Breeding transgressive lines of pepper for resistance to Phytophthora capsici in a recurrent selection system

Summary In order to increase the resistance level of pepper (Capsicum annuum) to Phytophthora capsici the main sources of genetic resistance were intercrossed. The parents included 7 varieties from different origins of partial resistance to P. capsici and to other soilborne pathogens. Two cycles of recurrent selection were performed by 7 plant breeders who screened the plants at two developmental stages with various isolates and at two temperatures. Analysis of data revealed a strong influence of the testing conditions on the evaluation of the plants. The level of resistance of the progenies further depended on the pathogen isolate used to screen the plants. Improvement of the whole population was more significant during the first cycle and lines with enhanced level of resistance as compared to the parents i.e. transgressive lines were fixed from the two cycles. These lines were resistant to much higher inoculum concentrations than the original parents and the expression of resistance was stable at high temperature (32°C). These new sources of polygenic resistance were included into a new breeding program to improve the agronomic characters.     

Somaclonal variation for resistance to Verticillium dahliae in potato (Solanum tuberosum L.) plants regenerated from callus

Summary Plant tissue culture is recognized as an important tool to generate useful genetic variability for crop improvement. Regenerated plants from callus induced from stem explants of Solanum tuberosum cv Désirée were assessed by in vitro selection, for resistance to Verticillium dahliae. This fungus is the causal agent of Verticillium wilt, a serious vascular wilt disease both in crops and wild species.The rate of in vitro multiplication by single node cuttings was used as a parameter of screening in two selection cycles with different concentrations of V. dahliae filtrate. One resistant clone was selected and then evaluated by inoculation in the growth chamber. Induced damage, and morphological traits (dry weight, leaf area and tuber production) were estimated. The selected clone was comparable to the resistant control, cv Kondor.The results suggest that genetic variation induced in tissue culture cound be utilized to generate disease resistance.     

Transmission of Verticillium wilt resistance to tetraploid potato via unilateral sexual polyploidization

Summary Verticillium wilt is a serious disease of potato and is caused by the soil-borne fungi Verticillium dahliae and V. albo-atrum. No major cultivar is resistant to this disease. Two diploid interspecific potato clones, C287 and C545, were previously identified with consistently high levels of Verticillium wilt resistance and are thought to have the same genotype for the loci that confer resistance to V. dahliae stem colonization. The purpose of this study was to determine whether resistance to V. dahliae stem colonization could be transferred to the tetraploid level in potato via unilateral sexual polyploidization (USP). Progenies in eight families obtained by crossing C287 and C545 to two tetraploid breeding clones, S438 and S440, and the cultivar ‘Atlantic’ were planted in a V. dahliae-infested field and a field without a significant V. dahliae infestation. Resistance was evaluated relative to C545 and C287. There were differences among families for stem colonization and yield. No differences between the mean stem colonization of C545 and C287 progeny were detected. Family differences due to the tetraploid parents indicate that they contributed resistance to the progeny. Stem colonization data from this experiment were consistent with the proposed complementary two-gene model for Verticillium wilt resistance in the diploid parents. Unilateral sexual polyploidization is an effective method for transferring V. dahliae stem colonization resistance to the tetraploid level.     

In vitro techniques for selecting wheat (Triticum aestivum L.) for Fusarium-resistance. II. Culture filtrate technique and inheritance of Fusarium-resistance in the somaclones

Summary Calli of resistant, intermediary and susceptible wheat (Triticum aestivum L.) varieties were selected using culture filtrates of Fusarium graminearum and F. culmorum and the regenerants were evaluated for resistance up to R₃. Czapek-Dox broth medium was inoculated with mycelia of Fusarium isolates and incubated for 2–6 weeks. Filtrates were added to MS callus growing medium, then 5 weeks-old calli were transferred onto this medium (MST) for 4–5 weeks. MST containing 30% filtrate was found to be suitable for selection. Resistant calli were transferred again to fresh MST for further two selection cycles. The surviving calli produced less fertile regenerated lines (R₀) than the non-selected ones. Among 18 R₁ lines tested for Fusarium-resistance in the seedling stage by artificial inoculation in the greenhouse, two (11.1%) were significantly more resistant, one (5.6%) was more susceptible than the original cultivar and the rest (83.3%) behaved similarly to the donor plants. Among unselected R₃ lines of three varieties, practically the same number of resistant plants were found as among the related selected ones. When the R₃ selfed generations obtained through double-layer and culture filtrate selection techniques were tested for Fusarium-resistance, 35.7% of the lines were found to be more resistant than the original cultivars, none was more susceptible and 64.3% had a reaction similar to that of the source materials. Thus, inheritance of the disease reaction was not stable in all cases. Success of in vitro selection for Fusarium-resistance depended also on the genotype, and toxin analysis showed that although being effective, the selective media contained deoxynivalenol only exceptionally. In selecting wheat for Fusarium-resistance in vitro, the culture filtrate technique proved better than the double-layer procedure.     

Improvement of resistance to Fusarium head blight by recurrent selection in an intermating breeding spring wheat population using the dominant male-sterile gene ms 2

Four cycles of recurrent selection for FHB resistance were conducted in an intermating wheat breeding population using the dominant male-sterile gene ms ₂ during 1987–1991.Five cycles of phenotypic mass selection for male-sterile plants were evaluated using the soil-surface inoculation method in Experiment I. Experiment II evaluated changes in FHB scores during five cycles of progeny selection for fertile plants using the single-floret inoculation method. In Experiment I, the average level of FHB response increased to MR level in C₄, compared to MS level in C₀. The numbers of infected spikelets and diseased kernels decreased 0.32 and 2.68 per cycle, respectively. In Experiment II, the average level of FHB response increased to R level in C₄F₁. The numbers of infected spikelets and diseased kernels decreased 0.93 and 4.58 per cycle, respectively. In both experiments, the largest selection gains were realized in the first cycle. The frequencies of R and MR individuals were increased significantly. The frequencies of individuals with FHB response equal and/or superior to Sumai 3 were increased to 5–8% in C₄ and 25% in C₄F₁after the fourth cycle. Agronomic traits tended to be slightly improved in selected populations. Compared to 2% in C₀, about 34% of lines superior in both FHB resistance and agronomic traits in C₄F₁ were selected to enter the conventional breeding program for further evaluation. Sixty three semidwarf lines superior in both FHB resistance and yield potential were selected from the F₅ generations derived from C₁F₁ to C₄F₁. From them, two resistant cultivars with high-yielding potential were developed and commercialized in the Lower Yangtze Valley. Recurrent selection appears to be highly effective and feasible in shifting the average FHB response of the intermating population in the desirable direction, thereby enhancing the frequency of resistant individuals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transgressive segregation for resistance to yellow rust in wheat

Summary Crosses were made between wheat varieties Joss Cambier, Nord Desprez and Maris Bilbo, all classified as susceptible to yellow rust in field tests, and between Cappelle Desprez and Maris Huntsman, both classified as moderately and durably resistant. Selection for resistance to yellow rust among the progeny was carried out using races of Puccinia striiformis able to overcome all the known race-specific components of resistance in both parents of each cross. Lines with greater resistance than in both parents were obtained from each cross, those with greatest resistance being obtained from the cross between the moderately resistant parents. Three lines selected for resistance from the cross of Joss Cambier with Nord Desprez and one from the cross of Cappelle Desprez with Maris Huntsman, together with the parents, were tested in the field with 12 races of P. striiformis. Nord Desprez possessed a previously undetected race-specific component. The selected lines also displayed race-specific resistance, some of which was clearly related to race-specificity of the parents, and a component of resistance, greater than in both parents, that was effective against all 12 races. The possible origin and potential durability of this transgressive level of resistance is discussed. It is suggested that such transgressive resistance is more likely to be durable if it is derived from parents that have shown durable resistance.     

Effect of common bunt on the frost resistance and winter hardiness of wheat (Triticum aestivum L.) lines containing Bt genes

In order to determine the effects of bunt inoculation on frost resistance and winter hardiness in lines containing resistance genes, the bunt [Tilletia foetida (Wallroth) Liro, T. caries (DC.) Tulasne] susceptibility of wheat lines containing bunt resistance genesBt1 to Bt10 and the effect of the year on the degree of infection were studied over six years from 1991 to 1997 in an artificial inoculation nursery. Uninoculated and artificially inoculated wheat plants were tested for frost resistance in the phytotron in 1995 and in the field in boxes in three years from 1994/95 to 1996/97. The line withBt10 was very resistant, lines with Bt5, Bt6, Bt8 and Bt9 were resistant, the line with Bt4 was moderately resistant, those with Bt2 and Bt3 were moderately susceptible, the line with Bt1 was susceptible and the line with Bt7 was very susceptible to the local bunt population in Hungary. Bunt incidence also varied over years. The frost resistance of the Bt lines was generally lower after bunt inoculation than that of uninoculated plants. The increased frost kill in inoculated plants was not correlated with the extent of varietal susceptibility to bunt. Some lines with resistance, namely those with Bt5 (1.6% infection), Bt8 (0.6%) and Bt10 (0.0%), suffered significantly greater frost kill in the young plant stage as the result of bunt inoculation. By contrast, the Bt7line had excellent frost resistance and winter hardiness but suffered the greatest extent of bunt infection, whereas the Bt6 line had good frost resistance and good bunt resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic analysis of resistance to Karnal bunt (Tilletia indica (Mitra)) in bread wheat

Summary An analyis of an F1-based incomplete diallel was conducted involving 11 parents with different levels of resistance to Karnal bunt (Tilletia indica (Mitra)). It demonstrated that general combining ability (GCA) and thus additive or additive × additive gene effects were very important in the inheritance of resistance, accounting for 86.9% of the variation. Further analysis concentrated on F3 lines derived from individual random F2 plants from crosses with resistant varieties having the highest negative GCA effects. It was shown that the varieties Weaver and W499 have single dominant genes of resistance, which are different from each other, and which differ from a single allelic gene in varieties K342 and Cruz Alta. The majority of the crosses did not demonstrate a relationship between Karnal bunt infection and the number of days to heading. Resistant F3 lines varied in the number of days to heading from 80 to 100.     

Two unnecessary powdery mildew resistance genes in a synthetic rye population are neutral on fitness

A synthetic winter rye population was produced with two race-specific powdery mildew resistance genes, one dominant (Rm1) and the other (rm2) recessive, each at a frequency of about 0.50. The population was advanced by open-pollination in an isolated plot under mildew-free conditions for eight years. Samples of generations Syn-0 through Syn-7 were inoculated in the laboratory with two mildew isolates, one avirulent to either resistance gene, the other virulent to Rm1 and avirulent to rm2, to discriminate resistant and susceptible phenotypes. From the proportions of resistant plants, frequencies of Rm1 and rm2 were calculated and the fitness of carriers of resistance alleles was estimated in relation to carriers of susceptibility alleles at the two loci using continuous models and linear regression analyses. Frequencies of the two resistance genes oscillated only weakly over the eight generations. Coefficients of selection against Rm1-and rm2rm2 genotypes were –0.04 and –0.02, respectively, and not significantly different from zero. Thus the two resistance genes were selectively neutral. It is concluded that pyramiding of major powdery mildew resistance genes in rye varieties should not reduce their yield potential in the absence of mildew.     

Influence of plant age,disease incubation time,and host plant resistance on Verticillium wilt symptom expression in lucerne (Medicago sativa)

Summary The most economical means to control Verticillium wilt in lucerne, caused by Verticillium albo-atrum, is through resistant cultivars. Most Verticillium wilt resistance evaluations use some form of root-cut-soak inoculation procedure. In Trial 1, three lucerne cultivars were inoculated at eight plant ages and plant symptoms were scored at ten successive weekly intervals. Trial 2 consisted of two cultivars inoculated at ten plant ages and symptoms were evaluated at eight weekly intervals. In an experiment to study the genetics of resistance to this pathogen, four plants showing no visual symptoms were selected from each of four germplasms. The selected populations were crossed in diallel fashion. The population crosses plus the parents were evaluated for resistance to V. albo-atrum. Results from Trial 1 and Trial 2 showed disease severity increased with increasing inoculation age and with increasing scoring age. Cultivar symptom expression interacted with inoculation age, but not with scoring age. The diallel analysis supported resistance to Verticillium wilt being conferred by additive gene action, as heterosis was not detected and general combining ability mean square was 3.75 times larger than the specific combining ability mean square.     

Genetics and population improvement of resistance to bacterial soft rot in Chinese cabbage

Resistance to bacterial soft rot caused by Erwinia carotovora subsp. carotovora is a quantitative trait. The narrow-sense heritability was from 42% to 60% in the studied populations. Griffing's diallel analysis and generation mean analysis indicated that additive genetic effects were most significant, and the resistant sources involved in this study appeared to have different genes or alleles. Recurrent phenotypic selection was used for improving the resistance level by combining different resistance genes from the selected genotypes of Chinese cabbage. After 3 cycles of recurrent selection, the level of resistance increased markedly. When the cycle 3 population was compared with the cycle 0 population, the mean disease severity rating was reduced 38% from 6.3 to 3.9, and the percentage of plant survival increased from 65% to 97% based on the mean of two years experiments using mist-chamber seedling inoculation methods. The improvement of resistance was also demonstrated in field assays. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of the yield potential and yield components of F1 and F2 hybrids of crosses between wheat (Triticum aestivum L.) and spelt (Triticum spelta L.)

Summary Crosses of wild barley (Hordeum vulgare ssp. spontaneum and Hordeum vulgare ssp. agriocrithon) with Hordeum vulgare ssp. vulgare were used to select high yielding grain types under dryland Mediterranean conditions. No special difficulties were faced in making the crosses, in eliminating the brittle rachis genes from the grain types or in selecting 6-rowed types in crosses between 2-rowed wild barley and 6-rowed ssp. vulgare varieties. Brittle rachis genotypes, present in the segregating populations were used in developing self-reseeding permanent pastures for dry areas. The best selections were tested in seven trials during 1989–92 and some of them outyielded their parents and also the best improved check variety by 13–22%. Indications for transgressive segregation were obtained for grain yield, straw yield, total biological yield, harvest index and volume weight. The crude protein content of some of the selections was significantly higher than that of the checks. For breeding programs aiming at large seeds, special ssp. spontaneum lines should be used as parents. High grain yield was positively correlated with high straw yield, total biological yield, earliness in heading date, high harvest index and negatively with volume weight. It was concluded that unexploited useful genes, even when not directly observed in wild barley, could be transfered easily into high yielding genotypes by breeding.     

The genetic basis of scald resistance in western Canadian barley cultivars

Summary The genetic basis of resistance to scald (Rhynchosporium secalis) within barley breeding populations is poorly understood. The design of effective genetically based resistance strategies is predicated on knowledge of the identity of the resistance genes carried by potential parents. The resistance exhibited by a broad selection of western Canadian barley lines was investigated by evaluating their reactions to five R. secalis isolates. Results were compared to the resistance exhibited by previously characterized lines. This comparison, combined with pedigree analysis indicated that there are two different resistance genes present inwwestern Canadian cultivars. These genes were shown to be independent through analysis of a segregating population derived from a cross between Falcon and CDC Silky. This evidence, along with observed linkage of the gene in CDC Silky with an allele specific amplicon developed for a Rhynchosporium secalis resistance locus on chromosome 3, provides evidence that the gene in Falcon is the Rh2 gene derived from Atlas, and the gene (s) in CDC Silky is located within the Rh/Rh3/Rh4 cluster and is similar to the Rh gene in Hudson.     

Effects of recurrent selection for resistance to scab (Gibberella zeae) in wheat

Summary Scab caused by Gibberella zeae Petch., in common wheat, is one of the most severe diseases in China. A source population C0, bred for scab resistance, was developed through three cycles of multiple-parent crossing and intercrossing by means of the dominant male-sterile gene Ta1 (Ms2), according to Wu's scheme. Phenotypic recurrent selection methods for increasing the resistance to scab-infection of spikelets and seeds with the male-sterile plants were carried out simultaneously in Nanjing and Shanghai and at Jianyang, Fujian Province, for three cycles. The generations from C0 to C3 and two check cultivars were evaluated, using a randomized block design, under conditions of an artificially induced epidemic of scab during 1988–1990. The results indicate that there were significant differences in the resistance to scab between these generations. On average, the percentages of diseased spikelets and seeds of the male-fertile plants were reduced by 9% and 10%, respectively. The frequency of resistant plants was distinctly enhanced by recurrent selection. Analysis of variance showed that no significant differences existed between cycles of recurrent selection in agronomic characters such as plant height, spikes per plant, spike length, numbers of spikelets and seeds per spike, weight of seeds per spike and 100-kernel weight, days to heading and to maturity. Except for plant height, most of these traits tended to be slightly improved with improvement of resistance in the gene pool. The variance for resistance in the generations was decreased under selection. Recurrent selection for scab resistance using the dominant male-sterile gene Ta1 (Ms2) was both an effective and feasible breeding method for producing this character in wheat.     

Resistance in Cucumis sativus L. to Tetranychus urticae Koch. 5. raising the resistance level by the exploitation of transgression

summary After crossing partially resistant varieties some lines with a markedly higher resistance level were selected. This transgression for resistance indicates a polygenic inheritance of the resistance. On the most resistant F₅ lines selected, oviposition was reduced by 50 to 60% and the economic damage threshold was reached 5 to 8 weeks later than in the susceptible control. The level of acceptance was not influenced by the selection.The selection of individual F₂ plants was hampered by low heritabilities, whereas the heritabilities of F₃- and subsequent line means were generally high enough. Selection for higher resistance levels was attended by an unintended increase in cucurbitacin content. This is explained by linkage of genes for resistance and bitterness rather than by identity of these genes.     

Genetics of stem rust resistance in the spring wheat cultivar Thatcher and the enhancement of stem rust resistance by <Emphasis Type="Italic">Lr34</Emphasis>

Three recombinant inbred line populations from the crosses RL6071/Thatcher, RL6071/RL6058 (Thatcher Lr34), and Thatcher/RL6058, were used to study the genetics of stem rust resistance in Thatcher and TcLr34. Segregation of stem rust response in each population was used to determine the number of genes conferring resistance, as well as the effect of the leaf rust resistance gene Lr34 on stem rust resistance. The relationship between resistance in seedling and adult plants was also examined, and an attempt was made to identify microsatellite markers linked to genes that were effective in adult plants. In field plot tests at least three additive resistance genes segregated in the RL6071/RL6058 population, whereas two resistance genes segregated in the RL6071/Thatcher population. The presence of the gene Lr34 permitted the expression of additional stem rust resistance in Thatcher-derived lines both at the seedling and adult plant stages. Seedling resistance to races TPMK and RKQQ was significantly associated with resistance in adult plants, whereas seedling resistance to races QCCD and QCCB may have made a minor contribution. The seedling resistance genes Sr16 and Sr12 may have contributed to resistance in adult plants. A molecular marker linked to resistance in adult plants was identified on chromosome 2BL.     

Pedigree selection for Gibberella ear rot resistance in maize

The pedigree method is often used for developing inbred lines in maize (Zea mays L.). This study was conducted to assess the effectiveness of pedigree selection for improving resistance to Gibberella ear rot in four maize populations. Selection was based on the severity of ear rot symptoms after inoculation with macroconidial suspensions of Fusarium graminearum (Schwabe) into the silk channel (for two populations) and into the developing kernels (for two other populations). Samples of the selfed families (S₁ to S₅), recovered from remnant seed from the selection programs, were evaluated for disease resistance during three years, using inoculation and evaluation protocols similar to those used during selection. Among-family selection was effective in both of the populations selected after silk inoculation and in one of the populations selected after kernel inoculation. Responses to selection were more evident in later than in earlier generations for both types of inoculation. Changes in the estimated genetic gain over generations were consistent with changes in the variances among families, which tended to increase in early generations and to decrease in later generations. Selection after kernel inoculation seemed to have been more effective than selection after silk inoculation in developing families with more stable resistance. Based on the results obtained here, it seems that responses to family selection could be accelerated without increasing operational costs by increasing selection intensity in later generations and inoculating fewer plants per family.     

Sources of resistance to Verticillium wilt in Capsicum

Summary A total of 125 novel accessions of Capsicum annuum L. and Capsicum baccatum L. were screened for sources of resistance to Verticillium wilt (Verticillium dahliae Kleb). A soil infestation method with 2000 microsclerotia of V. dahliae per gram of planting medium, and a soil temperature of 25±3°C, identified 27 Capsicum accessions with resistance to Verticillium wilt. P.I. 215699 (a mixture of Capsicum baccatum var. microcarpum and Capsicum annuum), P.I. 535616 (Capsicum annuum), and P.I. 555614 (Capsicum annuum), had the lowest disease severity and the highest percentage of resistant plants.     

Identification of segregation-distortion-neutral alleles to improve pollen fertility of indica-japonica hybrids in rice (Oryza sativa L.)

The genetics of resistance to Ascochyta blight (Ascochyta fabae f. sp. fabae) was studied in two populations of faba bean (Vicia faba). Plants of a resistant population, ILB 752, and a susceptible one, NEB 463, were screened for their reaction to the pathogen and the results were quantified on a scale of 0–5. Crosses were made between plants both within and between accessions and the F₁ and F₂ generations assessed in a field trial 21 and 45 days after inoculation. Disease scores were greater at 45 days than at 21 days and they were not significantly affected by the presence of susceptible spreader rows in part of the trial. ILB 752 carried a major dominant gene conferring resistance while NEB 463 carried the recessive allele for susceptibility. Furthermore, a minority of plants of NEB 463 appeared to carry at least one pair of complementary recessive genes, also conferring resistance. Most of the plants of ILB 752 were homozygous for the dominant resistance gene and a few were heterozygous. Reciprocal crosses behaved identically, indicating the absence of maternal effects in the expression of Ascochyta blight resistance in faba beans. The results show that it is important to confirm the level of heterozygosity for the resistance genes in this partially outbreeding species before crossing is commenced. The major dominant gene for resistance, identified in ILB 752, has clear potential for use in breeding for Ascochyta blight resistance. The minor genes identified in NEB 463 also show the potential for accumulating resistance through mass selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

Expression of barley yellow dwarf virus and Russian wheat aphid resistance genes in and fertility of spring wheat × triticale hybrids and backcross lines

Summary The Barley Yellow Dwarf Virus disease (BYDV) and the Russian wheat aphid (RWA) Diuraphis noxia (Mordvilko) have caused significant losses to wheat and barley in several areas of the world. Important sources of resistance to both BYDV and RWA have been found in Triticale. Different generations of interspecific wheat x Triticale crosses were produced and the progenies were screened for BYDV and RWA tolerance. Plants with equal chromosome numbers showed different levels of fertility. A significant correlation was observed between pollen fertility and seed set in primary florets (r=0.57). In generaL, pollen fertility, seed set and the number of euploid plants (2n=42) increased from one generation to the next. The expression of BYDV tolerance varied from population to population. Additive effects were predominant in F₁ and some backcross populations. A dominant effect of rye tolerance genes was also observed in few populations. A monogenic trait or a quantitative (polygenic) character would not agree with the observed segregation patterns. The heritability of this oligogenic tolerance was quite different between populations and in many populations the tolerance genes were only partially expressed. Some transgressive segregation for tolerance and sensitivity was demonstrated. The genes controlling tolerance to RWA in Triticale lines, Muskox 658 and Nord Kivu were not expressed in advanced lines resistant to BYDV. This indicates that tolerance genes for BYDV and RWA in these lines are located on different chromosomes.