum Mill.) crops in temperate regions as this spider mite has a very large capacity for population increase and causes severe tomato yield losses. There is no described tomato cultivar fully resistant to this pest, although resistant accessions have been reported within the green-fruited tomato wild species L. pennellii (Corr.) D’Arcy and L. hirsutum Humb. & Bonpl. We observed a L. pimpinellifolium (Jusl.) Mill. accession, ‘TO-937’, which seemed to be completely resistant to mite attacks and we crossed it with the susceptible L. esculentum cultivar. ‘Moneymaker’ to obtain a family of generations consisting of the two parents, the F₁, the F₂, the BC₁ to L. esculentum, and the BC₁ to L. pimpinellifolium. This family was evaluated for mite resistance in a polyethylene greenhouse using an experimental design in 60 small complete blocks distributed along 12 double rows. Each block consisted of five F₂ plants in one row and one plant of each of the two parents, the F₁, the BC₁ to L. esculentum, and the BC₁ to L. pimpinellifolium in the adjacent row. Plants at the 10–15 leaf stage were artificially infested by putting on them two pieces of French bean leaf heavily infested with T. urticae. After two months, evaluations of infestation were made by visual observation of mite nets and leaf damage. Plants that were free of signs of mite reproduction on the top half were considered as resistant, plants with silky nets only on their basal leaves, intermediate, and plants with mite reproduction on both basal and top canopies were scored as susceptible. Dominance for resistance appeared because all the ‘To-937’, BC₁ to L. pimpinellifolium, and F₁ plants were resistant. Not all ‘Moneymaker’ plants behaved as susceptible because 35% of plants were intermediate. In the BC₁ to L. pimpinellifolium and the F₂, most plants were scored as resistant, only 7 % BC₁ and 3 % F₂ plants were intermediate, and a single F₂ plant (0.3 %) was susceptible. With these figures, resistance seemed to be controlled by either four or two genes according to whether segregation in the BC₁ or in the F₂, respectively, were considered. These results could in part be explained because of appearance of negative interplot interference due to the high frequency of resistant genotypes within most of the generations. Therefore, the family was evaluated again but using a different experimental design. In the new experiment, 16 ‘TO-937’, 17 ‘Moneymaker’, 17 F₁, 37 BC₁ to L. pimpinellifolium, 38 BC₁ to L. esculentum, and 125 F₂ plants were included. Each of these test plants was grown besides a susceptible ‘Moneymaker’ auxilliary plant that served to keep mite population high and homogeneous in the greenhouse. Negative interplot interference was avoided with this design and all the ‘TO-937’, F₁, and BC₁ to L. pimpinellifolium plants were resistant, all ‘Moneymaker’ test plants were susceptible, and 52 % BC₁ to L. esculentum and 25 % F₂ plants were susceptible, which fitted very well with the expected for resistance governed by a single dominant gene. The simple inheritance mode found will favour sucessful introgression of mite resistance into commercial tomatoes from the very close relative L. pimpinellifolium.     

Multiple field and glasshouse assessments increase the reliability of linkage mapping of the Vf source of scab resistance in apple

 Apple scab, caused by the fungus Venturia inaequalis (Cke.) Wint., is an important disease in commercial apple production. A mapping population of 155 individuals, derived from a cross between the apple varieties ‘Prima’ (resistant)בFiesta’ (susceptible), was scored for response to the disease in replicated field and glasshouse trials throughout Europe. Twenty data sets were selected and cluster analysis was used to form a consensus score for the population fitting a 1 : 1 segregation ratio of resistance:susceptibility. The progeny were scored with molecular markers. A detailed map covering 54 cM of the ‘Prima’ linkage group containing the Vf gene for scab resistance was constructed using 24 molecular markers linked to the resistance gene. One isoenzyme marker (Pgm-1), six RFLP markers and 17 RAPD markers formed a linkage group with the consensus measure of resistance to scab. Four marker bridges were established with the corresponding ‘Fiesta’ linkage group with additional markers (one isozyme, one RFLP, three RAPD and one AFLP). A low chi-square value indicated a good fit of the marker ordering, which was in close agreement with previously reported linkage positions for some of the markers and Vf. Differences were observed in the ability of different scoring methods to resolve susceptible and resistant classes. The results obtained for the consensus classification of resistance to scab for the population may suggest the presence of virulent inocula at some sites, which could overcome the Vf gene for resistance. The consequences of relying on individual scoring occasions for studying Vf scab resistance are discussed in the context of linkage analysis, conventional breeding selection, and marker-assisted selection. Received: 23 July 1997 / Accepted: 31 October 1997     

VARIETAL RESISTANCE OF LETTUCE TO ATTACK BY THE LETTUCE ROOT APHID, PEMPHIGUS BURSARIUS (L.)

Striking varietal differences in susceptibility to attack by the lettuce root aphid, Pemphigus bursarius L., were first found in lettuces grown at Wellesbourne in 1955. Subsequent work has confirmed that White Favourite and Imperials E-4 and 19551-M are highly resistant; Imperial 45634-M, Continuity and Iceberg are markedly resistant. Midas, Salad Bowl and Imperial 4164 each appear to be a mixture of susceptible and resistant plants.
Immigrant winged forms of P. bursarius showed no preference for colonizing any particular variety of lettuce, and it seems that resistance to attack results from antibiosis.
It is suggested that varietal differences in the composition of root sap may account for the differences in susceptibility to lettuce root aphid, but analyses of water and alcohol extracts of root sap from resistant and susceptible lettuce varieties have not shown consistent differences.