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  • 11
    Electronic Resource
    Electronic Resource
    Variation for virulence in Erysiphe fischeri from Senecio vulgaris (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 42 (1993), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Variation for virulence was investigated in two populations of Erysiphe fischeri, the cause of powdery mildew of the common annual weed Senecio vulgaris (groundsel). Infection types were recorded on detached leaf segments from 50 inbred lines of S. vulgaris following inoculation with 24 single-conidial-chain isolates of E. fischeri (12 each from two UK sites; one, at Glasgow, located about 480 km north of the other, at Wellesbourne). Mean infection types for each isolate/line combination were categorized by applying several upper limits below which combinations were considered to be incompatible. Regardless of the limit applied, numerous specific virulence and resistance phenotypes could be discriminated. Virulence phenotypes were complex and all isolates were capable of colonizing and reproducing on the majority of groundsel lines. However, all isolates were completely avirulent (no reproduction) on at least one groundsel line, so no isolate was universally virulent. Plants of several groundsel lines exhibiting different resistance phenotypes were exposed to natural infection at Wellesbourne to act as mildew traps, to examine the virulence characteristics of the pathogen population. Common components of the fungus population colonized groundsel lines exhibiting resistance to only a few or none of the single-conidial-chain isolates. Rarer components of the pathogen population colonized groundsel lines with resistance to the majority or all of the isolates previously tested. These latter components of the fungus population also detected resistance in groundsel lines previously recorded as susceptible to all isolates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1993.tb01543.x
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  • 12
    Electronic Resource
    Electronic Resource
    Genetic relationship between races of Pseudomonas syringae pv.pisi and cultivars of Pisum sativum (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 38 (1989), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Isolates of Pseudomonas syringae pv. pisi from the UK and overseas were categorized into six races on the basis of their reactions to a range of differential pea (Pisum sativum) cultivars. Race 2 was predominant among the isolates examined and this probably reflects its relative international importance. A previously uncharacterized race (race 6) was virulent on all cultivars tested. Resistance to races 1-5 was widespread in commercial cultivars and breeding lines with more than 75% showing resistance to one or more races. A preliminary study of the inheritance of resistance indicated that for races 1, 2 and 3, resistance was controlled by different dominant genes. The genetic basis for the relationship between races of P. syringae pv. pisi and pea cultivars was explained in terms of a gene-for-gene relationship involving five matching gene pairs. With further clarification of the genetics of resistance this host-pathogen association will meet most of the requirements of a model system for the study of the genetic and molecular basis of pathogenicity and host specificity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1989.tb02155.x
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  • 13
    Electronic Resource
    Electronic Resource
    Studies on the inheritance of resistance to metalaxyl in Bremia lactucae and on the stability and fitness of field isolates (1988)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 37 (1988), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: One hundred and eighty-one sexual progeny isolates from three F1, one F2, five BC1 and one BC2 generations of crosses involving two metalaxyl-resistant and two metalaxyl-sensitive isolates of Bremia lactucae were assessed for response to metalaxyl. In most cases, isolates were also tested for virulence phenotype and sexual compatibility type. Isolates could be classified into three phenotypes (sensitive, resistant and intermediate) on the basis of their response to fungicide. Sensitive progeny isolates were similar to the sensitive parents and did not sporulate on lettuce seedlings grown in the presence of 〉 1.0 μg/ml metalaxyl. Resistant progeny isolates, in common with the resistant parents, sporulated as readily on lettuce seedlings grown in the presence of 100 μg/ml metalaxyl as on untreated seedlings. Progeny isolates which expressed an intermediate phenotype not observed in field samples were also recovered. This phenotype was more variable but isolates so classified were able to sporulate on seedlings grown in the presence of 100 μg/ml metalaxyl but exhibited a latent period (time from inoculation to appearance of sporulation) which was on average twice as long as that observed on seedlings grown in the absence of fungicide.Segregation patterns suggested that response to metalaxyl is controlled at a single locus exhibiting incomplete dominance but that the expression of response in the heterozygous condition may be affected by modifier genes. There was evidence that the metalaxyl-resistant parental field isolates were heterokaryotic (possibly carrying nuclei homozygous and heterozygous at the locus controlling metalaxyl response). After 10-30 asexual generations in the absence of fungicide, the response to metalaxyl of resistant field isolates began to approach that of an intermediate type. This phenomenon did not occur with a resistant sexual progeny isolate (presumed homokaryotic) and is additional evidence for the heterokaryosis of field isolates. The relative fitness of a range of isolates was examined by observing changes in the proportions of spores over three asexual generations after they were initially mixed in a 1:1 ratio with mutant isolates carrying a microscopically visible genetic marker (large lipid droplets). Most field isolates were more fit than the mutant isolates used, regardless of whether they were resistant or sensitive to metalaxyl, and there was no evidence that resistant isolates carried any fitness deficit in comparison with the sensitive isolates examined. In contrast, sexual progeny isolates (particularly from BC1 generations) were often less fit than the mutant isolates used, but there was no evidence that this was associated with response to fungicide. These findings are discussed in relation to the mode of action of metalaxyl and disease control strategy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1988.tb02069.x
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  • 14
    Electronic Resource
    Electronic Resource
    The genetic control and expression of specificity in Bremia lactucae (lettuce downy mildew) (1984)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 33 (1984), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The inheritance of specific virulence in Bremia lactucae was studied in crosses involving 12 heterothallic isolates of the fungus. In one cross, virulence to eight of the II specific resistance factors examined segregated in the F1 generation. Although there were exceptions, most of the data were consistent with the hypothesis that pathogenicity was controlled by independent single loci with avirulence dominant to virulence. Linkage between loci determining virulence on R2 and R11 was confirmed. Loci controlling virulence on R5 and R8 also appeared to be linked but these may be identical R-factors. Contrary to a previous suggestion, the locus determining virulence to R10 was independent of that for R5/R8. The expression of virulence to three R-factors (Rl, R4 and R5/R8) was influenced by independent second loci. The presence of a dominant allele at the second locus inhibited avirulence. The expression of avirulence on R6 seemed to be influenced by modifier genes and environment in some isolates. Although the cultivars Mildura, Bourguignonne, Sucrine and Captain were originally thought to contain a single resistance factor, these data suggest that Sucrine carries R5/R8 in addition to R10 whilst Mildura may carry Rl in addition to R3. The stock of Bourguignonne appeared to be a mixture of resistance genotypes. The data add additional support to the suggestion that Capitan (Rll) may carry two resistance factors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1984.tb01334.x
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  • 15
    Electronic Resource
    Electronic Resource
    The occurrence, characteristics and distribution in the United Kingdom of resistance to phenylamide fungicides in Bremia lactucae (lettuce downy mildew) (1987)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 36 (1987), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Failure to control Bremia lactucae (lettuce downy mildew) with metalaxyl in an intensive lettuce-producing region of Lancashire at the end of 1983 was shown to be due to the occurrence of a high level of resistance to this fungicide (isolates capable of growth at 〈 100 μg/ml metalaxyl). During most of 1984, metalaxyl-resistant isolates were obtained from numerous sites but all within a 20-km radius of the initial outbreak. Thereafter, at the end of 1984 and during 1985, metalaxyl-resistant isolates were recovered from most major lettuce-producing regions in the UK with protected crops more affected than field crops. AH metalaxyl-resistant isolates tested were identical in their response to fungicide, sexual compatibility type (B2) and virulence phenotype, probably representing a clone from a single origin. The resistant pathotype was virulent on resistance factors R 1-10 and 12-15 but lacked virulence for R 11 and 16-18. This was also the most common virulence phenotype among sensitive isolates collected at the same time. Cross-resistance to other phenylamide fungicides was demonstrated but isolates resistant and sensitive to phenylamide showed a similar response to the unrelated systemic fungicides propamocarb and fosetyl-Al. An F1 sexual progeny isolate from a cross between a phenylamide-sensitive and a phenylamide-resistant isolate (presumed heterozygous at the locus or loci regulating response to phenylamide fungicides) exhibited an intermediate response to phenylamide fungicides. No isolates of this type were obtained from the field. At the high concentrations affecting spore germination, phenylamide fungicides exhibited lower activity against a resistant isolate compared with a sensitive isolate. The findings are discussed in relation to future control strategies, the population biology of the fungus and possible directions for lettuce breeding programmes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1987.tb02237.x
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  • 16
    Electronic Resource
    Electronic Resource
    Further characterization of field resistance in lettuce to Bremia lactucae (downy mildew) (1985)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 34 (1985), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The development of five Bremia lactucae isolates on the field-resistant crisphead lettuce cultivar Iceberg was compared under field conditions with that on the more susceptible crisphead cultivar Ithaca. With each of five isolates, the epidemic developed more rapidly on Ithaca than Iceberg; lesions on Ithaca were more numerous, larger and sporulated more profusely than on Iceberg. There was no evidence for adaptation of isolates to Iceberg, which provides some additional evidence that the resistance of this cultivar could be non-differential (horizontal). There was, however, evidence of non-specific differences between isolates in their quantitative virulence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1985.tb01397.x
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  • 17
    Electronic Resource
    Electronic Resource
    Genetics of specific resistance in pea (Pisum sativum) cultivars to seven races of Pseudomonas syringae pv. pisi (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Plant pathology 44 (1995), S. 0 
    Details
    ISSN: 1365-3059
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Seven races of Pseudomonas syringae pv. pisi were distinguished using eight differential cultivars of pea (Pisum sativum). Segregation among F2 populations of crosses between differential cultivars sequentially inoculated with races of P.s. pv. pisi provided evidence for four and possibly six putative resistance(R)/avirulence(A) gene pairs. R1, R2 and R3 are dominant resistance alleles at single loci, R4 is a dominant allele at a single locus which exhibits variable expression possibly dependent on genetic background. There is evidence that R3 and R4 are at linked loci. Homology tests provided proof of the occurrence of the alleles R2, R3 and R4 in more than one cultivar. Two other alleles, R5 and R6, were postulated to explain the observed segregation ratios in certain crosses.It can be inferred that P.s. pv. pisi races 2, 3 and 4 each carry a different single a virulence gene, race 6 carries no apparent avirulence genes, and race 7 carries at least A2, A3 and A4. Race 1 carries Al, A3, A4 and possibly A6; race 5 carries A2, A4 and possibly A5 and A6.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3059.1995.tb02721.x
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  • 18
    Electronic Resource
    Electronic Resource
    Evidence for a race-specific resistance factor in some lettuce (Lactuca sativa L.) cultivars previously considered to be universally susceptible to Bremia lactucae regel (1981)
    Springer
    Theoretical and applied genetics 60 (1981), S. 185-189 
    Details
    ISSN: 1432-2242
    Keywords: Bremia lactucae ; Lettuce ; Race-specific resistance ; Downy mildew ; Host/pathogen genetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Previously undetected race-specific resistance to Bremia lactucae (downy mildew) was located in many lettuce cultivars hitherto considered to be universally susceptible to this disease. This resistance factor(s) may also be widely distributed in other cultivars known to carry combinations of already recognised factors R1 to R11. Specific virulence to match this resistance is almost invariably present in pathogen collections. This situation may be either a relic of the evolutionary history of the B. lactucae — L. sativa asssociation or may reflect a rare mutation in B. lactucae for avirulence on all but a few specialised L. sativa genotypes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00264527
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  • 19
    Electronic Resource
    Electronic Resource
    An association between resistance to root aphid (Pemphigus bursarius L.) and downy mildew (Bremia lactucae Regel) in lettuce (1980)
    Springer
    Euphytica 29 (1980), S. 483-488 
    Details
    ISSN: 1573-5060
    Keywords: Lactuca sativa ; lettuce ; Pemphigus bursarius ; root aphid ; Bremia lactucae ; downy mildew ; resistance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary Many lettuce cultivars (Lactuca sativa L.) with high resistance to lettuce root aphid (Pemphigus bursarius L.) also carried the gene Dm-6 for specific resistance to downy mildew (Bremia lactucae Regel). This suggests the possibility of linkage between this gene and root aphid resistance. The origin of this association is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00025149
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  • 20
    Electronic Resource
    Electronic Resource
    The location and characteristics of novel sources of resistance to Bremia Lactucae Regel (downy mildew) in wild Lactuca L. Species (1981)
    Springer
    Euphytica 30 (1981), S. 659-668 
    Details
    ISSN: 1573-5060
    Keywords: Bremia lactucae ; downy mildew ; Lactuca species ; lettuce ; resistance ; physiologic races
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Summary Twenty-one accessions of 3 wild Lactuca species which could be hybridised with L. sativa, the cultivated lettuce, were inoculated at different stages of plant development with 3 multivirulent isolates of Bremia lactucae. Nineteen sources of resistance to B. lactucae, not attributable to the previously recognised resistance factors 1–11 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying R11. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific. Tests on segregating F2 populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/18) appears to be inherited as a single dominant gene, which is sometimes incomplete in expression and allelic to either Dm6 or R7. The segregation patterns for resistance in PI 281876 did not give readily interpretable ratios. To assess the frequency of occurrence in B. lactucae populations of virulence factors to overcome this novel resistance, 11 of the novel sources of resistance were inoculated with numerous collections of the pathogen from the UK, Czechoslovakia and elsewhere and found to show a high level of resistance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00038794
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