Increasing productivity in sweet almond using selected clones of bitter almond

Summary Eight controlled crosses were made between seven different sweet-kerneled almond cultivars (Prunus dulcis [Mill.] D. A. Webb), and five crosses were made between sweet x highly floriferous bitter almond clones. Sweet x bitter progenies out-yielded the sweet x sweet matings by more than three to one in the first year of production. This was attributable to greater cropping efficiency rather than to tree size or precocity. There was no pleiotropic association or association due to linkage between bitterness and yield. The selected Alnem (bitter) clones appear to be potentially useful progenitors for increasing almond yield capacity by conventional breeding methodology.     

Response to Selection for Resistance to Macrophomina and Xanthomonas and its Association with Seed Colour in Cowpea*

In cowpea, black seeded types are supposedly resistant to Macrophomina. A study was undertaken to verify the association of seed colour with resistance to Macrophomina and Xanthomonas and to examine the response over three cycles of selection for such resistance along with seed yield, determinate habit and synchronous flowering in five black-and three white-seeded cowpea varieties. Selection was done between and within progenies under heavy natural incidence of both pathogens supplemented by artificial inoculation of Macrophomina using the toothpick method at flower primordial stage. Among 414 single plant progenies in the first cycle all 193 white-seeded progenies from three different varieties were highly susceptible to both pathogens, with considerable variation among the black-seeded ones. All the black- and white-seeded progenies derived from the same variety‘Floricream’were highly susceptible. Multivariate analysis for five characters in the ten best progenies after two selection cycles revealed significant genetic diversity between the progenies. Selection was successful with the isolation of three productive progenies resistant: to both diseases. The results indicated that seed colour had no influence on resistance m this material and that simultaneous improvement of seed yield, plant type and disease resistance could be achieved.     

Inheritance of resistance to the peach root‐knot nematode (Meloidogyne floridensis) in interspecific crosses between peach (Prunus persica) and its wild relative (Prunus kansuensis)

The peach root‐knot nematode, Meloidogyne floridensis (MF), infects majority of available nematode‐resistant peach rootstocks which are mostly derived from peach (Prunus persica) and Chinese wild peach (P. davidiana). Interspecific hybridization of peach with its wild relative, Kansu peach (P. kansuensis), offers potential for broadening the resistance spectrum in standard peach rootstocks. We investigated the inheritance of resistance to MF in segregating populations of peach (‘Okinawa’ or ‘Flordaguard’) × P. kansuensis. A total of 379 individuals from 13 F₂ and BC₁F₁ families were challenged with a pathogenic MF isolate “MFGnv14” and were classified as resistant (R) or susceptible (S) based on root galling intensity. Segregation analyses in F₂ progeny revealed the involvement of a major locus with a dominant or recessive allele determining resistance in progeny segregating 3R:1S and 1R:3S, respectively. Testcrosses with a homozygous‐susceptible peach genotype (‘Flordaguard’ or ‘UFSharp’) confirmed P. kansuensis as a source of new resistance and the heterozygous allelic status of P. kansuensis at the locus conferring resistance to MF. We propose a single‐locus dominant/recessive model for the inheritance of resistance.     

Selection of Diploid Nematode-Resistant Sugar Beet from Monosomic Addition Lines

Monosomic sugar beet/wild beet addition lines (2n = 19) with full resistance against the beet cyst nematode have been characterized in different ways. Within the B. procumbens and B. webbiana addition lines three groups could be classified according to their isozymes pattern, growth habit, transmission rate, and resistance level. It is assumed that B. procumbens and B. webbiana each possess three different chromosomes which carry genes for nematode resistance. In the offspring of the addition lines diploid translocation types appear at very low frequencies, Isozyme pattern or growth type of the resistant plants were used for selecting diploid types in the offspring of monosomic addition lines. Effective selection could be made in progenies of susceptible sugar beets pollinated by addition lines because the pollen transmission of the alien chromosome is very low. Using these methods 7 nematode-resistant sugar beet lines could be selected. The transmission rates of the resistance gene ranged from 70.6% to 100%. Threw heterozygous progenies showed a 1:1 segregation indicating monogenic dominant inheritance of resistance. The level of resistance was as high as in the addition lines.     

Breeding for quantitative resistance to potato cyst nematode (Globodera pallida) in tetraploid potatoes using dihaploids and unreduced gametes

Summary Dihaploids were produced from tetraploids resistant to potato cyst nematode (Globodera pallida (Stone)). High levels of resistance were found in the dihaploids and three were used to produce tetraploid progenies by crossing them with susceptible tetraploid cultivars. One dihaploid, PDH505, produced more highly resistant offspring than the other two, PDHs 417 and 418. The latter gave progenies whose levels of resistance were similar to those obtained from susceptible dihaploids crossed with resistant tetraploids.The differences between the progenies of the resistant dihaploids were probably due to different modes of unreduced gamete formation (PDH505 producing gametes by first division restitution (FDR) and PDHs 417 and 418 by second division restitution (SDR)) although cytological studies would be necessary to confirm this. The methods by which dihaploids could be utilised in a tetraploid potato breeding programme are discussed in relation to the mode of unreduced gamete formation.     

Nematode Resistance and Meiotic Abnormality in Diploid Sugar Beet Selected from Interspecific Beta vulgaris×B. procumbens Hybrids*

Two nematode-resistant trisomic lines which were derived from interspecific Beta, vulgaris × B. procumbens hybrids were intercrossed or backcrossed with susceptible diploid sugar beer and their progenies were screened for nematode resistance. The transmission rate of resistance varied from 1.5 % to 47.6 % with an average of 20.4 % in the progenies of individual insomics derived from the two trisomic lines. Eleven resistant diploads were selected with a frequency of 0.2 %. These resistant diploids were classified into two groups, i.e., one group showed relatively high transmission rates of resistance with an average of 25.4 % and the other extremely low with an average of 1.2 % in their backcrossed and s el fed progenies., Meiotic chromosome behavior in a resistant diploid group with high transmission rates was considerably normal as compared to that in a resistant diploid group with low transmission rates. Chromatid bridges and acertric fragments were detected in 93 % of resistant diploids and in 46 % of susceptible diploids. Two different sized fragments occurred in resistant diploids, while only a smaller fragment was present in susceptible diploids. A frequency of sporocytes with bridges-fragments was 17.4% at anaphase I and 13.9 % at anaphase II in resistant diploids, while in susceptible diploids a frequency was 2.9 % and 5.3 % at the respective stages. These results suggest that at least two paracentric inversions are present in resistant diploids, one of which is linked to nernatode resistance and may be responsible for the low transmission rate of resistance.     

Inheritance of partial resistance to Myzus persicae in lettuce

Summary The genetics of partial resistance of lettuce to Myzus persicae was studied using F₁ and F₂ generations of two crosses between a susceptible and partially resistant accession (Norden x Batacer and Liba x Norden) and three crosses in which both parents were partially resistant (Batavia la Brillante x Batacer, Batacer x Liba and CGN4741 x Batacer). Partial resistance to M. persicae inherited quantitatively, without important dominance effects. Only in the cross Batacer x Liba were significant departures of the F₁ and F₂ from the midparent found, which were probably caused by epistatic effects. Reciprocal F₁s had similar resistance levels, indicating the absence of cytoplasmic or other maternal effects. Estimates of broad-sense heritability ranged from 0.34 to 0.61. The results indicated that lines with an improved resistance level can be obtained from crosses between partially resistant accessions, preferably by line selection or the application of indirect marker aided selection.Abbreviations PR partial resistance, partially resistant - S susceptibility, susceptible     

Evaluation of globe artichoke and related germplasm for resistance to artichoke latent virus

Summary A total of 280 globe artichoke (Cynara scolymus L.) clones were assessed for reaction to naturally occurring artichoke latent virus (ALV) in field trials. Also, the reaction of 17 seed progenies of crosses which produced ALV-resistant clones in the field, 20 self progenies of parental genotypes common to these crosses, self progenies of 17 artichoke cultivars, and 7 accessions ofC. cardunculus andC. syriaca were evaluated in the glasshouse following mechanical inoculation with ALV. The local and systemic presence of ALV, and the relative virus content in infected plants were determined by ELISA.All cultivars and most of the otherC. scolymus material tested (clones and seed progenies) proved to be highly susceptible to ALV. Ten progenies were less susceptible to mechanical inoculation as shown by the moment the virus could be detected, and a slow rate of accumulation of viral antigen. Some plants in two seed progenies possessed apparently a high resistance as could be concluded from the failure of ALV to spread or to multiply in plant.Accessions ofC. cardunculus andC. syriaca were also highly susceptible to ALV. However, an interspecific hybridC. cardunculus x C. syriaca presented the highest level of resistance, the plants remained free of local and systemic infection. Ratios of resistant to susceptible plants in self progenies of resistant plants suggested that this resistance was dominant.     

Evaluation of resistance to verticillium wilt in diploid,wild potato interspecific hybrids

Summary Verticillium wilt (V. albo-atrum Reinke & Berthold or V. dahliae Kleb) threatens potato (Solanum tuberosum L.) production in most growing areas of the world. Genetic resistance offers the most cost-effective and environmentally-sound control measure. However, there is a dearth of genetic and breeding information on resistance to verticillium wilt in potato, because of obscure parentage of some standard cultivars and the complex segregation at the tetraploid level. The wide range of genetic variability in wild relatives of potatoes can be potentially useful as a source of disease resistance, such as verticillium wilt resistance. Six diploid, wild, interspecific Solanum hybrids involving resistant x resistant and susceptible x resistant crosses, were assayed for verticillium wilt resistance under greenhouse conditions to evaluate their potential as sources of verticillium wilt resistance. The cross between S. gourlayi Oka. and S. chacoense Bitt. and its reciprocal had the most resistant progenies based on mean colony counts. No simple mode of inheritance can be proposed based on the observed segregation ratios. However, the continuous distributions observed on verticillium wilt disease response among hybrid families indicate that inheritance of resistance may be polygenic and complex. In addition, skewness of colony count distributions toward the resistance parents were characteristic of all resistant x susceptible crosses suggesting that resistance may be dominant. By contrast, the susceptible x susceptible cross showed a more normal distribution. Overall, the cross between S. gourlayi and S. chacoense showed the most potential as a source of verticillium wilt resistance.     

The importance of wild potato species resistant to the potato cyst nematode,Globodera pallida,pathotypes P4A and P5A,in potato breeding. I. Resistance studies

Summary Seven wild diploid potato species, Series Tuberosa, representing 1023 clones were screened for resistance to the potato cyst nematode, Globodera pallida. Over 25% of the clones were resistant to pathotype P₄A and almost 30% were resistant to pathotype P₅A. The resistance in hybrid progenies of these and other resistant species with cultivated potatoes was evaluated, and over 2200 seedlings were screened. High frequencies of resistance (>50%) to P₄A were found in progenies with Solanum leptophyes, S. vernei, S. gourlayi and S. capsicibaccatum, whereas resistance to P₅A was found in these species as well as S. sparsipilum. The importance of nematode resistant wild species for potato breeding is discussed.     

Genetical analysis of resistance to Mycosphaerella pinodes in pea seedlings

Summary In studies of the inheritance of resistance, pea seedlings of seven lines in which stems and leaves were both resistant to Mycosphaerella pinodes were crossed with a line in which they were both susceptible. With seven of the crosses resistance was dominant to susceptibility. When F₂ progenies of five crosses were inoculated on either stems or leaves independently, phenotypes segregated in a ratio of 3 resistant: 1 susceptible indicating that a single dominant gene controlled resistance. F₂ progenies of one other cross gave ratios with a better fit to 9 resistant: 7 susceptible indicating that two co-dominant genes controlled resistance. The F₂ progeny of another cross segregated in complex ratios indicating multigene resistance.When resistant lines JI 97 and JI 1089 were crossed with a susceptible line and leaves and stems of each F₂ plant were inoculated, resistance phenotypes segregated independently demonstrating that leaf and stem resistance were controlled by different genes. In two experiments where the F₂ progeny of the cross JI 97×JI 1089 were tested for stem and leaf resistance separately, both characters segregated in a ratio of 15 resistant:1 susceptible indicating that these two resistant lines contain two non-allelic genes for stem resistance (designated Rmp1 and Rmp2) and two for leaf resistance (designated Rmp3 and Rmp4). Evidence that the gene for leaf resistance in JI 1089 is located in linkage group 4 of Pisum sativum is presented.     

Variation in forage grasses as hosts of the root-knot nematode Meloidogyne naasi (Franklin) and selection for resistance in ryegrass

Summary Reactions of 13 grasses to Meloidogyne naasi varied with species; ryegrasses, fescues and their hybrids were generally susceptible and cocksfoot and timothy resistant. Marked variation in host resistance levels occurred between genotypes within cultivars.Selection of single plants, followed by tests on replicate tillers, identified resistant and susceptible genotypes in both Italian and perennial ryegrass cultivars. Resistant plants had few nematode-induced galls and fewer females and eggs than susceptibles. There was more or less continuous variation, with many genotypes intermediate between extremes of resistance and susceptibility. Selected resistant and susceptible genotypes are of use in assessing variation in nematode populations and as controls for breeding and selection programmes.     

甘薯抗茎线虫病基因的遗传分析及SCAR标记

以甘薯抗茎线虫病品种徐781和感茎线虫病品种徐薯18的杂交F1分离群体的174株单株为材料,对甘薯抗茎线虫病基因的遗传进行了分析。结果表明,徐781的抗茎线虫病为单基因控制,推测其基因型为Rrrrrr,徐薯18的基因型为rrrrrr。采用BSA-RAPD相结合的方法,筛选940条随机引物,发现10条引物在抗、感池间表现多态性。用这10条引物检测两亲本及建池单株,发现只有引物OPP03在8株抗池单株中扩增出一条在8株感池单株中所没有的特异条带,认为该标记与甘薯抗茎线虫病基因连锁。根据该标记对F1代174个单株的扩增结果,利用Mapmaker3.0软件计算遗传距离,表明该标记与抗茎线虫病基因间的遗传距离为14.2cM。将该片断回收、克隆、测序,表明其长度为878bp,该标记命名为OPP03878。根据测序结果,设计1对特异引物,进行特异性扩增,成功地将OPP03878标记转化为SCAR标记,用亲本及分离群体验证表明该分子标记稳定性好。     

Genetic control of resistance to the aphid Brevicoryne brassicae in the wild species Brassica fruticulosa

Initial studies have shown variable resistance in Brassica fruticulosa to the aphid Brevicoryne brassicae The aim of this work was to fix high levels of resistance to B. brassicae in true breeding lines of B. fruticulosa and obtain data on the genetic control of resistance. Plants from initially variable B. fruticulosa accessions were selfed to produce inbred resistant and susceptible lines that were studied in three separate experiments to determine the extent to which resistance to B. brassicae had been fixed. Results from three experiments using successive generations of resistant and susceptible inbred lines showed that continued selection resulted in resistant inbred lines that supported an average of three aphids per plant compared with an average of 96 aphids per plant for susceptible inbred lines. Data collected from an experiment determining the resistant phenotype of lines including the selfed progenies and the F₁ and F₂ progeny of a cross between resistant and susceptible individual plants indicated that the resistance was not controlled by a single gene.     

Resistance of almond cultivars to Plum pox virus (sharka)

Sharka, a disease caused by Plum pox virus (PPV), mainly affects some Prunus species, including apricot, peach and plum, and to a lesser degree, sweet cherry and sour cherry. In almond, different PPV isolates have been transmitted experimentally to the ‘Aï’ cultivar. In this study, the resistance of 10 almond cultivars to a Dideron PPV isolate was evaluated in controlled conditions by grafting the cultivars on to inoculated GF305 peach rootstocks. The results demonstrated a high level of resistance to PPV in all the almond cultivars assayed. They did not show any symptoms and were ELISA and RT‐PCR negative, despite the strong symptoms observed in their GF305 rootstocks. The implications of these results for the dispersion of PPV, and the potential role of almond as a source of resistance to PPV in other Prunus species such as peach, are also discussed.     

Recurrent selection for downy-mildew resistance in pearl millet

One population of pearl millet (Pennisetum glaucum (L.) R. Br.) highly susceptible to downy mildew (Sclerospora graminicola (Sacc.) Schroet.) was subjected to two cycles of recurrent selection for downy mildew resistance using a modified greenhouse screening method. The response to selection was evaluated under greenhouse and field conditions using 50 random S₁ progenies and 50 random full-sib progenies from each cycle bulk. Significant progress over cycles of selection was observed in all evaluation trials. These results demonstrated that, in a susceptible population, recurrent selection effectively increased the level of resistance to downy mildew. The modified greenhouse method for assessing resistance to downy mildew effectively differentiated genotypes and had the advantages of greater rapidity and suitability for use throughout the year, independent of season. A rapid decline of genotypic variance was observed in advanced cycles of selection, indicating that a small number of genes controls downy-mildew resistance in this population. The comparison of genotypic and error variance components from S₁ progenies and full-sib progenies suggested that full-sib progenies can be used successfully in recurrent selection for increased downy-mildew resistance.     

Evaluation of resistance to sharka (Plum pox virus) of several Prunus rootstocks

Sharka (Plum pox virus, PPV) severely affects the production of most Prunus species in the areas affected by the disease. In this study, the resistance of 15 Prunus rootstocks to a Dideron type isolate of PPV was evaluated under controlled conditions in an insect‐proof greenhouse. After four cycles of study,‘GF677’ almond x peach hybrid,‘Myrobolan 29C plum and ‘L2’ cherry did not show any symptoms and were ELISA‐DASI and RT‐PCR negative. These were considered resistant to PPV. The rest of the rootstocks assayed showed symptoms of sharka (confirmed by ELISA‐DASI or RT‐PCR), although the level of susceptibility was different for each rootstock.‘GF305’ peach, ‘Puebla de Soto’ plum and ‘Real Fino’ apricot, were highly susceptible to PPV, showing strong sharka symptoms and being ELISA and RT‐PCR positive. ‘Marianna 2624’ plum,‘AC 9921‐07’ hybrid and ‘CP‐2’ plum showed susceptibility to PPV confirmed by ELISA‐DASI and RT‐PCR positives.‘Nemaguard’ and ‘Nemared’ hybrids, ‘Torinel’ plum and ‘STN2’ hybrid showed an intermediate susceptibility to PPV with slight sharka symptoms and were ELISA‐DASI and RT‐PCR positive. Whereas, ‘Montclar’ peach and ‘Evrica’ hybrid showed moderate resistance to PPV with slight sharka symptoms and were ELISA positive but RT‐PCR negative. The results open new possibilities in the search for different sources of resistance to PPV within Prunus.     

Single-gene resistance to powdery mildew in zucchini squash (Cucurbita pepo)

Cucurbita pepo (pumpkin, squash,gourd) is an economically important species that is susceptible to the cucurbit powdery mildew fungus, Podosphaera xanthii(syn. Sphaerotheca fuliginea). ‘True French’, an open-pollinated cultivar of the Zucchini Group of C. pepo, was crossed with an unnamed powdery-mildew resistant straight neck-type accession, the resistance of which was apparently derived from an interspecific cross with a resistant wild species of Cucurbita,and resistant plants were selected in the F₂ generation. This was followed by six cycles of backcross-pedigree selection for resistance, and resulted in the development of an accession true-breeding for resistance to powdery mildew and nearly isogenic to ‘True French’. The resistant and susceptible near-isogenics were crossed and seeds of the filial and backcross generations were produced. Plants of the parental accessions and their progenies were grown together in a controlled-environment chamber, exposed to the pathogenic fungus, and scored as resistant, partially resistant, or susceptible 27–33 days after sowing. The results indicated that resistance is conferred by a single incompletely dominant gene, designated Pm-0. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quantitative evaluation of the degree of sprout leaf bending of rice cultivars using P-type Fourier descriptors and principal component analysis

For decades, seedling peaches have been used as the standard rootstock in California almond orchards. Vigorous, deep rooted trees are needed in almond orchards for maximum yields and to withstand the annual tree-shaking at harvest. Currently, researchers are actively evaluating rootstocks for almonds in field trials and in various screening protocols. In this study, seedling rootstocks, obtained from male-sterile advanced generation peach-almond (PEAL) hybrid mother trees, were compared with ‘Nemaguard’ peach seedlings for emergence in the nursery row, trunk caliper at propagation time, and end of season dormant above ground tree weight. Seedling emergence was affected significantly (P ≤ 0.05) by seed source, as was trunk caliper and end of season dormant above ground tree weight. Trunk caliper and dormant above ground tree weight were also affected significantly (P ≤ 0.05) by planting year. Results obtained in this study demonstrate the enhanced first year growth from seedlings of advanced generation PEAL hybrids, as compared to ‘Nemaguard’ seedlings. Seedling emergence in the rootstock bed was not affected significantly by planting year. Advanced generation PEAL hybrid seedlings were ready for June-budding at an earlier date compared to ‘Nemaguard’ seedlings, providing the potential for larger-sized finished nursery stock by the end of the growing season. Due to the male-sterile status of the advanced generation PEAL mother trees, bloom periods of several root-knot nematode resistant rootstock cultivars were examined for their degree of synchronicity with the mother trees. The examined rootstocks and mother trees varied in both chill hour and post-chill heat requirements necessary to effect bloom. ‘Flordaguard’ peach rootstock began bloom in advance of the male-sterile mother trees, whereas the bloom period of ‘Tsukuba No. 4’ occurred well after, suggesting they would not be effective synchronous pollenizers for consistent hybrid seed production. Based on more limited flowering period data, better bloom synchronicity was achieved with a Tsukuba No. 4 X Flordaguard hybrid.     

Performance of upland coloured cotton germplasm lines in line × tester crosses

Anthracnose is a serious disease affecting dry bean production especially in the cool highland areas worldwide. The objective of this research was to study the inheritance of anthracnose resistance in market-class dry beans. A complete diallel set of crosses was generated from nine diverse parents comprising six resistant and three susceptible to anthracnose. The F₁ and F₂ crosses and parents were artificially inoculated with Colletotriclum lindenumthianum Race-767 in a growth room. There was significant variation for anthracnose resistance among genotypes. General combining ability (GCA) and specific combining ability effects were significant for resistance, indicating importance of both additive and non-additive effects, respectively. Preponderance of GCA effects (66%) suggested that additive effects were more important than non-additive effects (24%), which were also reflected by high heritability estimates (70%), and suggested that simple selection or backcrossing would be useful for improving the resistance in market class varieties. The study was not conclusive on whether epistatic gene action played a major role, but if available it might have biased the dominance gene effects. Reciprocal effects (10%) were not significant (P > 0.05), suggesting that cytoplasmic genes did not play a major role in modifying anthracnose resistance. Parental lines G2333, AB136, NAT002, and NAT003 showed highly negative GCA effects qualifying them as suitable parents for transferring resistance genes to their progenies. A few major genes, 1–3, displaying partial dominance conditioned anthracnose resistance, suggesting a possibility of using marker-assisted selection to improve anthracnose resistance in market-class dry beans.