Sources Of Partial Resistance Research Articles

Advanced backcross QTL analysis and comparative mapping with RIL QTL studies and GWAS provide an overview of QTL and marker haplotype diversity for resistance to Aphanomyces root rot in pea (Pisum sativum).

Aphanomyces euteiches is the most damaging soilborne pea pathogen in France. Breeding of pea resistant varieties combining a diversity of quantitative trait loci (QTL) is a promising strategy considering previous research achievements in dissecting polygenic resistance to A. euteiches. The objective of this study was to provide an overview of the diversity of QTL and marker haplotypes for resistance to A. euteiches, by integrating a novel QTL mapping study in advanced backcross (AB) populations with previous QTL analyses and genome-wide association study (GWAS) using common markers. QTL analysis was performed in two AB populations derived from the cross between the susceptible spring pea variety "Eden" and the two new sources of partial resistance "E11" and "LISA". The two AB populations were genotyped using 993 and 478 single nucleotide polymorphism (SNP) markers, respectively, and phenotyped for resistance to A. euteiches in controlled conditions and in infested fields at two locations. GWAS and QTL mapping previously reported in the pea-Aphanomyces collection and from four recombinant inbred line (RIL) populations, respectively, were updated using a total of 1,850 additional markers, including the markers used in the Eden x E11 and Eden x LISA populations analysis. A total of 29 resistance-associated SNPs and 171 resistance QTL were identified by GWAS and RIL or AB QTL analyses, respectively, which highlighted 10 consistent genetic regions confirming the previously reported QTL. No new consistent resistance QTL was detected from both Eden x E11 and Eden x LISA AB populations. However, a high diversity of resistance haplotypes was identified at 11 linkage disequilibrium (LD) blocks underlying consistent genetic regions, especially in 14 new sources of resistance from the pea-Aphanomyces collection. An accumulation of favorable haplotypes at these 11 blocks was confirmed in the most resistant pea lines of the collection. This study provides new SNP markers and rare haplotypes associated with the diversity of Aphanomyces root rot resistance QTL investigated, which will be useful for QTL pyramiding strategies to increase resistance levels in future pea varieties.

Adaptation of Phytophthora nicotianae to Multiple Sources of Partial Resistance in Tobacco.

Host resistance is an important tool in the management of black shank disease of tobacco. Race development leads to rapid loss of single-gene resistance, but the adaptation by Phytophthora nicotianae to sources of partial resistance from Beinhart 1000, Florida 301, and the Wz gene region introgressed from Nicotiana rustica is poorly characterized. In greenhouse environments, host genotypes with quantitative trait loci (QTLs) conferring resistance from multiple sources were initially inoculated with an aggressive isolate of race 0 or race 1 of P. nicotianae. The most aggressive isolate was selected after each of six host generations to inoculate the next generation of plants. The race 0 isolate demonstrated a continuous gradual increase in disease severity and percentage root rot on all sources of resistance except the genotype K 326 Wz/-, where a large increase in both was observed between generations 2 and 3. Adaptation by the race 0 isolate on Beinhart 1000 represents the first report of adaptation to this genotype by P. nicotianae. The race 1 isolate did not exhibit significant increases in aggressiveness over generations but exhibited a large increase in aggressiveness on K 326 Wz/- between generations 3 and 4. Molecular characterization of isolates recovered during selection was completed via double digest restriction-site associated DNA sequencing, but no polymorphisms were associated with the observed changes in aggressiveness. The rapid adaptation to Wz resistance and the gradual adaptation to other QTLs highlights the need to study the nature of Wz resistance and to conduct field studies on the efficacy of resistance gene rotation for disease management.

Impacts of Continued Exposure to a Susceptible Host Genotype on Aggressiveness of Phytophthora nicotianae Isolates Adapted to Multiple Sources of Partial Resistance.

Pathogen adaptation can threaten the durability of partial resistance. Mixed plantings of susceptible and partially resistant varieties may prolong the effectiveness of partial resistance, but little is known about how continued exposure to a susceptible genotype can change the aggressiveness of pathogen isolates adapted to a source of partial resistance. The objective of this study was to examine the effects of continued exposure to a highly susceptible tobacco genotype on isolates of Phytophthora nicotianae that had been adapted to partial resistance. Isolates of P. nicotianae previously adapted to two sources of partial resistance were continually exposed to either the original host of adaptation or a susceptible host. After six generations of host exposure, isolates obtained from the partially resistant and the susceptible hosts were compared for their aggressiveness on the resistant host and for differences in expression of genes associated with pathogenicity and aggressiveness. Results suggested that exposure to the susceptible tobacco genotype reduced aggressiveness of isolates adapted to partial resistance in K 326 Wz/- but not of isolates adapted to partial resistance in Fla 301. Quantification of pathogenicity-associated gene expression using qRT-PCR suggested the rapid change in aggressiveness of isolates adapted to Wz-sourced partial resistance may have resulted from modification in gene expression in multiple genes.

Long-term resistance in promising rubber tree genotypes as a breeding source for improving South American leaf blight management under high disease incidence in the Colombian Amazon

South American Leaf Blight (SALB), caused by Pseudocercospora ulei, is the most important disease in Hevea brasiliensis grown in South America. This study aimed to evaluate long-term resistance in the field for 11 years in 10 rubber tree genotypes (nine hybrid clones and one primary clone) in a large-scale clonal trial (LSCT) and under controlled and nursery conditions. Resistance was evaluated using two resistance components: type of reaction (TR, ranging from 1 to 6) related to asexual sporulation, and stroma density (ST, ranging from 0 to 4), which is linked to sexual sporulation. Resistance status ranking (RSR) based on the maximum scores of the variables was adapted to classify the genotypes (completely resistant, partially resistant or highly susceptible) over time, and against four indigenous isolates of P. ulei. Eight of the ten clones exhibited partial resistance in the field, and clones FDR 5597, GU 198 and FDR 5788 were more resistant with TR ≤ 4 and ST ≤ 2. These clones were completely resistant in the laboratory/nursery with TR < 3 and ST = 0. Only clone MDF 180 did not exhibit the sexual phase of the pathogen (i.e., ST = 0) in the field or in the nursery. Clones CDC 56 and IAN 873 (control) were highly susceptible, with TR > 5 and ST > 2. In this study, sources of partial resistance were evidenced for the effective management of SALB under conditions of high P. ulei incidence in the Amazon.

Genome-wide association multi-locus and multi-variate linear mixed models reveal two linked loci with major effects on partial resistance of apricot to bacterial canker

BackgroundDiseases caused by Pseudomonas syringae (Ps) are recognized as the most damaging factors in fruit trees with a significant economic and sanitary impact on crops. Among them, bacterial canker of apricot is exceedingly difficult to control due to a lack of efficient prophylactic measures. Several sources of partial resistance have been identified among genetic resources but the underlying genetic pattern has not been elucidated thus far. In this study, we phenotyped bacterial canker susceptibility in an apricot core-collection of 73 accessions over 4 years by measuring canker and superficial browning lengths issued from artificial inoculations in the orchard.In order to investigate the genetic architecture of partial resistance, we performed a genome-wide association study using best linear unbiased predictors on genetic (G) and genetic x year (G × Y) interaction effects extracted from linear mixed models.Using a set of 63,236 single-nucleotide polymorphism markers genotyped in the germplasm over the whole genome, multi-locus and multi-variate mixed models aimed at mapping the resistance while controlling for relatedness between individuals.ResultsWe detected 11 significant associations over 7 candidate loci linked to disease resistance under the two most severe years. Colocalizations between G and G × Y terms indicated a modulation on allelic effect depending on environmental conditions. Among the candidate loci, two loci on chromosomes 5 and 6 had a high impact on both canker length and superficial browning, explaining 41 and 26% of the total phenotypic variance, respectively. We found unexpected long-range linkage disequilibrium (LD) between these two markers revealing an inter-chromosomal LD block linking the two underlying genes. This result supports the hypothesis of a co-adaptation effect due to selection through population demography. Candidate genes annotations suggest a functional pathway involving abscisic acid, a hormone mainly known for mediating abiotic stress responses but also reported as a potential factor in plant-pathogen interactions.ConclusionsOur study contributed to the first detailed characterization of the genetic determinants of partial resistance to bacterial canker in a Rosaceae species. It provided tools for fruit tree breeding by identifying progenitors with favorable haplotypes and by providing major-effect markers for a marker-assisted selection strategy.

Characterization of Phytophthora nicotianae Resistance Conferred by the Introgressed Nicotiana rustica Region, Wz, in Flue-Cured Tobacco.

Black shank, caused by Phytophthora nicotianae, is one of the most important diseases affecting tobacco worldwide and is primarily managed through use of host resistance. An additional source of resistance to P. nicotianae, designated as Wz, has been introgressed into Nicotiana tabacum from N. rustica. The Wz gene region confers high levels of resistance to all races, but has not been characterized. Our study found Wz-mediated resistance is most highly expressed in the roots, with only a slight reduction in stem-lesion size in Wz genotypes compared with susceptible controls. No substantial relationships were observed between initial inoculum levels and disease development on Wz genotypes, which is generally consistent with qualitative or complete resistance. Isolates of P. nicotianae adapted for five host generations on plants with the Wz gene caused higher disease severity than isolates adapted on Wz plants for only one host generation. Wz-adapted isolates did not exhibit increased aggressiveness on genotypes with other sources of partial resistance, suggesting pathogen adaptation was specific to the Wz gene. To reduce potential for pathogen population shifts with virulence on Wz genotypes, Wz should be combined with other resistance sources and rotation of varying black shank resistance mechanisms is also recommended.

Adaptation to Partial Resistance to Powdery Mildew in the Hop Cultivar Cascade by Podosphaera macularis.

The hop cultivar Cascade has been grown in the Pacific Northwestern U.S. and elsewhere with minimal input for management of powdery mildew (Podosphaera macularis) for nearly 15 years due to the putatively quantitative resistance in this cultivar. While partial resistance is generally thought to be more durable than qualitative resistance, in 2012, powdery mildew was reported on Cascade in Washington State. Field surveys conducted during 2013 to 2016 indicated increasing prevalence of powdery mildew on Cascade, as well as an increasing number of fungicide applications applied to this cultivar in Washington State. Nearly all isolates of P. macularis tested were able to infect Cascade in laboratory inoculations. However, the greatest number of colonies, most conidia produced, and the shortest latent period was only observed with isolates derived originally from Cascade, as compared with other isolates derived from other cultivars. Further, the enhanced aggressiveness of these isolates was only manifested on Cascade and not six other susceptible cultivars, further indicating a specific adaptation to Cascade by the isolates. There was no evidence of a known major R-gene in Cascade, as seven isolates of P. macularis with contrasting virulence all infected Cascade. Among 158 isolates obtained from hop yards planted to Cascade, only two (1.3%) were able to infect the cultivar Nugget, which possesses the resistance factor termed R6, indicating that isolates of P. macularis virulent on Nugget are largely distinct from those adapted to Cascade. Further, race characterization indicated Cascade-adapted isolates of P. macularis were able to overcome R-genes Rb, R3, and R5, but not other known R-genes. Therefore, multiple R-genes and other sources of partial resistance are expected to provide resistance to Cascade-adapted strains of the fungus. Given the plasticity of the powdery mildew fungus, breeding strategies for powdery mildew need to consider the potential for adaptation to both qualitative and partial resistance in the host.

Delayed Foliar Symptoms Caused by Verticillium dahliae as an Alternative Resistance Trait in Iceberg Lettuce

Verticillium wilt caused by Verticillium dahliae Kleb. is an economically damaging disease of iceberg lettuce on the Central Coast of California. Foliar wilting symptoms that manifest near or at peak market maturity (MM) lead to collapse of the head, making it unmarketable. Complete resistance to race 1 of the pathogen is known, but adequate levels of resistance are not available against race 2. Additional mechanisms or traits that reduce foliar symptoms (FS) are needed to lessen economic losses from this disease. Since the disease affects leaves, the harvested product, identification of iceberg cultivars that delay the onset of FS past peak MM could reduce yield loss from the disease. The goal of this research was to identify iceberg lettuce germplasm with delayed onset of FS. Diverse iceberg cultivars were evaluated in replicated field experiments for MM, FS severity, and adaptation. A few winter-adapted cultivars showed fewer FS past MM and seem to be promising candidates for breeding. These cultivars are not adapted to the California Central Coast where the disease currently predominates. Further studies will determine the usefulness of this trait for breeding improved cultivars for use in V. dahliae–infested fields. Developing new cultivars that combine currently available sources of partial resistance against race 2 with delayed onset of FS could lead to reduced crop losses should race 2 of V. dahliae become widespread.

Novel Sources of Partial Resistance against Phytophthora sojae in Soybean PI 399036

With the rapid emergence of novel pathogen pathotypes, durable sources of resistance against Phytophthora sojae in soybean [Glycine max (L.) Merrill] are needed to manage Phytopthora stem and root rot (PRR). This study used an inoculum source composed of multiple isolates of the pathogen to characterize partial resistance (PR) to P. sojae in plant introduction (PI) 399036. Two recombinant inbred line (RIL) F5:7 populations developed by crossing PI 399036 (high PR), with two germplasm lines (low PR), AR2 (AX20925) and AR3 (AX20931), were screened. Corrected dry weights (CDW—root and shoot) and percentage root rot data were collected. Inclusive composite interval mapping identified one putative quantitative trait locus (QTL) on chromosome 2 by all disease assessment parameters in the AX20925 population that explained 6 to 14% of the total phenotypic variation and five putative minor QTL on 5 chromosomes (3, 6, 12, 15, and 19) that individually explained 5 to 9% of the phenotypic variation. In the AX20931 population, two QTL, mapped to chromosome 9 and 17, were commonly identified by CDW while the QTL identified on chromosome 9 accounted for 21 to 30% of the total phenotypic variation. Additional QTL identified by CDW on chromosomes 9, 13, 14, and 15 individually accounted for 5 to 8% of the phenotypic variation. PI 399036 contributed the favorable alleles for the majority of the identified QTL. This study demonstrates the usefulness of PI 399036 as a novel source for breeding for resistance to PRR and that an inoculum mixture of P. sojae isolates with varying pathotypes can be used to identify putative QTL for PR to P. sojae using the rice (Oryza sativa) method.

Biological characterisation of Globodera pallida from Idaho

The white potato cyst nematode, Globodera pallida, was recently found in the USA and was identified using molecular and morphometric assessment. This paper examined some biological characteristics of the Idaho population of G. pallida. The samples of the original cysts showed that they had low rates of juvenile hatch and, consequently, low rates of reproduction when compared with glasshouse-reared populations. Virulence tests using sources of partial resistance from Solanum vernei and S. tuberosum ssp. Andigena CPC 2802 compared the Idaho population to European and South American populations of G. pallida. The Idaho population showed a virulence pattern comparable to other European populations in the Pa2/3 virulence group. The overall level of reproduction of the Idaho population on the resistant cultivars was the lowest of all the populations tested, suggesting that it is not highly virulent. These results are compatible with molecular characterisation, which indicates that this population is similar to the European gene pool.

New consistent QTL in pea associated with partial resistance to Aphanomyces euteiches in multiple French and American environments.

Partial resistances, often controlled by quantitative trait loci (QTL), are considered to be more durable than monogenic resistances. Therefore, a precursor to developing efficient breeding programs for polygenic resistance to pathogens should be a greater understanding of genetic diversity and stability of resistance QTL in plants. In this study, we deciphered the diversity and stability of resistance QTL to Aphanomyces euteiches in pea towards pathogen variability, environments and scoring criteria, from two new sources of partial resistance (PI180693 and 552), effective in French and USA infested fields. Two mapping populations of 178 recombinant inbred lines each, derived from crosses between 552 or PI 180693 (partially resistant) and Baccara (susceptible), were used to identify QTL for Aphanomyces root rot resistance in controlled and in multiple French and USA field conditions using several resistance criteria. We identified a total of 135 additive-effect QTL corresponding to 23 genomic regions and 13 significant epistatic interactions associated with partial resistance to A.euteiches in pea. Among the 23 additive-effect genomic regions identified, five were consistently detected, and showed highly stable effects towards A.euteiches strains, environments, resistance criteria, condition tests and RIL populations studied. These results confirm the complexity of inheritance of partial resistance to A.euteiches in pea and provide good bases for the choice of consistent QTL to use in marker-assisted selection schemes to increase current levels of resistance to A.euteiches in pea breeding programs.

Diallel analysis of Fusarium head blight resistance in genetically diverse winter wheat germplasm

New sources of partial resistance to Fusarium head blight (FHB) in wheat have been identified over the past decade; however, little is known of their breeding value. A 20 parent partial diallel that included resistant genotypes from the U.S., Europe, China and South America was used to evaluate the potential of these sources of resistance as parents in wheat breeding programs. Eight plants replication−1 of each of 190 crosses and 20 parents were point-inoculated with Fusarium graminearum under greenhouse conditions in two replicated experiments. Both general (GCA) and specific combining ability (SCA) were significant. Most of the variance for FHB severity was associated with additive genes; however, estimates for SCA ranged from highly negative to highly positive in both resistant × resistant and resistant × susceptible crosses which suggest that improving FHB resistance through gene pyramiding strategies based on additive genetic variation may be complicated by interaction effects that condition FHB resistance.

Response of Dry Bean Genotypes to Fusarium Root Rot, Caused by Fusarium solani f. sp. phaseoli, Under Field and Controlled Conditions.

Fusarium root rot of dry bean (Phaseolus vulgaris), caused by Fusarium solani f. sp. phaseoli, is a major yield-limiting disease in North Dakota and Minnesota. Although a few sources of partial resistance are available, most commercial cultivars grown in this region are susceptible, especially in the red kidney bean market class. This study evaluated three methods of screening for resistance to Fusarium root rot. A sand-cornmeal inoculum layer method, spore suspension method, and paper towel method were used to evaluate 11 dry bean genotypes for resistance to Fusarium root rot under growth-chamber conditions. These same genotypes were also evaluated in field trials at Fargo, ND, and Park Rapids and Perham, MN, in 2005. In all trials, the small red genotype VAX 3 was found to have a consistently high level of resistance to Fusarium root rot and could be used as a source of resistance by dry bean breeders. Correlation analyses between field and growth-chamber root rot ratings indicated that all three growth-chamber methods had significantly (P ≤ 0.05) positive correlations with field results from Perham and Fargo, which suggests that all three methods could be used to screen germplasm efficiently for resistance to Fusarium root rot.

Partial resistance to leaf rust in a collection of ancient Spanish barleys.

A collection of 569 Spanish barley accessions was screened for resistance to leaf rust (Puccinia hordei Otth) in the field at Córdoba during the 2000-2001 season. The level of resistance ranged from very low to very high. In 14% of the accessions the relative AUDPC (L94 = 100%) was lower than 10%. Selected accessions that were most resistant in the field, were tested in the seedling stage under controlled conditions. Macroscopic components of resistance indicated that six lines had a high level of partial resistance close to check cv. Vada and one line a similar level of partial resistance. Histological studies indicated that the resistance was based on a high percentage of early aborted colonies and reduction in colony size without plant cell necrosis. Three of the selected lines showed high percentage of plant cell necrosis associated with established colonies, which indicates a combination of prehaustorial resistance with late acting incomplete posthaustorial resistance. Although the new barley varieties already incorporate some partial resistance, new sources of partial resistance like these are needed to improve durability of the resistance.

Sources of Partial Resistance to Fusarium Root Rot in the Pisum Core Collection

Fusarium root rot, caused by Fusarium solani f. sp. pisi, is one of the most important fungal diseases of pea and is found in most pea-growing areas around the world. Currently, no commercial cultivars are resistant to this pathogen. Availability of new sources of partial resistance could provide another tool for managing Fusarium root rot. In all, 387 accessions from the Pisum core collection were evaluated for resistance to Fusarium root rot in two independent experiments. Nonparametric analysis of variance conducted on ranks of disease severity for each accession indicated that the two experiments corresponded well. Forty-four plant introduction lines with a disease severity rating of 2.5 or less on a 0-to-5 scale (where 5 = completely rotted) were selected as being partially resistant to root rot. Immunity to Fusarium root rot was not found. Comparison of disease resistance data for Aphanomyces root rot and Fusarium root rot showed a weak, but significant and positive correlation. A complete listing of the data for the partial resistance of all accessions tested can be found at the National Plant Germplasm System website, United States Department of Agriculture-Agricultural Research Service.

Evaluation of Soybean Resistance to Sclerotinia Stem Rot Using Reciprocal Grafting.

Sclerotinia stem rot of soybean is one of the major soybean diseases in the north central region of the United States. One disease management option is to plant cultivars that have resistance. Some sources of partial resistance have been identified, but information pertaining to the nature of resistance is limited. The objective of this study was to determine if the expression of resistance is dictated by shoots of resistant plants and if this can be altered by using resistant and susceptible soybean genotypes grafted in different shoot and rootstock combinations of self-, single-, or double-shoot grafts. After successful grafts were made, several experiments were conducted using different inoculation techniques and soybean genotypes. In one experiment, cotyledons were inoculated with a plug of fungal mycelium, plants were incubated in a mist chamber for 23 h, and plant survival was recorded over time. Based on seven grafting combinations of cross- and self-grafted plants using two soybean cultivars, grafts with NKS19-90 (partially resistant) as shoots had greater (P ≤ 0.05) plant survival at 3, 4, and 5 days after inoculation than the other graft combinations. In another experiment, a total of 17 graft combinations were generated using resistant plant introductions and two susceptible cultivars. Resistant self-grafts of the plant introductions had greater (P ≤ 0.05) plant survival (mean = 75%) than self-grafts of the susceptible cultivars (mean = 15%) at 5 days after inoculation. Inter-genotypic grafts with resistant shoots had greater (P ≤ 0.05) plant survival (mean = 65%) than those in reciprocal combinations (mean = 8%) 5 days after inoculation. A cut stem inoculation method was used to test graft combinations of one resistant and two susceptible cultivars. Grafts with susceptible shoots of cvs. Williams 82 and Asgrow 2242 had greater (P < 0.05) lesion lengths (mean = 13.2 cm) than shoots of NKS19-90 (mean = 9.2 cm) regardless of the rootstock 15 days after inoculation. In a double-graft experiment, shoots of both NKS19-90 and Williams 82 were grafted to either NKS19-90 or Williams 82 rootstocks. Regardless of the rootstock, the shoots of Williams 82 died while shoots of NKS19-90 survived. For all the experiments, resistance was greater when the grafted shoot came from a resistant source on a susceptible rootstock compared with the reciprocal combination regardless of the type of grafting technique or inoculation method.

Evaluation of sources of partial resistance to mildew in barley using enzyme-linked immunosorbent assay and other assessment methods

Twenty-nine spring barley genotypes ranging from primitive landraces to modern cultivars were characterised for partial resistance to powdery mildew on detached leaves and in the field. Detached seedling and adult plant leaves were produced under spore-free glasshouse conditions and were assessed for infection frequency (IF) and then total mildew biomass using enzyme-linked immunosorbent assay (ELISA), and thereby biomass per colony (BMC), following controlled infection. Field plants were assessed visually for infection and for total mildew biomass using ELISA. Several genotypes were identified as having below average IF and BMC on both seedling and adult detached leaves and field scores of leaf area infected with mildew. IF at the seedling stage was highly correlated with IF at adult plant stage and with mildew biomass in the field. Adult plant IF was highly correlated with visual field scores. BMC data were often negatively correlated with other assessments.

Sources of partial resistance to powdery mildew in cowpea

Sources of partial resistance to powdery mildew in cowpea

Partial Resistance to Northern Leaf Blight and Stewart's Wilt in Sweet Corn Germ Plasm

Sources of partial resistance to northern leaf blight (NLB) and Stewart's wiltwere identified in sweet corn germ plasm. Inbreds that were classified resistant to NLB in evaluations from 1988 to 1990 included IL545a, IL611a, IL676a, IL6677a, IL685d, IL731a, and IL797a. Inbreds that were classified resistant to Stewart's wilt in both 1988 and 1989 included IL126b, IL676a, IL766a, IL722a, IL772b, IL776a, and IL797a. Inbred lines with IL677a in their pedigree ten to be more resistant to NLB and Stewart's wilt than the populations of inbreds evaluated for these two diseases