Twenty-one accessions of 3 wild Lacfuca 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 l-l 1 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying Rl 1. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific.Tests on segregating F, populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/lS) 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.

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.

The inheritance of specific virulence in Bremia lactucae was studied by hybridizing two isolates which differed in their virulence on host cultivars carrying the resistance factors Rl, R2, R4, R6 and Rll. Avirulence on Rl, R2, R4 and Rl 1 was dominant to virulence and the segregation conformed to that expected for alleles at single loci, although reactions on R4 were not always clearly defined. The loci conditioning avirulence on R2 and Rll were probably linked. It was difficult to classify the reactions of isolates on R6 and the ratios obtained could not readily be interpreted in Mendelian terms. Since no segregation was observed for reaction on R3, R5, R7, R8, R9 and RIO' it was concluded that the genes determining virulence on these factors occurred at the same loci in both isolates. Sexual compatibility type (Bl and B2) segregated with a ratio of 1:1.

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SUMMARYData are presented on the segregation of resistance to four races of Bremia lactucae in the F2 progenies of crosses involving 15 resistant and various susceptible lettuce cultivars. Most of these data and those recently published by other workers fit the systematic model for the genetics of race specific resistance to B. lactucae proposed by Crute & Johnson(1976).Seven different dominant resistance genes of major effect were found. There was also evidence of a pair of dominant genes with complementary effect, one of which may sometimes be effective on its own. Two of the genes may be linked and another may have two different alleles for resistance at the same locus. The resistance conferred by these genes is specified in relation to two British, two Dutch and four French races of the fungus. Resistance genotypes are proposed for 16 cultivars.