Chemical interactions with the herbicide propanil on propanil-resistant barnyardgrass

We are examining the interaction of compounds with the herbicide propanil to find synergistic or additive actions that can increase efficacy against propanil-resistant barnyardgrass [Echinochloa crus-galli] (R-BYG) without substantial injury to rice. Field tests (herbicidal injury) and laboratory tests (chlorophyll quantification in excised leaves; measurement of chlorophyll fluorescence to determine PSII inhibition) have been conducted on R-BYG and rice tissue exposed to various rates of propanil and additive. Important synergistic interactions on R-BYG in laboratory and field tests were found with propanil plus either the herbicides anilophos or piperophos, or the insecticide carbaryl. In laboratory tests, the insecticide methiocarb and PPG-124 (p-chlorophenyl N-methylcarbamate) were highly effective synergists with propanil on R-BYG. Other important interactions occurred with certain concentrations/application rates when propanil was combined with the herbicides quinclorac, thiobencarb, molinate, or pendimethalin (field tests). Combinations of these or other chemicals with propanil may provide additive or synergistic action useful to control R-BYG without increasing rice injury. Such mixtures might also prevent or delay the development of propanil resistance in this weed species.     

Selection for quantitative trait loci associated with resistance to Stewart's wilt in sweet corn

The objectives of this research were to identify quantitative trait loci (QTL) for Stewart's wilt resistance from a mapping population derived from a sweet corn hybrid that is highly resistant to Pantoea stewartii and to determine if marker-based selection for those QTL could substantially improve Stewart's wilt resistance in a population derived from a cross of resistant lines and a highly susceptible sweet corn inbred. Three significant QTL for Stewart's wilt resistance on chromosomes 2 (bin 2.03), 5 (bin 5.03), and 6 (bin 6.06/6.07) explained 31% of the genetic variance in a population of 110 F(3:4) families derived from the sweet corn hybrid Bonus. The three QTL appeared to be additive in their effects on Stewart's wilt ratings. Based on means of families that were either homozygous or heterozygous for marker alleles associated with the resistance QTL, the QTL on chromosomes 2 and 6 appeared to have dominant or partially dominant gene action, while the QTL on chromosome 5 appeared to be recessive. A population of 422 BC(2)S(2) families was derived from crosses of a sweet corn inbred highly susceptible to Stewart's wilt, Green Giant Code 88 (GG88), and plants from two F(3:4) families (12465 and 12467) from the Bonus mapping population that were homozygous for marker alleles associated with Stewart's wilt resistance at the three QTL. Mean Stewart's wilt ratings for BC(2)S(2) families were significantly (P < 0.05) lower for families that were homozygous for the bnlg1902 marker allele (bin 5.03) from resistant lines 12465 or 12467 than for families that were heterozygous at this marker locus or homozygous for the bnlg1902 marker allele from GG88. Resistance associated with this QTL was expressed only if F(3:5) or BC(2)S(2) families were homozygous for marker alleles associated with the resistant inbred parent (P(1)). Marker alleles identified in the F(3:5) mapping population that were in proximity to the resistance QTL on chromosomes 2 and 6 were not polymorphic in crosses of GG88 with 12465 and 12467. Selection for other polymorphic marker loci adjacent to these two regions did not improve Stewart's wilt resistance of BC(2)S(2) families.     

Modelling the interactions between phenology and insecticide resistance genes in the codling moth Cydia pomonella

In the codling moth Cydia pomonella (L), insecticide resistance genes have been associated with pleiotropic effects affecting phenology. In this paper, we investigated whether an increase in the frequency of insecticide resistance in field populations of C pomonella was likely to entail significant divergences in the temporal occurrence of both susceptible and insecticide-resistant individuals. For this purpose, we built a phenological model that provided suitable predictions of the distinct and diverging seasonal evolutions of populations of a susceptible and two insecticide-resistant (at two and three loci) homozygous genotypes of C pomonella. Model simulations for each genotype were further compared with pheromone trap catches recorded in a field insecticide-treated population over an 8-year period (from 1992 to 2000), which reflected the progressive annual increase in the frequency of resistance in southeastern France. We found a significant delay in field adult emergence relative to those predicted by the homozygous susceptible model, and the magnitude of such a delay was positively correlated with increasing frequencies of insecticide resistance in the sampled field population of C pomonella. Adult emergence predicted in the theoretical population that was homozygous for resistance at two loci converged with those recorded in the field during the investigated 8-year period. This suggested that the pleiotropic effects of resistance were likely to result in a significant phenological segregation of insecticide-resistant alleles in the field. The results of this study emphasized the potential for pest populations exposed to chemical selection to evolve qualitatively with respect to phenology. This may raise critical questions regarding the use of phenological modelling as a forecasting tool for appropriate resistance management strategies that would take into account the diverging seasonal evolutions of both insecticide resistance and susceptibility.     

The inheritance of virulence of Phytophthora infestans to potato

Of 31 matings between isolates of P. infestans from several countries, six yielded enough progeny for analysis of inheritance of the virulence phenotype. Virulence was determined in vitro after inoculation of detached leaflets of nine differential lines of potato, each carrying a different gene for resistance. Parents of three matings carried an isozyme marker (glucosephosphate isomerase) which allowed the hybridity of most progeny to be confirmed. Apparently non-hybrid progeny from all three matings were probably selfs or apomicts; these were discarded. The inheritance of virulence in two sib-cross and one backcross family was determined. Patterns of inheritance in F₁ and F₂ indicated the presence of a gene-for-gene interaction in which alleles of a single locus in the pathogen conditioned virulence or avirulence on each differential. Although the hypothesis that avirulence alleles were dominant and virulence alleles were recessive was supported by many of the data, unexpected segregations were obtained. Alternative hypotheses to explain the latter included low aggressiveness in a proportion of the progeny, a second locus inhibiting avirulence in one parent, a different locus in each parent determining avirulence/virulence on one R-gene, and dominance of some alleles determining virulence. Avirulent field isolates appeared to be heterozygous ( A vravr ) rather than homozygous ( Avr Avr ) at avirulence loci. A somatic segregation from avirulence to virulence at three avirulence loci was postulated for one parental isolate. Evidence for linkage of these three loci suggested that the observed somatic segregation resulted from mitotic crossing-over.     

Characterization of a resistance locus (Pfs-1) to the spinach downy mildew pathogen (Peronospora farinosa f. sp. spinaciae) and development of a molecular marker linked to Pfs-1

Downy mildew is a destructive disease of spinach worldwide. There have been 10 races described since 1824, six of which have been identified in the past 10 years. Race identification is based on qualitative disease reactions on a set of diverse host differentials which include open-pollinated cultivars, contemporary hybrid cultivars, and older hybrid cultivars that are no longer produced. The development of a set of near-isogenic open-pollinated spinach lines (NILs), having different resistance loci in a susceptible and otherwise common genetic background, would facilitate identification of races of the downy mildew pathogen, provide a tool to better understand the genetics of resistance, and expedite the development of molecular markers linked to these disease resistance loci. To achieve this objective, the spinach cv. Viroflay, susceptible to race 6 of Peronospora farinosa f. sp. spinaciae, was used as the recurrent susceptible parent in crosses with the hybrid spinach cv. Lion, resistant to race 6. Resistant F(1) progeny were subsequently backcrossed to Viroflay four times with selection for race 6 resistance each time. Analysis of the segregation data showed that resistance was controlled by a single dominant gene, and the resistance locus was designated Pfs-1. By bulk segregant analysis, an amplified fragment length polymorphism (AFLP) marker (E-ACT/M-CTG) linked to Pfs-1 was identified and used to develop a co-dominant Sequence characterized amplified region (SCAR) marker. This SCAR marker, designated Dm-1, was closely linked ( approximately 1.7 cM) to the Pfs-1 locus and could discriminate among spinach genotypes that were homozygous resistant (Pfs-1Pfs-1), heterozygous resistant (Pfs-1pfs-1), or homozygous susceptible (pfs-1pfs-1) to race 6 within the original mapping population. Evaluation of a wide range of commercial spinach lines outside of the mapping population indicated that Dm-1 could effectively identify Pfs-1 resistant genotypes; the Dm-1 marker correctly predicted the disease resistance phenotype in 120 out of 123 lines tested. In addition, the NIL containing the Pfs-1 locus (Pfs-1Pfs-1) was resistant to multiple races of the downy mildew pathogen indicating Pfs-1 locus may contain a cluster of resistance genes.     

Regulation of insecticide resistance-related cytochrome P-450 expression in Drosophila melanogaster

Two protein bands, resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, account for most of the cytochrome P-450 in Drosophila melanogaster. P-450-A is ubiquitous among strains tested; whereas P-450-B is unique to certain strains. Dimethylnitrosamine demethylase activity is associated with P-450-B. Biochemical and genetic analyses showed that genes located on chromosome II were required for P-450-B expression. These genes were mapped within an interval that includes a major insecticide-resistance locus. Regulatory loci on chromosome III were required for maximum expression of P-450-B. One of these regulatory loci was mapped at another major resistance locus on chromosome III. There was a good correlation between P-450-B expression and resistance to phenylurea among the different strains tested. These results indicate that Drosophila can be a useful model to study the molecular mechanisms of insecticide resistance.     

增效剂的增效作用与棉铃虫抗药性水平之间的关系研究

以不同抗药性水平的棉铃虫三龄幼虫试虫，分别测定增效磷（ＳＶ１），八氯二丙醚（Ｓ２）月桂氮Ｚｕｏ酮（ＤＣ），ＩＳ－９６１对久效磷（ｍｏｎｏｃｒｏｔｏｐｈｏｓ）灭多威（ｍｅｔｈｏｍｙｌ），氰戊菊酯（ｆｅｎｖａｌｅｒａｔｅ）的增效作用，结果表明：４种增效剂的增效作用，除与被测定药剂种类有关外，还与棉铃虫抗药性水平密切相关，其增效作用一般随抗性水平的提高而增加，但增加的幅度低于抗药性提高的幅度。     

Using synergists to detect multiple insecticide resistance in field populations of rice stem borer

The rice stem borer, Chilo suppressalis (Walker), an important insect pest of rice in China, has developed resistances to several classes of insecticides in field. In order to investigate multiple resistance mechanisms, synergistic tests were conducted with the Ruian (RA) population and Lianyungang (LYG) population, two representative populations to different insecticides. Results showed that diethyl maleate (DEM), S,S,S-tributyl phosphorotrithioate (DEF) and piperonyl butoxide (PBO), had no significant synergistic or inhibitory effect on the high level of resistance to monosultap (313.4-fold) and moderate level to chlorpyrifos (36.9-fold) in Ruian field population from the year 2011 (RA11). DEF significantly synergized the activity of triazophos in RA11 population (536.8-fold), with synergism ratio of 1.92. DEF and PBO significantly suppressed 43.3% and 40.4% of resistance to fipronil in RA11 population (48.4-fold), respectively, with the synergistic ratios of 1.76 and 1.69. When pretreated with PBO, the activity of deltamethrin against RA11 population were significantly synergized, with synergism ratio of 9.57, and with reducing resistance levels from 152.5- to 15.9-fold. The results of this study indicated that resistance to several classes of insecticide among the field populations of C. suppressalis might be provided by the combination of the multiple resistance mechanisms. Metabolic resistance mechanism might be the major reason for the evolution for resistance to deltamethrin and fipronil, while resistance to monosultap, triazophos and chlorpyrifos is not associated with metabolic resistance.     

增效混剂延缓棉铃虫对菊酯农药抗性机制的研究

用同位素标记体壁穿透测定法及不同种群神经敏感性电生理测定法,进行了增效混剂延缓棉铃虫对菊酯抗性机制的初步研究。结果表明,与抗性生物汰选相一致,显示出药剂对增效菊酯混剂汰选种群比对菊酯单剂汰选种群的棉铃虫体壁穿透率高、神经敏感性也高的特点。证明了混剂延缓棉铃虫抗药性确与体壁穿透率和神经敏感性有关,有机制存在,这就为使用增效混剂进行棉铃虫抗性治理的措施打下了部分理论基础。     

Climate change and the genetics of insecticide resistance

Changes in global temperature and humidity as a result of climate change are producing rapid evolutionary changes in many animal species, including agricultural pests and disease vectors, leading to changes in allele frequencies of genes involved in thermotolerance and desiccation resistance. As some of these genes have pleiotropic effects on insecticide resistance, climate change is likely to affect insecticide resistance in the field. In this review, we discuss how the interactions between adaptation to climate change and resistance to insecticides can affect insecticide resistance in the field using examples in phytophagous and hematophagous pest insects, focusing on the effects of increased temperature and increased aridity. We then use detailed genetic and mechanistic studies in the model insect, Drosophila melanogaster, to explain the mechanisms underlying this phenomenon. We suggest that tradeoffs or facilitation between adaptation to climate change and resistance to insecticides can alter insecticide resistance allele frequencies in the field. The dynamics of these interactions will need to be considered when managing agricultural pests and disease vectors in a changing climate. © 2019 Society of Chemical Industry     

Metabolic mechanisms of insecticide resistance in the western flower thrips, Frankliniella occidentalis (Pergande)

The interactions between six insecticides (methiocarb, formetanate, acrinathrin, deltamethrin, methamidophos and endosulfan) and three potential synergists (piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF) and diethyl maleate (DEM)) were studied by topical exposure in strains selected for resistance to each insecticide, and in a susceptible strain of Frankliniella occidentalis (Pergande). In the susceptible strain PBO produced appreciable synergism only of formetanate, methiocarb and methamidophos. Except for endosulfan, PBO synergized all the insecticides to varying degrees in the resistant strains. A very high level of synergism by PBO was found with acrinathrin, which reduced the resistance level from 3344- to 36-fold. PBO slightly synergized the carbamates formetanate (4.6-fold) and methiocarb (3.3-fold). PBO also produced a high synergism of deltamethrin (12.5-fold) and methamidophos (14.3-fold) and completely restored susceptibility to both insecticides. DEF did not produce synergism with any insecticide in the resistant strains and DEM was slightly synergistic to endosulfan (3-fold). These studies indicate that an enhanced detoxification, mediated by cytochrome P-450 monooxygenases, is the major mechanism imparting resistance to different insecticides in F occidentalis. Implications of different mechanisms in insecticide resistance in F occidentalis are discussed.     

Segregation of non‐target‐site resistance to herbicides in multiple‐resistant Alopecurus myosuroides plants

Non‐target‐site resistance (NTSR) comprises a set of mechanisms conferring resistance to multiple modes of action. Investigation of the number of loci involved in NTSR will aid in the understanding of these resistance mechanisms. Therefore, six different multiple herbicide‐resistant Alopecurus myosuroides plants with different herbicide history were crossed in two generations with a susceptible wild type. Seeds from the backcrossing generation were studied for their segregation rate for resistance to five herbicides with four different modes of action (HRAC groups C₂, A, B and K₃). Taking into account that NTSR is a set of quantitative traits, the numbers of loci controlling NTSR were estimated using a normal mixture model fitted by the NLMIXED procedure of SAS. Each herbicide was controlled by a different number of loci comparing the six plants. In most of the cases, chlorotoluron resistance was controlled by one locus, whereas resistance to fenoxaprop‐P‐ethyl needed one or two loci. Resistance to pinoxaden was in all plants conferred by two loci. Cross‐resistance of fenoxaprop‐P‐ethyl and pinoxaden was found in all backcrossings, indicating that at least one of the two loci is responsible for both resistances. Resistance to mesosulfuron + iodosulfuron was conferred by a minimum of two loci. Results indicated that a minimum of five different loci can be involved in a multiple NTSR plant. Furthermore, the plant‐specific accumulation of NTSR loci was demonstrated. Such behaviour should be taken into account when evaluating the development and further spread of herbicide resistance.     

Insecticide resistance management strategies against the western flower thrips, Frankliniella occidentalis

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an economically important pest of a wide range of crops grown throughout the world. Insecticide resistance has been documented in many populations of WFT. Biological and behavioural characteristics and pest management practices that promote insecticide resistance are discussed. In addition, an overview is provided of the development of insecticide resistance in F. occidentalis populations and the resistance mechanisms involved. Owing to widespread resistance to most conventional insecticides, a new approach to insecticide resistance management (IRM) of F. occidentalis is needed. The IRM strategy proposed consists of two parts. Firstly, a general strategy to minimise the use of insecticides in order to reduce selection pressure. Secondly, a strategy designed to avoid selection of resistance mechanisms, considering cross-resistance patterns and resistance mechanisms.     

Pyramiding of partial disease resistance genes has a predictable,but diminishing,benefit to efficacy

Partial resistance genes often need to be ‘pyramided’ into a crop cultivar to obtain commercially acceptable levels of disease resistance. Analysis of data from four different wheat mapping populations, segregating for partial resistance to four contrasting foliar pathogens, showed a diminishing benefit to disease control from increasing the numbers of resistance loci in wheat lines. To test whether a general function could describe the efficacy benefit from pyramiding, a simple multiplicative survival model (MSM) was used to predict disease severities on mapping population lines carrying various combinations of two, three or four resistance loci. The effectiveness of each resistance locus was expressed as the disease severity in lines carrying resistance alleles at one locus, as a proportion of the severity in lines carrying no detectable resistance alleles. The predicted severity from any given combination of multiple resistance loci was calculated as the product of the proportional severities for the relevant single loci. A regression line fitted to transformed observed against predicted values explained 93% of the variation, with a slope and intercept not significantly different from 1 and 0, respectively. MSM may therefore provide a simple method to test contrasting types of partial resistance and search for synergistic combinations. The analysis suggests that diminishing returns are a general feature of partial resistance to foliar pathogens in wheat, a finding that is likely to apply to other crop pathosystems. Identifying and combining ever more QTL is likely to provide limited gains. The consequences of these findings for QTL analysis are described.     

A Gene-for-Gene Relationship Underlying the Species-Specific Parasitism of Avena/Triticum Isolates of Magnaporthe grisea on Wheat Cultivars

ABSTRACT To elucidate genetic mechanisms of the species-specific parasitism of Magnaporthe grisea, a Triticum isolate (pathogenic on wheat) was crossed with an Avena isolate (pathogenic on oat), and resulting F(1) progeny were subjected to segregation analyses on wheat cvs. Norin 4 and Chinese Spring. We found two fungal loci, Pwt3 and Pwt4, which are involved in the specific parasitism on wheat. Pwt3 operated on both cultivars while Pwt4 operated only on 'Norin 4'. Using the cultivar specificity of Pwt4, its corresponding resistance gene was successfully identified in 'Norin 4' and designated as Rmg1 (Rwt4). The presence of the corresponding resistance gene indicated that Pwt4 is an avirulence locus. Pwt3 was assumed to be an avirulence locus because of its temperature sensitivity. We suggest that gene-for-gene interactions underlie the species-specific parasitism of M. grisea.     

Variation in resistance to the root-knot nematode Meloidogyne incognita in tomato genotypes bearing the Mi gene

Root-knot nematodes ( Meloidogyne spp.) are among the main pathogens of tomato ( Lycopersicon esculentum ) worldwide. Plant resistance is currently the method of choice for controlling these pests and all the commercially available resistant cultivars carry the dominant Mi gene, which confers resistance to the three main species Meloidogyne arenaria , M. incognita and M. javanica . However the emergence of virulent biotypes able to overcome the tomato resistance gene may constitute a severe limitation to such a control strategy. To date, little was known of the possible influence of the homozygous vs heterozygous allelic state of the Mi locus, or the tomato genetic background, on the expression of the resistance. In order to test both these factors, the resistance was evaluated of a large panel of L. esculentum genotypes (selected from the Vilmorin germplasm stock collection) to seven M. incognita lines avirulent or virulent against the Mi gene. Plant resistance was estimated by counting the egg masses on the root systems after inoculation with second-stage juveniles (J₂). Reproduction of the nematodes was similar or, more often, significantly higher on heterozygous tomato genotypes than on homozygous ones, suggesting a possible dosage effect of the Mi gene. Data also indicated that the tomato genetic background had a major effect on the variations observed in nematode reproduction, especially when tomato genotypes were heterozygous for the Mi gene. These results have important consequences in terms of breeding strategies and durability of the resistance conferred by the Mi gene.     

Polygenic powdery mildew disease resistance in Arabidopsis thaliana: quantitative trait analysis of the accession Warschau-1

To further the understanding of the natural genetic diversity for disease resistance to powdery mildew ( Erysiphe cichoracearum ) in Arabidopsis thaliana , quantitative trait loci analysis was undertaken on recombinant inbred lines derived from a cross between the resistant accession Warschau-1 and the susceptible Columbia-0. Powdery mildew grew less well on Warschau-1, but the resistance was not associated with a specific block in the infection sequence. Two potential powdery mildew disease-resistance loci were identified and mapped, one with a major effect and one with a minor effect on disease resistance. The two loci acted in an additive manner to confer resistance, and together they explained 65% of the variation in resistance. In addition, the major powdery mildew disease-resistance locus was genetically mapped to the bottom of chromosome III, a region containing the powdery mildew resistance loci RPW7 , RPW8 and RPW10 . Unlike resistance mediated by the RPW8 locus in the accession Moscow-1, resistance in Warschau-1 was not correlated with the hypersensitive response, highlighting the influence of genetic background or environmental factors on the expression of disease resistance. Together with the powdery mildew resistance loci described in other studies, these results suggest that A. thaliana is a useful source of natural powdery mildew disease resistance, which potentially can be utilized in fundamental studies and as a tool for applied studies.     

Whole-genome QTL analysis of Stagonospora nodorum blotch resistance and validation of the SnTox4-Snn4 interaction in hexaploid wheat

Necrotrophic effectors (also known as host-selective toxins) are important determinants of disease in the wheat-Stagonospora nodorum pathosystem. To date, five necrotrophic effector-host gene interactions have been identified in this system. Most of these interactions have additive effects while some are epistatic. The Snn4-SnTox4 interaction was originally identified in a recombinant-inbred population derived from a cross between the Swiss winter wheat cultivars 'Arina' and 'Forno' using the S. nodorum isolate Sn99CH 1A7a. Here, we used a recombinant-inbred population consisting of 121 lines developed from a cross between the hexaploid land race Salamouni and the hexaploid wheat 'Katepwa' (SK population). The SK population was used for the construction of linkage maps and quantitative trait loci (QTL) detection using the Swiss S. nodorum isolate Sn99CH 1A7a. The linkage maps developed in the SK population spanned 3,228 centimorgans (cM) and consisted of 441 simple-sequence repeats, 9 restriction fragment length polymorphisms, 29 expressed sequence tag sequence-tagged site markers, and 5 phenotypic markers. The average marker density was 6.7 cM/marker. Two QTL, designated QSnb.fcu-1A and QSnb.fcu-7A on chromosome arms 1AS and 7AS, respectively, were associated with disease caused by the S. nodorum isolate Sn99CH 1A7a. The effects of QSnb.fcu-1A were determined by the Snn4-SnTox4 interaction and accounted for 23.5% of the phenotypic variation in this population, whereas QSnb.fcu-7A accounted for 16.4% of the phenotypic variation for disease but was not associated with any known effector sensitivity locus. The effects of both QTL were largely additive and collectively accounted for 35.7% of the total phenotypic variation. The results of this research validate the effects of a compatible Snn4-SnTox4 interaction in a different genetic background, and it provides knowledge regarding genomic regions and molecular markers that can be used to improve Stagonospora nodorum blotch resistance in wheat germplasm.     

Genetic Analysis of Metalaxyl Insensitivity Loci in Phytophthora infestans Using Linked DNA Markers

ABSTRACT Previous studies indicated that incompletely dominant loci determine insensitivity by oomycetes to phenylamide fungicides such as metalaxyl. To compare the bases of insensitivity in different strains of the late blight pathogen, Phytophthora infestans, crosses were performed between sensitive isolates and isolates from Mexico, the Netherlands, and the United Kingdom that displayed varying levels of insensitivity. Segregation analyses indicated that metalaxyl insensitivity was determined primarily by one locus in each isolate, and that two of the isolates were heterozygous and the other homozygous for the insensitive allele. Metalaxyl insensitivity was also affected by the segregation of additional loci of minor effect. DNA markers linked to insensitivity were obtained by bulked segregant analysis using random amplified polymorphic DNA (RAPD) markers and the Dutch and Mexican crosses. By studying the linkage relationships between these markers and the insensitivity in each cross by RAPD or restriction fragment length polymorphism analysis, it appeared that the same chromosomal locus conferred insensitivity in the Mexican and Dutch isolates. However, a gene at a different chromosomal position was responsible for insensitivity in the British isolate.     

The identification of a gene for race-specific resistance to Peronospora parasitica (downy mildew) in Brassica napus var. oleifera (oilseed rape)

The oilseed rape cultivar Cresor was resistant to 14 isolates of Peronospora parasitica derived from crops of Brassica napus in the UK. Segregation for resistance to one isolate among F₂ plants and F₃ progeny of crosses between Cresor and the susceptible cultivars Victor and Jet Neuf indicated that resistance was controlled by a single gene. There was evidence that genetic background and environment could influence the phenotypic expression of this resistance. Two sexual progeny isolates derived from a homothallic isolate of P. parasitica avirulent on Cresor were completely virulent on this cultivar. This suggested that the parental isolate was heterozygous at a matching locus or loci for avirulence and demonstrated the race-specific nature of the resistance.