Susceptible F2 Plants Research Articles

Hereditary and molecular validity of pleiotropic resistance to rusts and powdery mildew in hexaploid wheat

To study the genetic basis of pleiotropic resistance to rusts and powdery mildew in hexaploid wheat, three Egyptian cultivars, Misr-1, Sids-13 and Sakha-94 were crossed to three CIMMYT cultivars carrying pleiotropic resistance genes i.e., Pavon-76 (Sr2/Yr30/Lr27, Lr46/Yr29/Sr58/Pm39), Jupateco-73-R (Lr34/Yr18/Sr57/Pm38) and Seri-82 (Lr26/Sr31/Yr9). In F2 progeny test, no segregation was found in four crosses, Pavon-76/Misr-1, Pavon-76/Sids-13, Jupateco-73-R/Sids-13, and Jupateco-73-R/Sakha-94, indicating the dominance of pleiotropic adult plant resistance (PAPR). Segregation ratios of the other five crosses fitted the expected ratios indicating the absence of these genes in the tested Egyptian cultivars. There was a difference between each of the two parents that was controlled by digenic recessive or dominance. Molecular screening revealed that three PAPR genes Sr2/Yr30/Lr27, Lr46/Yr29/Sr58/Pm39 and Lr34/Yr18/Sr57/Pm38 were present in the majority of resistant F2 plants, while, race-specific gene Lr26/Sr31/Yr9 was present mostly in susceptible F2 plants. PAPR genes should be considered for marker-assisted wheat breeding.

Fine Mapping of the Wheat Leaf Rust Resistance Gene LrLC10 (Lr13) and Validation of Its Co-segregation Markers.

Wheat leaf rust, caused by the fungus Puccinia triticina Eriks. (Pt), is a destructive disease found throughout common wheat production areas worldwide. At its adult stage, wheat cultivar Liaochun10 is resistant to leaf rust and the gene for that resistance has been mapped on chromosome 2BS. It was designated LrLC10 and is the same gene as cataloged gene Lr13 by pedigree analysis and allelism test. We fine-mapped it using recessive class analysis (RCA) of the homozygous susceptible F2 plants derived from crosses using Liaochun10 as the resistant, male parent. Taking advantage of the re-sequencing data of Liaochun10 and its counterpart susceptible parent, we converted nucleotide polymorphisms in the LrLC10 interval between the resistant and susceptible parents into molecular markers to saturate the LrLC10 genetic linkage map. Four indel markers were added in the 1.65 cM map of LrLC10 flanked by markers CAUT163 and Lseq22. Thirty-two recombinants were identified by those two markers from the 984 F2 homozygous susceptible plants and were further genotyped with additional ten markers. LrLC10 was finally placed in a 314.3 kb region on the Chinese Spring reference sequence (RefSeq v1.0) that contains three high confidence genes: TraesCS2B01G182800, TraesCS2B01G182900, and TraesCS2B01G183000. Sequence analysis showed several variations in TraesCS2B01G182800 and TraesCS2B01G183000 between resistant and susceptible parents. One KASP marker and an indel marker were designed based on the differences in those two genes, respectively, and were validated to be diagnostic co-segregating markers for LrLC10. Our results both improve marker-assisted selection and help with the map-based cloning of LrLC10.

Identification of a novel recessive gene for resistance to powdery mildew (Blumeria graminis f. sp. hordei) in barley (Hordeum vulgare)

AbstractOne of the most important diseases of barley (Hordeum vulgare) is powdery mildew, caused by Blumeria graminis f. sp. hordei. Spring barley line 173‐1‐2 was selected from a Moroccan landrace and revealed broad‐spectrum resistance to powdery mildew. The objective of this study was to map and characterize the gene for seedling powdery mildew resistance in this line. After crossing with the susceptible cultivar ‘Manchuria’, genetic analysis of F2 and F3 families at the seedling stage revealed powdery mildew resistance in line 173‐1‐2 conditioned by a single recessive gene. Molecular analysis of non‐segregating homozygous resistant and homozygous susceptible F2 plants conducted on the DArTseq platform (Diversity Arrays Technology Pty Ltd) identified significant markers which were converted to allele‐specific PCR markers and tested among 94 F2 individuals. The new resistance gene was mapped on the long arm of chromosome 6H. No other powdery mildew recessive resistance gene has been located on 6H so far. Therefore, we concluded that the 173‐1‐2 barley line carries a novel recessive resistance gene designated as mlmr.

Identifying a novel powdery mildew resistance gene in a barley landrace from Morocco

Powdery mildew is a barley foliar disease that causes great loss in yield. Because of the limited number of effective resistance genes, efforts to identify new sources of resistance are frequently focused on genetically diversified landraces. The goal of this study was to characterise the powdery mildew resistance gene in barley line 2553-3 selected from the Moroccan landrace. Phytopathological testing against a set of differential pathogen isolates revealed different pattern responses of this gene from those of other known resistance genes. F2 and F2:3 (2553-3 × Manchuria) mapping populations were employed to investigate resistance inheritance. Two approaches were applied for the linkage analysis: in the first approach, 22 resistant and 21 susceptible homozygous F2 plants genotyped by the DArTseq platform (Diversity Arrays Technology, Pty. Ltd.) were used; in the second, 94 F2 plants were genotyped by converted DArTseq markers and SSRs. Both analyses delineated a new resistance gene on the short arm of chromosome 2H. The authors propose MlMor as a gene symbol for newly characterized powdery mildew resistance genes in barley line 255-3-3. The results presented herein provide a good foundation for the development of closer linkage markers and MAS breeding.

Mapping a Resistance Gene to Puccinia graminis f. sp. tritici in the Bread Wheat Cultivar 'Matlabas'.

Puccinia graminis f. sp. tritici race TTKSF+ was collected from the South African wheat cultivar 'Matlabas' in 2010. F2 and F3 populations derived from a Matlabas × Line 37-07 cross segregated for a single resistance gene to race TTKSF that is avirulent to Matlabas. In screening genomic DNA bulks of susceptible or resistant F2 plants with simple sequence repeat (SSR) markers, three chromosome arm 2BS markers and one multilocus marker amplified alleles present only in the resistant bulks and Matlabas. Additional 2B-specific SSR markers, incorporating markers spanning regions containing Sr9h, SrWLR, Sr28, and Sr47, were screened in the parental lines and mapped in the F2 population. Linkage and QTL mapping showed that the gene is located between Xbarc160 in the centromeric region and Xgwm47 on the long arm of chromosome 2B. When 2B-specific SNP markers were mapped, the area of interest was delimited to a 15.3 cM region on chromosome arm 2BL, with XIWA543-HRM and Xgwm47 as flanking loci. Matlabas, Webster, and related Sr9h lines all produced a similar, low infection type to race TTKSF, but were susceptible to race TTKSF+. Phenotypic data and allelic studies suggested that stem rust resistance in Matlabas was derived from an Sr9h source.

Quantitative resistance against Bemisia tabaci in Solanum pennellii: Genetics and metabolomics.

The whitefly Bemisia tabaci is a serious threat in tomato cultivation worldwide as all varieties grown today are highly susceptible to this devastating herbivorous insect. Many accessions of the tomato wild relative Solanum pennellii show a high resistance towards B. tabaci. A mapping approach was used to elucidate the genetic background of whitefly-resistance related traits and associated biochemical traits in this species. Minor quantitative trait loci (QTLs) for whitefly adult survival (AS) and oviposition rate (OR) were identified and some were confirmed in an F2 BC1 population, where they showed increased percentages of explained variance (more than 30%). Bulked segregant analyses on pools of whitefly-resistant and -susceptible F2 plants enabled the identification of metabolites that correlate either with resistance or susceptibility. Genetic mapping of these metabolites showed that a large number of them co-localize with whitefly-resistance QTLs. Some of these whitefly-resistance QTLs are hotspots for metabolite QTLs. Although a large number of metabolite QTLs correlated to whitefly resistance or susceptibility, most of them are yet unknown compounds and further studies are needed to identify the metabolic pathways and genes involved. The results indicate a direct genetic correlation between biochemical-based resistance characteristics and reduced whitefly incidence in S. pennellii.

Identification and mapping stripe rust resistance gene YrLM168a using extreme individuals and recessive phenotype class in a complicate genetic background.

The identification and characterization of resistance genes effective against stripe rust of wheat is beneficial for modern wheat breeding programs. Molecular markers to such genes facilitate their deployment. The variety Milan has resistance that is effective against the predominant stripe rust races in the Sichuan region. Two resistant and two susceptible F8 lines from a cross between Milan and the susceptible variety Chuannong 16 were used to investigate inheritance of the Milan resistance. Three F2 populations were developed from crosses between the resistant lines and their susceptible sibling lines (LM168a×LM168c, LM168c×LM168a, LM168b×LM168d) and used for genetic analysis and molecular mapping of the genes for resistance. The stripe rust resistance in LM168a and LM168b was conferred by a single dominant gene, temporarily designated as YrLM168a. Forty-five extreme susceptible plants from the F2 families of LM168d×LM168b were genotyped with 836 simple sequence repeat (SSR) markers to map YrLM168a. YrLM168a was mapped in chromosome 6BL. The nearest flanking markers Xwmc756 and Xbarc146 were 4.6 and 4.6cM away from the gene at both sides, respectively. The amplification results of twenty extreme resistant (IT 0) and susceptible (IT 4) F2 plants of LM168c×LM168a and LM168a×LM168c with marker Xwmc756 further validated the mapping results. The study suggested that extreme individuals and recessive phenotype class can be successfully used for mapping genes, which should be efficient and reliable. In addition, the flanking markers near YrLM168a should be helpful in marker-assisted breeding.

Identification and mapping of Sr46 from Aegilops tauschii accession CIae 25 conferring resistance to race TTKSK (Ug99) of wheat stem rust pathogen.

Mapping studies confirm that resistance to Ug99 race of stem rust pathogen in Aegilops tauschii accession Clae 25 is conditioned by Sr46 and markers linked to the gene were developed for marker-assisted selection. The race TTKSK (Ug99) of Puccinia graminis f. sp. tritici, the causal pathogen for wheat stem rust, is considered as a major threat to global wheat production. To address this threat, researchers across the world have been devoted to identifying TTKSK-resistant genes. Here, we report the identification and mapping of a stem rust resistance gene in Aegilops tauschii accession CIae 25 that confers resistance to TTKSK and the development of molecular markers for the gene. An F2 population of 710 plants from an Ae. tauschii cross CIae 25 × AL8/78 were first evaluated against race TPMKC. A set of 14 resistant and 116 susceptible F2:3 families from the F2 plants were then evaluated for their reactions to TTKSK. Based on the tests, 179 homozygous susceptible F2 plants were selected as the mapping population to identify the simple sequence repeat (SSR) and sequence tagged site (STS) markers linked to the gene by bulk segregant analysis. A dominant stem rust resistance gene was identified and mapped with 16 SSR and five new STS markers to the deletion bin 2DS5-0.47-1.00 of chromosome arm 2DS in which Sr46 was located. Molecular marker and stem rust tests on CIae 25 and two Ae. tauschii accessions carrying Sr46 confirmed that the gene in CIae 25 is Sr46. This study also demonstrated that Sr46 is temperature-sensitive being less effective at low temperatures. The marker validation indicated that two closely linked markers Xgwm210 and Xwmc111 can be used for marker-assisted selection of Sr46 in wheat breeding programs.

Molecular mapping of leaf rust resistance gene LrNJ97 in Chinese wheat line Neijiang 977671

Neijiang 977671 and 19 near-isogenic lines with known leaf rust resistance genes were inoculated with 12 pathotypes of Puccinia triticina for postulation of leaf rust resistance genes effective at the seedling stage. The reaction pattern of Neijiang 977671 differed from those of the lines with known leaf rust resistance genes used in the test, indicating that Neijiang 977671 may carry a new leaf rust resistance gene(s). With the objective of identifying and mapping the new gene for resistance to leaf rust, F1 and F2 plants, and F2:3 families, from Neijiang 977671×Zhengzhou 5389 (susceptible) were inoculated with Chinese P. triticina pathotype FHNQ in the greenhouse. Results from the F2 and F2:3 populations indicated that a single dominant gene, temporarily designated LrNJ97, conferred resistance. In order to identify other possible genes in Neijiang 977671 other eight P. triticina pathotypes avirulent on Neijiang 977671 were used to inoculate 25 F2:3 families. The results showed that at least three leaf rust resistance genes were deduced in Neijiang 977671. Bulked segregant analysis was performed on equal amounts of genomic DNA from 20 resistant and 20 susceptible F2 plants. SSR markers polymorphic between the resistant and susceptible bulks were used to analyze the F2:3 families. LrNJ97 was linked to five SSR loci on chromosome 2BL. The two closest flanking SSR loci were Xwmc317 and Xbarc159 at genetic distances of 4.2 and 2.2cM, respectively. At present two designated genes (Lr50 and Lr58) are located on chromosome 2BL. In the seedling tests, the reaction pattern of LrNJ97 was different from that of Lr50. Lr50 and Lr58 were derived from T. armeniacum and Ae. triuncialis, respectively, whereas according to the pedigree of Neijiang 977671 LrNJ97 is from common wheat. Although seeds of lines with Lr58 were not available, it was concluded that LrNJ97 is likely to be a new leaf rust resistance gene.

Plant metabolomics: Resolution and quantification of elusive peaks in liquid chromatography–mass spectrometry profiles of complex plant extracts using multi-way decomposition methods

Previous studies on LC–MS metabolomic profiling of 127 F2 Barbarea vulgaris plants derived from a cross of parental glabrous (G) and pubescent (P) type, revealed four triterpenoid saponins (hederagenin cellobioside, oleanolic acid cellobioside, epihederagenin cellobioside, and gypsogenin cellobioside) that correlated with resistance of plants against the insect herbivore, Phyllotreta nemorum. In this study, for the first time, we demonstrate the efficiency of the multi-way decomposition method PARAllel FACtor analysis 2 (PARAFAC2) for exploring complex LC–MS data. PARAFAC2 enabled automated resolution and quantification of several elusive chromatographic peaks (e.g. overlapped, elution time shifted and low s/n ratio), which could not be detected and quantified by conventional chromatographic data analysis. Raw LC–MS data of 127 F2 B. vulgaris plants were arranged in a three-way array (elution time point×mass spectra×samples), divided into 17 different chromatographic intervals and each interval were individually modeled by PARAFAC2. Three main outputs of the PARAFAC2 models described: (1) elution time profile, (2) relative abundance, and (3) pure mass spectra of the resolved peaks modeled from each interval of the chromatographic data. PARAFAC2 scores corresponding to relative abundances of the resolved peaks were extracted and further used for correlation and partial least squares (PLS) analysis. A total of 71 PARAFAC2 components (which correspond to actual peaks, baselines and tails of neighboring peaks) were modeled from 17 different chromatographic retention time intervals of the LC–MS data. In addition to four previously known saponins, correlation- and PLS-analysis resolved five unknown saponin-like compounds that were significantly correlated with insect resistance. The method also enabled a good separation between resistant and susceptible F2 plants. PARAFAC2 spectral loadings corresponding to the pure mass spectra of chromatographic peaks matched well with experimentally recorded mass spectra (correlation based similarity >95%). This enabled to extract pure mass spectra of highly overlapped and low s/n ratio peaks.

Molecular Mapping of a Gene for Resistance to Stripe Rust in Wheat Variety PIW138

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most damaging diseases of common wheat (Triticum aestivum L.). Wheat variety PIW138 introduced from Pakistan is resistant to the currently prevailing Pst race CYR32 in China. In this study, the bulked segregant analysis (BSA) method and simple sequence repeat (SSR) markers were used to map the stripe rust resistance gene in PIW138. The resistant and susceptible DNA bulks were prepared from the segregating F2 population of the cross between Thatcher, a susceptible variety as the female parent, and PIW138 as the male parent. The segregation of resistant and susceptible F2 plants inoculated with CYR32 indicated that single dominant gene determined the reactions of PIW138 line and temporarily designated as YrP138. Total 200 SSR primers were screened, and 4 SSR markers, Xwmc52, Xbarc61, Xgwm268, and Xgwm153, on chromosome 1B were found to be polymorphic between the resistant and the susceptible DNA bulks as well as their parents. Genetic linkage was tested on the segregating F2 population with 259 plants, including 196 resistant and 63 susceptible plants. All 4 SSR markers were linked to the stripe rust resistance gene in PIW138. The genetic distances of Xwmc52, Xbarc61, Xgwm268, and Xgwm153 to the resistance gene were 29.8, 6.2, 6.8, and 8.2 cM, respectively.

Genetic and physical mapping of blast resistance gene Pi-42(t) on the short arm of rice chromosome 12

We have identified, genetically mapped and physically delimited the chromosomal location of a new blast resistance gene from a broad spectrum resistant genotype ‘DHR9’. The segregation analysis of an F2 progeny of a cross between a susceptible cv. ‘HPU741’ and the resistant genotype ‘DHR9’ suggested that the resistance was conditioned by a single dominant gene. A RAPD marker, OPA82000, linked to the resistance gene was identified by the linkage analysis of 109 F2 individuals. By chromosomal landing of the sequence of RAPD marker on the sequence of reference cv. Nipponbare, the gene was mapped onto rice chromosome 12. Further linkage analysis with two polymorphic simple sequence repeat (SSR) markers, RM2529 and RM1337 of chromosome 12, confirmed the chromosomal localization of the resistance gene. Based on linkage analysis of 521 susceptible F2 plants and comparative haplotype structure analysis of the parental genotypes with SSR and sequence tagged site (STS) markers developed from the Nipponbare PAC/BAC clones of chromosome 12, the resistance gene was delimited within a 2 cM interval defined by STS marker, STS5, on the telomeric side and SSR marker, RRS6 on the centromeric side. By aligning the sequences of linked markers on the sequence of cv. Nipponbare, a ~4.18 Mb cross-over cold region near the centromere of chromosome 12 was delineated as the region of blast resistance gene. In this region, six putatively expressed NBS-LRR genes were identified by surveying the equivalent genomic region of cv. Nipponbare in the TIGR Whole Genome Annotation Database (http://www.tigr.org). NBS-LRR locus, LOC_Os12g18374 situated in BAC clone OJ1115_G02 (Ac. No. AL772419) was short-listed as a potential candidate for the resistance gene identified from DHR9. The new gene was tentatively designated as Pi-42(t). The markers tightly linked to gene will facilitate marker-assisted gene pyramiding and cloning of the resistance gene.

Molecular mapping of rice blast resistance gene Pi-1(t) in the elite indica variety Samba mahsuri

Samba mahsuri (BPT 5204) is a cultivar of the medium slender grain indica variety of Oryza sativa grown across India for its high yield and quality. However, this cultivar is susceptible to several diseases and pests including rice blast. The analysis of near isogenic lines indicated the presence of a resistance gene, Pi-1(t) in the donor cultivar C101LAC which is highly resistant to the rice blast fungus Magnaporthe grisea (M. grisea). C101LAC was crossed with susceptible indica rice cultivar (BPT 5204) to generate the mapping population. A mendelian segregation ratio of 3:1 for resistant to susceptible F2 plants using bulk segregation analysis confirmed the presence of a major gene pi-1(t) by simple sequence repeats marker RM224 to the highly virulent blast isolate DRR 001.

Identification of SSR markers for a broad-spectrum blast resistance gene Pi20(t) for marker-assisted breeding

The Pi20(t) gene was determined to confer a broad-spectrum resistance against diverse blast pathotypes (races) in China based on inoculation experiments utilizing 160 Chinese Magnaporthe oryzae (formerly Magnaporthe grisea) isolates, among which isolate 98095 can specifically differentiate the Pi20(t) gene present in cv. IR24. Two flanking and three co-segregating simple sequence repeat (SSR) markers for Pi20(t), located near the centromere region of chromosome 12, were identified using 526 extremely susceptible F2 plants derived from a cross of Asominori, an extremely susceptible cultivar, with resistant cultivar IR24. The SSR OSR32 was mapped at a distance of 0.2 cM from Pi20(t), and the SSR RM28050 was mapped to the other side of Pi20(t) at a distance of 0.4 cM. The other three SSR markers, RM1337, RM5364 and RM7102, co-segregated with Pi20(t). RM1337 and RM5364 were found to be reliable markers of resistance conditioned by Pi20(t) in a wide range of elite rice germplasm in China. As such, they are useful tags in marker-assisted rice breeding programs aimed at incorporating Pi20(t) into advanced rice breeding lines and, ultimately, at obtaining a durable and broad spectrum of resistance to M. oryaze.

A high‐resolution map of the rice blast resistance gene Pi15 constructed by sequence‐ready markers

AbstractThe gene Pi15 for resistance of rice to Magnaporthe grisea was previously mapped to a ≈0.7‐cM region on chromosome 9. To further define the chromosomal region of the Pi15 locus, a contig spanning the locus was constructed, in silico, through bioinformatics analysis using a reference sequence of the cultivar ‘Nipponbare’. One simple sequence repeat marker adopted from the International Rice Microsatellite Initiative and six candidate resistance gene (CRG) markers, developed from gene annotation of the reference sequence of the contig, were used for linkage analysis in a mapping population consisting of 504 extremely susceptible F2 plants. The Pi15 locus was delimited to a ≈0.5‐cM region flanked by the markers CRG5 and CRG2 and co‐segregated with the markers BAPi15782, CRG3 and CRG4, which was physically converted to a 44‐kb interval.

Characterization of Wheat-Triticale Lines Resistant to Powdery Mildew, Stem Rust, Stripe Rust, Wheat Curl Mite, and Limitation on Spread of WSMV.

High yield potential and the wide adaptability of wheat-rye T1BL·1RS translocation lines are attractive to breeders. The wheat-rye lines Lankao 1, 3, 4, and 5 were resistant to a wide spectrum of wheat powdery mildew (Blumeria graminis f. sp. tritici) isolates from both China and Canada. They also were resistant to a mixture of wheat stem rust (Puccinia graminis f. sp. tritici) pathotypes (98WSR) and wheat stripe rust (P. striiformis f. sp. tritici) races from western Canada and China. Colonization of wheat curl mite (WCM) (Aceria tosichella) resulted in slower development of rolling and trapping leaves in the Lankao lines than in the WCM-susceptible check cultivars. The delayed development of Wheat streak mosaic (WSM) symptoms on Lankao lines was observed when transmitted by viruliferous WCM, even though they were susceptible to Wheat streak mosaic virus (WSMV). This effect of Lankao lines on limiting the spread of WSM was comparable with other known sources of WCM resistance. Sequential C-banding and genomic in situ hybridization analyses revealed the presence of a pair of T1BL·1RS translocated chromosomes in the Lankao lines. Segregation analysis of the F2 progeny plants derived from crosses between Lankao 4 and the susceptible wheat cvs. Mingxian 169 and Lovrin 13 indicated that a single dominant gene was responsible for the isolate-specific resistance against wheat powdery mildew in Lankao 4. Polymerase chain reaction analysis using an STS marker amplified rye chromatin in powdery mildew-resistant and -susceptible F2 plants of the Mingxian 169 × Lankao 4 cross demonstrated that the resistance of Lankao 4 was not controlled by a gene or genes located on the rye chromosome arm of T1BL·1RS. The resistance of the Lankao lines to diseases and limitation of the spread of WSMV, in combination with good quality and high yield potential, makes them useful for wheat improvement and production.

Soybean Aphid Resistance in Soybean Jackson Is Controlled by a Single Dominant Gene

The soybean aphid, Aphis glycines Matsumura, has become established as a serious pest of soybean, Glycine max (L.) Merr., since it was first found in North America in 2000 and has caused millions of dollars in economic losses. While the application of chemical insecticides is the only means to control the soybean aphid at present, genetic resistance to the aphid was recently discovered in soybean. A single dominant gene named Rag1 that controls resistance to the soybean aphid was found in the cultivar Dowling. Another cultivar found to have strong antibiosis‐type resistance to the soybean aphid was Jackson. The primary objective of this study was to determine the inheritance of resistance to the soybean aphid in Jackson. Segregation of resistance was analyzed in F2 and among F2–derived F3 (F2:3) families produced from crosses between Jackson and the susceptible soybean cultivar Loda. Segregation of F2 plants was 247 resistant to 97 susceptible and fit a 3:1 genetic ratio (P = 0.17). Segregation among F2:3 families was not clear because a number of susceptible F2 plants did not produce a sufficient amount of seed for progeny testing. Ignoring the susceptible class, the segregation of F2:3 families fit a 1:2 (all resistant/segregating) ratio. These results indicated that a single dominant gene controlled resistance in Jackson. There is no known genetic relationship between Jackson and Dowling. The genetic relationship between Rag1 in Dowling and the gene in Jackson is unknown.

Genetics of resistance to cotton leaf curl disease in Gossypium hirsutum

Twenty‐two cotton varieties were screened for resistance to cotton leaf curl disease (CLCuD), a disease of viral origin, using three procedures: field evaluation, whitefly transmission assay and graft inoculation. Viral infection of cotton varieties was determined by visual symptom assessment as well as dot‐blot and multiplex PCR diagnostic techniques. Crosses were made between the most susceptible variety (S‐12) and highly resistant varieties (CP‐15/2, LRA‐5166 and CIM‐443). All F1 plants of these crosses were resistant, showing dominant expression of the resistance as well as the absence of extrachromosomal inheritance. The F2 plants of the crosses CP‐15/2 × S12, LRA‐5166 × S‐12 and CIM‐443 × S12 exhibited a ratio of 13 resistant (symptomless) to three susceptible (with symptoms). Screening of the F2 generation for virus infection by multiplex PCR further subdivided the resistant class into those exhibiting a high level of resistance (HR; PCR‐negative) and those exhibiting resistance (R; symptomless, yet showing virus replication by PCR analysis). Hence, the final ratio was 3:10:3 (HR:resistant:susceptible). The F3 progeny of susceptible F2 plants segregated for resistance, indicating the probable presence of a suppressor gene (S). These findings are consistent with three genes being involved in G. hirsutum resistance to CLCuD, two for resistance (R1CLCuDhir and R2CLCuDhir) and a suppressor of resistance (SCLCuDhir).

A single recessive gene confers effective resistance to powdery mildew of field pea grown in northern New South Wales

Reactions to powdery mildew (Erysiphe pisi Syd.) of a range of varieties, advanced lines and pea germplasm from the Australian Temperate Field Crops Collection (ATFCC) were evaluated under field conditions at Narrabri, New South Wales. Thirteen resistant genotypes, including 3 M257 sister lines [M257-2-1 (Mukta), M257-3-6 and M257-5-1], Glenroy, Kiley and PSI 11 from Australian breeding programs, and LE 25 (ATC 1275), ATC 649, ATC 767, ATC 823, ATC 1036, ATC 1121 and ATC 1181 from the ATFCC, along with 8 susceptible lines Dinkum, Bohatyr, Jupiter, Greenfeast, Solara, P441-6, Trapper and Cressy Blue, were used to make resistant × resistant, resistant × susceptible and susceptible × susceptible combinations to examine the inheritance of resistance. Parental lines and F1 and F2 populations were evaluated in the field under natural disease epiphytotics at the Plant Breeding Institute, Narrabri, during the winters of 1996 and 1997. In 1996, resistant and susceptible F2 plants were selected from 5 crosses, and further examined as F3 families in 1997. No fully resistant line was identified among the parents. F1 reactions indicated that resistance was governed by recessive genes and there were no maternal effects. Segregation patterns in the F2 and F3 generations supported the hypothesis of resistance in the genotypes M257-5-1, Glenroy, Kiley, ATC 649 and ATC 1121 being conferred by the same single recessive gene.

Segregation distortion of Brassica carinata derived black rot resistance in Brassica oleracea

Three segregating F2 populations were developed by self-pollinating 3 black rot resistant F1 plants, derived from across between black rot resistant parent line 11B-1-12 and the susceptible cauliflower cultivar ‘Snow Ball’. Plants were wound inoculated using 4 isolates ofXanthomonas campestris pv. campestris (Xcc) race 4, and disease severity ratings of F2 plants from the three populations were scored. A total of 860 arbitrary oligonucleotide primers were used to amplify DNA from black rot resistant and susceptible F2 plants and bulks. Eight RAPD markers amplified fragments associated with completely disease free plants following black rot inoculation,which segregated in frequencies far lower than expected. Segregation of markers with black rot resistance indicates that a single, dominant major gene controls black rot resistance in these plants. Stability of this black rot resistance gene in populations derived from 11B-1-12 may complicate introgression into B. oleracea genotypes for hybrid production.