Undescribed wheat gene for partial leaf rust and stripe rust resistance from Thatcher derivatives RL6058 and 90RN249 carryingLr34

Inheritance of partial leaf rust and stripe rust resistance of a Thatcher wheat 90RN2491, earlier reported to carry two doses of the gene pairLr34-Yr18 and the reference line RL6058 (6*Thatcher/PI58548) for theLr34-Yr18 gene pair was studied against predominant and highly virulent Indian races. Thatcher derivatives 90RN2491 and RL6058 were intercrossed as well as crossed with the leaf rust and stripe rust susceptible Indian cultivar WL711. The F₁, F₂ and F₃ generations from these crosses were assessed for rust severity against leaf rust race 77-5 and stripe rust race 46S119. The F₂ and F₃ generations from the crosses of RL6058 and 90RN2491 with WL711, segregated 15 resistant : 1 susceptible (F₂) and 7 homozygous resistant : 8 segregating : 1 homozygous susceptible (F₃) ratios, respectively, both for leaf rust and stripe rust severity. Therefore, partial resistance against each of the leaf rust and stripe rust races in both RL6058 and 90RN2491 is ascribed to two independently inherited dominant genes. One of the two genes for leaf rust and stripe rust resistance in 90RN2491 and RL6058 isLr34 and the linked geneYr18, respectively. The second leaf rust resistance gene in both the Thatcher lines segregated independently of stripe rust resistance. Therefore, it is notLr34 and it remains unidentified.     

Inheritance of components of partial resistance of barley to Rhynchosporium secalis with particular reference to race specificity

Six spring barley cultivars with no known genes for resistance to specific virulences but varying in partial resistance to Rhynchosporium secalis, were crossed in all combinations (6 × 6 diallel including reciprocals). In addition to seeds from naturally selfed plants, seeds of all parent cultivars were also produced by artificial selfing (emasculation followed by pollination using pollen from the same cultivar). This ensured comparability between seeds of parents and F₁. Both sets of parents, F₁ and F₂ families were grown in the field as single spaced plants and inoculated at Zadoks growth stage 49 with spore suspensions (2 × 10⁶ spores ml^-1) of three races (pathotypes) of R. secalis (Zadoks, Chang & Konzak, 1974). Components of partial resistance, incubation period (ICP), infection frequency (IF) and spore production per lesion (SP/L) were assessed on each plant. There were highly significant differences for all three components of partial resistance in both sets of parent cultivars but rank order in both sets was similar as evidenced by correlation coefficients, r= 0.96 for ICP and IF and r= 0.87 for SP/L. All three components of partial resistance were strongly correlated with NIAB (National Institute of Agricultural Botany, Cambridge, UK) resistance ratings. Means of F₁ and F₂ families were correlated with mid-parent values for ICP and IF but not SP/L. No difference in aggressiveness was found between races but for each component of partial resistance there was a significant interaction between race and parent cultivar (artificial selfs) and, for IF and ICP, a significant interaction between race and F₁ family. There was no evidence of interaction between parent (natural selfs) and race nor between race and F₂ family. Examination of genetic control of resistance showed evidence of strong additive effects (combining ability) in both F₁ and F₂ for ICP and IF but not for SP/L. There was no evidence for maternal or reciprocal differences, but there was evidence for dominance effects although their nature differed between components of partial resistance and between F₁ and F₂ generations. In the F₁, but not the F₂ generation, several elements of dominance (direction, distribution of dominant genes between parent cultivars, specific combining ability) showed for ICP or IF (but not SP/L) significant interaction with race.     

The genetical control of incomplete forms of resistance to Erysiphe graminis in spring barley

Eight spring barley lines exhibiting incomplete resistance to powdery mildew in field trials were crossed and back-crossed to a common susceptible parent to produce the basic generations (P₁ P₂, F₁ F₂, B₂ B₂) required for a genetical investigation. By means of joint scaling tests, disease assessments on adult plants in a field trial were used to estimate the genetic parameters contributing to resistance. Lines with major genes, which appeared partially resistant due to the presence of corresponding virulence, were characterised by the presence of a significant dominance component in the analysis. Partially resistant lines which either lacked identifiable major genes or with major genes that were ineffective in the trial, but with good background resistance, could also be distinguished. Significant additive and epistatic effects were detected in all crosses. The prospect of enhancing partial resistance through hybridisation and selection is discussed.     

Molecular mapping of <Emphasis Type="Italic">Rym17</Emphasis>, a dominant and <Emphasis Type="Italic">rym18</Emphasis> a recessive barley yellow mosaic virus (BaYMV) resistance genes derived from <Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.

PK23-2, a line of six-rowed barley (Hordeum vulgare L.) originating from Pakistan, has resistance to Japanese strains I and III of the barley yellow mosaic virus (BaYMV). To identify the source of resistance in this line, reciprocal crosses were made between the susceptible cultivar Daisen-gold and PK23-2. Genetic analyses in the F₁ generation, F₂ generation, and a doubled haploid population (DH45) derived from the F₁ revealed that PK23-2 harbors one dominant and one recessive resistance genes. A linkage map was constructed using 61 lines of DH45 and 127 DNA markers; this map covered 1268.8 cM in 10 linkage groups. One QTL having a LOD score of 4.07 and explaining 26.8% of the phenotypic variance explained (PVE) for resistance to BaYMV was detected at DNA marker ABG070 on chromosome 3H. Another QTL having a LOD score of 3.53 and PVE of 27.2% was located at marker Bmag0490 on chromosome 4H. The resistance gene on chromosome 3H, here named Rym17, showed dominant inheritance, whereas the gene on chromosome 4H, here named rym18, showed recessive inheritance in F₁ populations derived from crosses between several resistant lines of DH45 and Daisen-gold. The BaYMV recessive resistance genes rym1, rym3, and rym5, found in Japanese barley germplasm, were not allelic to rym18. These results revealed that PK23-2 harbors two previously unidentified resistance genes, Rym17 on 3H and rym18 on 4H; Rym17 is the first dominant BaYMV resistance gene to be identified in primary gene pool. These new genes, particularly dominant Rym17, represent a potentially valuable genetic resource against BaYMV disease.     

Heritable,De Novo Resistance to Leaf Rust and Other Novel Traits in Selfed Descendants of Wheat Responding to Inoculation with Wheat Streak Mosaic Virus

Stable resistance to infection with Wheat streak mosaic virus (WSMV) can be evolved de novo in selfing bread wheat lines subjected to cycles of WSMV inoculation and selection of best-performing plants or tillers. To learn whether this phenomenon might be applied to evolve resistance de novo to pathogens unrelated to WSMV, we examined the responses to leaf rust of succeeding generations of the rust- and WSMV-susceptible cultivar ‘Lakin’ following WSMV inoculation and derived rust-resistant sublines. After three cycles of the iterative protocol five plants, in contrast to all others, expressed resistance to leaf and stripe rust. A subset of descendant sublines of one of these, ‘R1’, heritably and uniformly expressed the new trait of resistance to leaf rust. Such sublines, into which no genes from a known source of resistance had been introgressed, conferred resistance to progeny of crosses with susceptible parents. The F₁ populations produced from crosses between, respectively, susceptible and resistant ‘Lakin’ sublines 4-3-3 and 4-12-3 were not all uniform in their response to seedling inoculation with race TDBG. In seedling tests against TDBG and MKPS races the F₂s from F₁ populations that were uniformly resistant had 3∶1 ratios of resistant to susceptible individuals but the F₂s from susceptible F₁ progenitors were uniformly susceptible. True-breeding lines derived from resistant individuals in F₂ populations were resistant to natural stripe and leaf rust inoculum in the field, while the ‘Lakin’ progenitor was susceptible. The next generation of six of the ‘Lakin’-derived lines exhibited moderate to strong de novo resistance to stem rust races TPMK, QFCS and RKQQ in seedling tests while the ‘Lakin’ progenitor was susceptible. These apparently epigenetic effects in response to virus infection may help researchers fashion a new tool that expands the range of genetic resources already available in adapted germplasm.     

Genetics and molecular mapping of resistance to necrosis inducing race 5 of <Emphasis Type="Italic">Pyrenophora tritici-repentis</Emphasis> in tetraploid wheat

Race 5 of Pyrenophora tritici-repentis, causal agent of tan spot, induces two distinct symptoms, necrosis and chlorosis in susceptible tetraploid and hexaploid wheat, respectively. This study was conducted under controlled environmental conditions to determine the inheritance of resistance to P. tritici-repentis, race 5, in a tetraploid wheat population and to map the resistance genes. Additionally, the relationship between the resistance genes effective against necrosis inducing races 3 and 5 in tetraploid wheat was determined. A population of 98 recombinant-inbred lines (RIL) was developed from a cross between the resistant genotype Triticum turgidum # 283 (PI352519) and the susceptible durum cultivar Coulter. This RIL population was screened individually with race 3 and race 5 and molecular mapping of the resistance gene(s) in this population was conducted. Additionally, the F₂ and F_4:5 generations of this population were screened with race 5 to determine the genetic control of resistance. Plants were inoculated at the two-leaf stage and disease reaction was assessed based on 1 to 5 lesion-type rating scale eight days after inoculation. Segregation analysis of the F₂ generation and of the F_4:5 and F_6:7 families indicated that a single recessive gene controlled resistance to necrosis induced by race 5. Analysis of the mapping data of the T. turgidum # 283/Coulter RIL population indicate that a major gene, designated tsn5, controlling resistance to race 5 is located on the long arm of chromosome 3B. The tsn5 gene is 8.3 cM proximal to the gene tsn2 that controls resistance to necrosis induced by race 3.     

Genetic and molecular analysis of wheat tan spot resistance effective against <Emphasis Type="Italic">Pyrenophora tritici-repentis</Emphasis> races 2 and 5

Tan spot, a major foliar disease of wheat (Triticum aestivum L.), is caused by an ascomycete Pyrenophora tritici-repentis. Both culture filtrates and conidiospore inocula induce disease symptoms in susceptible wheat genotypes. The objectives of this study were to determine and map the genetic control of resistance to spore inocula and culture filtrates of P. tritici-repentis races 2 and 5. The F₁ and F₂ generations and an F_2:6 recombinant inbred lines (RIL) population were developed from a cross between the resistant ND 735 and the susceptible Steele-ND. Disease assessments of the segregating generations were done at the seedling stage using culture filtrates and spore inocula under controlled environmental conditions. Genetic and mapping analyses of the F₁ and F₂ generations and the RIL by both methods indicated that the same single recessive gene, Tsr1, located on chromosome 5BL, controlled resistance and insensitivity to necrosis induced by race 2. A second recessive gene, designated Tsr6, located on chromosome 2BS, conferred resistance/insensitivity to chlorosis induced by spore inocula or culture filtrates of race 5. Diversity Arrays Technology markers wPt-3049 (2.9 cM) and wPt-0289 (4.6 cM) were closely linked to Tsr1 and Tsr6, respectively. The results further indicated that culture filtrates can be used as surrogates for spore inoculation. Tsr1 and Tsr6 can be selected by marker-assisted selection in breeding for resistance to tan spot.     

Transfer of bacterial blight and blast resistance from the tetraploid wild rice Oryza minuta to cultivated rice,Oryza sativa

Summary Oryza minuta J. S. Presl ex C. B. Presl is a tetraploid wild rice with resistance to several insects and diseases, including blast (caused by Pyricularia grisea) and bacterial blight (caused by Xanthomonas oryzae pv. oryzae). To transfer resistance from the wild species into the genome of cultivated rice (Oryza sativa L.), backcross progeny (BC₁, BC₂, and BC₃) were produced from interspecific hybrids of O. sativa cv IR31917-45-3-2 (2n=24, AA genome) and O. minuta Acc. 101141 (2n=48, BBCC genomes) by backcrossing to the O. sativa parent followed by embryo rescue. The chromosome numbers ranged from 44 to 47 in the BC₁ progeny and from 24 to 37 in the BC₂ progeny. All F₁ hybrids were resistant to both blast and bacterial blight. One BC₁ plant was moderately susceptible to blast while the rest were resistant. Thirteen of the 16 BC₂ progeny tested were resistant to blast; 1 blast-resistant BC₂, plant 75-1, had 24 chromosomes. A 3 resistant: 1 susceptible segregation ratio, consistent with the action of a major, dominant gene, was observed in the BC₂F₂ and BC₂F₃ generations. Five of the BC₁ plants tested were resistant to bacterial blight. Ten of the 21 BC₂ progeny tested were resistant to Philippine races 2, 3, and 6 of the bacterial blight pathogen. One resistant BC₂, plant 78-1, had 24 chromosomes. The segregation of reactions of the BC₂F₂, BC₂F₃, and BC₂F₄ progenies of plant 78-1 suggested that the same or closely linked gene(s) conferred resistance to races 2, 3, 5, and 6 of the bacterial blight pathogen from the Philippines.     

Inheritance and mechanism of resistance to herbicides inhibiting acetolactate synthase in Sonchus oleraceus L.

A biotype of Sonchus oleraceus L. (Compositae) has developed resistance to herbicides inhibiting acetolactate synthase (ALS) following field selection with chlorsulfuron for 8 consecutive years. The aim of this study was to determine the inheritance and mechanism of resistance in this biotype. Determination of ALS activity and inhibition kinetics revealed that K_m and V_max did not vary greatly between the resistant and susceptible biotypes. ALS extracted from the resistant biotype was resistant to five ALS-inhibiting herbicides in an in vitro assay. ALS activity from the resistant biotype was 14 19, 2, 3 and 3 times more resistant to inhibition by chlorsulfuron, sulfometuron, imazethapyr, imazapyr and flumetsulam, respectively, than the susceptible biotype. Hybrids between the resistant and a susceptible biotype were produced, and inheritance was followed through the F₁, F₂ and F₃ generations. F₁ hybrids displayed a uniform intermediate level of resistance between resistant and susceptible parents. Three distinct phenotypes, resistant, intermediate and susceptible, were identified in the F₂ generation following chlorsulfuron application. A segregation ratio of 121 was observed, indicative of the action of a single, nuclear, incompletely dominant gene. F₃ families, derived from intermediate F₂ individuals, segregated in a similar manner. Resistance to herbicides inhibiting ALS in this biotype of S. oleraceus is due to the effect of a single gene coding for a resistant form of the target enzyme, ALS.     

DEVELOPMENT AND DECLINING OF RESISTANCE IN DIAMOND BACK MOTH PLUTELLA XYLOSTELLA L. RESISTANT STRAINS SELECTED BY DIMEHYPO AND CARTAP*

Abstract Two resistant strains of diamond back moth Plutella xylostella L. were treated with dimehy-po and cartap in succession, The susceptible strain had never contacted with any insecticieds since reared in the in sectary. The rearing method by using vermiculite and radish seedling was discribed by Chen et al. (1990) and Liuet al. (1993). Comparison between reared strains and field strain did not display any difference in biological characteristics. The resistance reached 178 fold in dimehypo resistant strain in F₈₅, and 87 fold in cartap resistant strain in F₈₀, Two high level resistant strains had formed. After termination of selection, the resistance declined from 167 to 57 fold in dimehypo resistant strain and from 74 to 16 fold in cartap resistant strain within five generations. The resistance of diamond back moth to the two insecticides was unstable at high level, but could be steady at quite lower degree for a long time. It seemed impossible to regain the same sensitivity as before selection for the two resistant strains after resistance declining.     

Mode of Inheritance of Amitraz Resistance in a Brazilian Strain of the Southern Cattle Tick, Boophilus microplus (Acari: Ixodidae)*

The southern cattle tick, Boophilus  microplus (Canestrini), has developed resistance to amitraz in several countries in recent years. A study was conducted at the USDA Cattle Fever Tick Research Laboratory in Texas to investigate the mode of inheritance of amitraz resistance with cross-mating experiments. The Muñoz strain, a laboratory reared acaricide-susceptible reference strain, was used as the susceptible parent and the Santa Luiza strain, originating in Brazil, was used as the resistant parent. A modified Food and Agriculture Organization Larval Packet Test was used to measure the levels of susceptibility of larvae of the parental strains, F₁, backcross, F₂, and F₃ generations. Results of reciprocal crossing experiments suggested that amitraz resistance was inherited as an incomplete recessive trait. There was a strong maternal effect on larval progeny’s susceptibility to amitraz in both the F₁ and the subsequent generations. The values of the degree of dominance were estimated at ?0.156 and ?0.500 for the F₁ larvae with resistant and susceptible female parents, respectively. Results of bioassays on larval progeny of the F₁ backcrossed with the resistant parent strain and that of the F₂ generations suggested that more than one gene was responsible for amitraz resistance in the Santa Luiza strain. Comparisons of biological parameters (engorged female weight, egg mass weight, and female-to-egg weight conversion efficiency index) indicated significant differences between different genotypes. The differences appeared to be heritable, but not related to amitraz resistance. Results from this study may have significant implications for the management of amitraz resistance.     

Inheritance pattern of downy mildew resistance in advanced generations of sorghum*

In a project aimed to incorporate downy mildew resistance into sorghum hybrid seed parents, we screened F₄ and F₅ families for resistance to the ICRISAT Centre isolate of the pathogen using a greenhouse seedling screening technique. The families originated from a cross of 296B (susceptible) and IS 18757 [(QL-3) resistant]. The F₄s were obtained from agronomic selection in F₂s and F₃s, and the F₅ families from advancing plants identified as resistant in segregating F₄ families. The resistant plants were more than double the number of susceptible plants in the F₄ and almost so in the F₅ suggesting that resistance to downy mildew was dominant. Of the four genetic models examined (a single-locus model and three two-locus models with complementary, inhibitory, and a combination of complementary and inhibitory interactions), the two-locus model with independent segregation and a combination of complementary and inhibitory inter-allelic interaction appeared to be most appropriate in explaining the segregation patterns within and among F₄ and F₅ families. Accordingly, for resistance to P. sorghi, the suggested genotypes for IS 18757 is Pl_aPl_aPl_bPl_b and for 296B is Pl_aPl_aPl_bPl_b.     

Study on inheritance of resistance to root aphid, Pemphigus bursarius, in lettuce

The inheritance of resistance to lettuce root aphid, Pemphigus bursarius, was studied in lettuce using the Wellesbourne cultivars Avondefiance and Avoncrisp as resistant parents and Borough Wonder and Webb's Wonderful as aphid-susceptible parents. All four cultivars were crossed in all possible combinations including reciprocals and the response to root aphid of plants in the P₁F₁F₂ and BC generations was assessed using apterae of P. bursarius from the lettuce cv. Iceberg. Resistance to attack was clearly inherited and the parents appeared to be homozygous for their resistance or susceptibility. In the F₁ generation, however, in all crosses between resistant and susceptible parents, segregation into susceptible, resistant and some slightly less resistant plants occurred. This and the highly significant differences in segregation between pairs of reciprocal crosses in the F₁ and other generations indicate that the inheritance of resistance to root aphid is controlled by extra-nuclear factors. Modifying genes might also be involved but there appears to be no linkage of root aphid resistance with resistance to downy mildew, for which the Wellesbourne lettuces were bred.     

Studies on inheritance of resistance and allelic relationships for strain 2 of pigeonpea sterility mosaic pathogen

Inheritance of resistance and allelic relationships were studied in three resistant pigeonpea sources for strain 2 of sterility mosaic pathogen. The resistant genotypes (ICP 7035, ICP 7349 and ICP 8850) were crossed with susceptible genotypes (BDN1 and LRG30) to determine the inheritance of resistance. The resistant and susceptible genotypes were also crossed among themselves to obtain information on their allelic relationships. Parents, F₁ and F₂ generations were sown in pots and screened using infector-hedge technique. Observations in parents, F₁ and F₂ generations, indicated dominance of resistance in certain crosses and the dominance of susceptibility in others. Disease reaction appeared to be governed by two independent non-allelic genes, with at least three multiple alleles, at one of the loci.     

Stripe Rust Resistance in Roegneria kamoji (Poaceae: Triticeae) and its Genetic Analysis

The Roegneria kamoji accession ZY 1007 was resistant to the mixed predominant races of Puccinia striiformis f.sp. tritici (Pst) in China based on field tests at adult‐plant stage. The seedling resistance evaluation of ZY 1007 showed that it was resistant to stripe rust physiological strains CYR29, CYR33 and PST‐V26, which were the predominant races of Pst in China. The female parent R. kamoji cv. Gansi No.1 (susceptible to Pst) was crossed with ZY 1007 (resistant to Pst). Parents, F₁ and F₂ populations were tested in a field inoculated with the mixed urediniospores. ZY 1007 and all the observed 11 F₁ hybrid plants were resistant, while plants of Gansi No.1 were susceptible. Among the 221 F₂ plants, 168 plants were resistant and 53 were susceptible, and the segregation of resistant and susceptible plants fits 3R:1S ratio (χ² = 0.074, P > 0.75). It confirmed that the resistance of stripe rust in ZY 1007 was controlled by a single dominant gene and temporarily designated as YrK1007.     

Genetics of adult plant stripe rust resistance in CSP44, a selection from Australian wheat

Wheat line CSP44, a selection from an Australian bread wheat cultivar Condor, has shown resistance to stripe rust in India since the last twenty years. Seedlings and adult plants of CSP44 showed susceptible infection types against stripe rust race 46S119 but displayed average terminal disease severity of 2.67 on adult plants against this race as compared to 70.33 of susceptible Indian cultivar, WL711. This suggests the presence of nonhypersensitive adult plant stripe rust resistance in the line CSP44. The evaluation of F₁, F₂ and F₃ generations and F₆ SSD families from the cross of CSP44 with susceptible wheat cultivar WL711 for stripe rust severity indicated that the resistance in CSP44 is based on two genes showing additive effect. One of these two genes isYr18 and the second gene is not yet described.     

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 F_2:3 families, from Neijiang 977671 × Zhengzhou 5389 (susceptible) were inoculated with Chinese P. triticina pathotype FHNQ in the greenhouse. Results from the F₂ and F_2:3 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 F_2:3 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 F_2:3 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 2.2 cM, 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.     

Inheritance of fenoxaprop-P-ethyl resistance in a blackgrass (Alopecurus myosuroides Huds.) population

A blackgrass population has developed resistance to fenoxaprop-P-ethyl following field selection with the herbicide for 6 consecutive years. Within this population, 95% of the individuals are also resistant to flupyrsulfuron. Both the inheritance(s) and the mechanism(s) of resistances were investigated by making crosses between the resistant and a susceptible biotype. The inheritance was followed through the F₁ and F₂ generations either by spraying the herbicide on seedlings at the three-leaf stage or using a seedling bioassay, based on coleoptile length. No maternal effects were evident in the fenoxaprop-P-ethyl responses of the F₁ plants, suggesting that the inheritance was nuclear. Some F₁ families treated with fenoxaprop-P-ethyl segregated in a 3:1 (resistant:susceptible) ratio, indicating that the resistance was conferred by two dominant and independent nuclear genes. This was confirmed by the 15:1 (R:S) ratio observed in the F₂ generation treated with fenoxaprop- P-ethyl. The use of selective inhibitors of herbicide de-toxifying enzymes (aminobenzotriazole, pyperonylbutoxide, malathion and tridiphane) with the F₂ plants suggested that each of the two genes may govern two different mechanisms of fenoxaprop-P-ethyl resistance: the ACCase mutation previously postulated and an enhanced herbicide metabolism, mediated by cytochrome P 450 mono-oxygenases (P 450) susceptible to malathion. The P 450 activity may also confer resistance to flupyrsulfuron. This study clearly indicates that two distinct mechanisms of resistance may co-exist in the same plant. Received: 18 August 2000 / Accepted: 6 December 2000     

A region of barley chromosome 6H harbors multiple major genes associated with net type net blotch resistance

Net type net blotch (NTNB), caused by Pyrenophora teres f. teres Drechs., is prevalent in barley growing regions worldwide. A population of 118 doubled haploid (DH) lines developed from a cross between barley cultivars ‘Rika’ and ‘Kombar’ were used to evaluate resistance to NTNB due to their differential reaction to various isolates of P. teres f. teres. Rika was resistant to P. teres f. teres isolate 15A and susceptible to isolate 6A. Conversely, Kombar was resistant to 6A, but susceptible to 15A. A progeny isolate of a 15A × 6A cross identified as 15A × 6A#4 was virulent on both parental lines. The Rika/Kombar (RK) DH population was evaluated for disease reactions to the three isolates. Isolate 15A induced a resistant:susceptible ratio of 78:40 (R:S) whereas isolate 6A induced a resistant:susceptible ratio of 40:78. All but two lines had opposite disease reactions indicating two major resistance genes linked in repulsion. Progeny isolate 15A × 6A#4 showed a resistant:susceptible ratio of 1:117 with the one resistant line also being the single line that was resistant to both 15A and 6A. An RK F₂ population segregated in a 1:3 (R:S) ratio for both 15A and 6A indicating that resistance is recessive. Molecular markers were used to identify a region on chromosome 6H that harbors the two NTNB resistance genes. This work shows that multiple NTNB resistance genes exist at the locus on chromosome 6H, and the recombinant DH line harboring the resistance alleles from both parents will be useful for the development of NTNB-resistant barley germplasm.     

Inheritance of downy mildew resistance, β-1,3-glucanases and peroxidases in pearl millet [Pennisetum glaucum (L.) R. Br.] crosses

The inheritance of resistance to downy mildew disease and the defense-related enzymes β-1,3-glucanase and peroxidase was studied in crosses of pearl millet using a generation-mean analysis. The study material comprised six generations (susceptible and resistant parents, F₁, F₂, BC₁ and BC₂) in three crosses. Seedlings from these generations were inoculated with the downy mildew pathogen Sclerospora graminicola and disease incidence was recorded. Analysis of constitutive levels of β-1,3-glucanase and peroxidase in the seedlings of different generations indicated that the resistant populations showed higher enzyme activities, while lower activities of the enzymes were recorded in the susceptible populations. In the generation-mean analysis, the significance of scaling tests revealed the existence of non-allelic interactions in the inheritance of resistance to downy mildew as well as with the enzymes. Among the gene effects, both additive and dominant effects were significant. All the non-allelic interaction effects were significant in the crosses. Studies on the isozyme patterns of the enzymes substantiated the results of the disease-incidence experiments in most of the generations. The results indicated that the inheritance of downy mildew disease resistance and the expression of β-1,3-glucanase and peroxidase in pearl millet is not only under the control of additive and dominant genes but are also governed by complex non-allelic interactions. Received: 30 April 2000 / Accepted: 17 October 2000